ExprConstant.cpp

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//===--- ExprConstant.cpp - Expression Constant Evaluator -----------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements the Expr constant evaluator.
//
// Constant expression evaluation produces four main results:
//
//  * A success/failure flag indicating whether constant folding was successful.
//    This is the 'bool' return value used by most of the code in this file. A
//    'false' return value indicates that constant folding has failed, and any
//    appropriate diagnostic has already been produced.
//
//  * An evaluated result, valid only if constant folding has not failed.
//
//  * A flag indicating if evaluation encountered (unevaluated) side-effects.
//    These arise in cases such as (sideEffect(), 0) and (sideEffect() || 1),
//    where it is possible to determine the evaluated result regardless.
//
//  * A set of notes indicating why the evaluation was not a constant expression
//    (under the C++11 / C++1y rules only, at the moment), or, if folding failed
//    too, why the expression could not be folded.
//
// If we are checking for a potential constant expression, failure to constant
// fold a potential constant sub-expression will be indicated by a 'false'
// return value (the expression could not be folded) and no diagnostic (the
// expression is not necessarily non-constant).
//
//===----------------------------------------------------------------------===//

#include "clang/AST/APValue.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/ASTDiagnostic.h"
#include "clang/AST/ASTLambda.h"
#include "clang/AST/CharUnits.h"
#include "clang/AST/CurrentSourceLocExprScope.h"
#include "clang/AST/CXXInheritance.h"
#include "clang/AST/Expr.h"
#include "clang/AST/OSLog.h"
#include "clang/AST/RecordLayout.h"
#include "clang/AST/StmtVisitor.h"
#include "clang/AST/TypeLoc.h"
#include "clang/Basic/Builtins.h"
#include "clang/Basic/FixedPoint.h"
#include "clang/Basic/TargetInfo.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/SmallBitVector.h"
#include "llvm/Support/SaveAndRestore.h"
#include "llvm/Support/raw_ostream.h"
#include <cstring>
#include <functional>

#define DEBUG_TYPE "exprconstant"

using namespace clang;
using llvm::APInt;
using llvm::APSInt;
using llvm::APFloat;
using llvm::Optional;

static bool IsGlobalLValue(APValue::LValueBase B);

namespace {
  struct LValue;
  struct CallStackFrame;
  struct EvalInfo;

  using SourceLocExprScopeGuard =
      CurrentSourceLocExprScope::SourceLocExprScopeGuard;

  static QualType getType(APValue::LValueBase B) {
    if (!B) return QualType();
    if (const ValueDecl *D = B.dyn_cast<const ValueDecl*>()) {
      // FIXME: It's unclear where we're supposed to take the type from, and
      // this actually matters for arrays of unknown bound. Eg:
      //
      // extern int arr[]; void f() { extern int arr[3]; };
      // constexpr int *p = &arr[1]; // valid?
      //
      // For now, we take the array bound from the most recent declaration.
      for (auto *Redecl = cast<ValueDecl>(D->getMostRecentDecl()); Redecl;
           Redecl = cast_or_null<ValueDecl>(Redecl->getPreviousDecl())) {
        QualType T = Redecl->getType();
        if (!T->isIncompleteArrayType())
          return T;
      }
      return D->getType();
    }

    if (B.is<TypeInfoLValue>())
      return B.getTypeInfoType();

    const Expr *Base = B.get<const Expr*>();

    // For a materialized temporary, the type of the temporary we materialized
    // may not be the type of the expression.
    if (const MaterializeTemporaryExpr *MTE =
            dyn_cast<MaterializeTemporaryExpr>(Base)) {
      SmallVector<const Expr *, 2> CommaLHSs;
      SmallVector<SubobjectAdjustment, 2> Adjustments;
      const Expr *Temp = MTE->GetTemporaryExpr();
      const Expr *Inner = Temp->skipRValueSubobjectAdjustments(CommaLHSs,
                                                               Adjustments);
      // Keep any cv-qualifiers from the reference if we generated a temporary
      // for it directly. Otherwise use the type after adjustment.
      if (!Adjustments.empty())
        return Inner->getType();
    }

    return Base->getType();
  }

  /// Get an LValue path entry, which is known to not be an array index, as a
  /// field declaration.
  static const FieldDecl *getAsField(APValue::LValuePathEntry E) {
    return dyn_cast_or_null<FieldDecl>(E.getAsBaseOrMember().getPointer());
  }
  /// Get an LValue path entry, which is known to not be an array index, as a
  /// base class declaration.
  static const CXXRecordDecl *getAsBaseClass(APValue::LValuePathEntry E) {
    return dyn_cast_or_null<CXXRecordDecl>(E.getAsBaseOrMember().getPointer());
  }
  /// Determine whether this LValue path entry for a base class names a virtual
  /// base class.
  static bool isVirtualBaseClass(APValue::LValuePathEntry E) {
    return E.getAsBaseOrMember().getInt();
  }

  /// Given a CallExpr, try to get the alloc_size attribute. May return null.
  static const AllocSizeAttr *getAllocSizeAttr(const CallExpr *CE) {
    const FunctionDecl *Callee = CE->getDirectCallee();
    return Callee ? Callee->getAttr<AllocSizeAttr>() : nullptr;
  }

  /// Attempts to unwrap a CallExpr (with an alloc_size attribute) from an Expr.
  /// This will look through a single cast.
  ///
  /// Returns null if we couldn't unwrap a function with alloc_size.
  static const CallExpr *tryUnwrapAllocSizeCall(const Expr *E) {
    if (!E->getType()->isPointerType())
      return nullptr;

    E = E->IgnoreParens();
    // If we're doing a variable assignment from e.g. malloc(N), there will
    // probably be a cast of some kind. In exotic cases, we might also see a
    // top-level ExprWithCleanups. Ignore them either way.
    if (const auto *FE = dyn_cast<FullExpr>(E))
      E = FE->getSubExpr()->IgnoreParens();

    if (const auto *Cast = dyn_cast<CastExpr>(E))
      E = Cast->getSubExpr()->IgnoreParens();

    if (const auto *CE = dyn_cast<CallExpr>(E))
      return getAllocSizeAttr(CE) ? CE : nullptr;
    return nullptr;
  }

  /// Determines whether or not the given Base contains a call to a function
  /// with the alloc_size attribute.
  static bool isBaseAnAllocSizeCall(APValue::LValueBase Base) {
    const auto *E = Base.dyn_cast<const Expr *>();
    return E && E->getType()->isPointerType() && tryUnwrapAllocSizeCall(E);
  }

  /// The bound to claim that an array of unknown bound has.
  /// The value in MostDerivedArraySize is undefined in this case. So, set it
  /// to an arbitrary value that's likely to loudly break things if it's used.
  static const uint64_t AssumedSizeForUnsizedArray =
      std::numeric_limits<uint64_t>::max() / 2;

  /// Determines if an LValue with the given LValueBase will have an unsized
  /// array in its designator.
  /// Find the path length and type of the most-derived subobject in the given
  /// path, and find the size of the containing array, if any.
  static unsigned
  findMostDerivedSubobject(ASTContext &Ctx, APValue::LValueBase Base,
                           ArrayRef<APValue::LValuePathEntry> Path,
                           uint64_t &ArraySize, QualType &Type, bool &IsArray,
                           bool &FirstEntryIsUnsizedArray) {
    // This only accepts LValueBases from APValues, and APValues don't support
    // arrays that lack size info.
    assert(!isBaseAnAllocSizeCall(Base) &&
           "Unsized arrays shouldn't appear here");
    unsigned MostDerivedLength = 0;
    Type = getType(Base);

    for (unsigned I = 0, N = Path.size(); I != N; ++I) {
      if (Type->isArrayType()) {
        const ArrayType *AT = Ctx.getAsArrayType(Type);
        Type = AT->getElementType();
        MostDerivedLength = I + 1;
        IsArray = true;

        if (auto *CAT = dyn_cast<ConstantArrayType>(AT)) {
          ArraySize = CAT->getSize().getZExtValue();
        } else {
          assert(I == 0 && "unexpected unsized array designator");
          FirstEntryIsUnsizedArray = true;
          ArraySize = AssumedSizeForUnsizedArray;
        }
      } else if (Type->isAnyComplexType()) {
        const ComplexType *CT = Type->castAs<ComplexType>();
        Type = CT->getElementType();
        ArraySize = 2;
        MostDerivedLength = I + 1;
        IsArray = true;
      } else if (const FieldDecl *FD = getAsField(Path[I])) {
        Type = FD->getType();
        ArraySize = 0;
        MostDerivedLength = I + 1;
        IsArray = false;
      } else {
        // Path[I] describes a base class.
        ArraySize = 0;
        IsArray = false;
      }
    }
    return MostDerivedLength;
  }

  // The order of this enum is important for diagnostics.
  enum CheckSubobjectKind {
    CSK_Base, CSK_Derived, CSK_Field, CSK_ArrayToPointer, CSK_ArrayIndex,
    CSK_Real, CSK_Imag
  };

  /// A path from a glvalue to a subobject of that glvalue.
  struct SubobjectDesignator {
    /// True if the subobject was named in a manner not supported by C++11. Such
    /// lvalues can still be folded, but they are not core constant expressions
    /// and we cannot perform lvalue-to-rvalue conversions on them.
    unsigned Invalid : 1;

    /// Is this a pointer one past the end of an object?
    unsigned IsOnePastTheEnd : 1;

    /// Indicator of whether the first entry is an unsized array.
    unsigned FirstEntryIsAnUnsizedArray : 1;

    /// Indicator of whether the most-derived object is an array element.
    unsigned MostDerivedIsArrayElement : 1;

    /// The length of the path to the most-derived object of which this is a
    /// subobject.
    unsigned MostDerivedPathLength : 28;

    /// The size of the array of which the most-derived object is an element.
    /// This will always be 0 if the most-derived object is not an array
    /// element. 0 is not an indicator of whether or not the most-derived object
    /// is an array, however, because 0-length arrays are allowed.
    ///
    /// If the current array is an unsized array, the value of this is
    /// undefined.
    uint64_t MostDerivedArraySize;

    /// The type of the most derived object referred to by this address.
    QualType MostDerivedType;

    typedef APValue::LValuePathEntry PathEntry;

    /// The entries on the path from the glvalue to the designated subobject.
    SmallVector<PathEntry, 8> Entries;

    SubobjectDesignator() : Invalid(true) {}

    explicit SubobjectDesignator(QualType T)
        : Invalid(false), IsOnePastTheEnd(false),
          FirstEntryIsAnUnsizedArray(false), MostDerivedIsArrayElement(false),
          MostDerivedPathLength(0), MostDerivedArraySize(0),
          MostDerivedType(T) {}

    SubobjectDesignator(ASTContext &Ctx, const APValue &V)
        : Invalid(!V.isLValue() || !V.hasLValuePath()), IsOnePastTheEnd(false),
          FirstEntryIsAnUnsizedArray(false), MostDerivedIsArrayElement(false),
          MostDerivedPathLength(0), MostDerivedArraySize(0) {
      assert(V.isLValue() && "Non-LValue used to make an LValue designator?");
      if (!Invalid) {
        IsOnePastTheEnd = V.isLValueOnePastTheEnd();
        ArrayRef<PathEntry> VEntries = V.getLValuePath();
        Entries.insert(Entries.end(), VEntries.begin(), VEntries.end());
        if (V.getLValueBase()) {
          bool IsArray = false;
          bool FirstIsUnsizedArray = false;
          MostDerivedPathLength = findMostDerivedSubobject(
              Ctx, V.getLValueBase(), V.getLValuePath(), MostDerivedArraySize,
              MostDerivedType, IsArray, FirstIsUnsizedArray);
          MostDerivedIsArrayElement = IsArray;
          FirstEntryIsAnUnsizedArray = FirstIsUnsizedArray;
        }
      }
    }

    void truncate(ASTContext &Ctx, APValue::LValueBase Base,
                  unsigned NewLength) {
      if (Invalid)
        return;

      assert(Base && "cannot truncate path for null pointer");
      assert(NewLength <= Entries.size() && "not a truncation");

      if (NewLength == Entries.size())
        return;
      Entries.resize(NewLength);

      bool IsArray = false;
      bool FirstIsUnsizedArray = false;
      MostDerivedPathLength = findMostDerivedSubobject(
          Ctx, Base, Entries, MostDerivedArraySize, MostDerivedType, IsArray,
          FirstIsUnsizedArray);
      MostDerivedIsArrayElement = IsArray;
      FirstEntryIsAnUnsizedArray = FirstIsUnsizedArray;
    }

    void setInvalid() {
      Invalid = true;
      Entries.clear();
    }

    /// Determine whether the most derived subobject is an array without a
    /// known bound.
    bool isMostDerivedAnUnsizedArray() const {
      assert(!Invalid && "Calling this makes no sense on invalid designators");
      return Entries.size() == 1 && FirstEntryIsAnUnsizedArray;
    }

    /// Determine what the most derived array's size is. Results in an assertion
    /// failure if the most derived array lacks a size.
    uint64_t getMostDerivedArraySize() const {
      assert(!isMostDerivedAnUnsizedArray() && "Unsized array has no size");
      return MostDerivedArraySize;
    }

    /// Determine whether this is a one-past-the-end pointer.
    bool isOnePastTheEnd() const {
      assert(!Invalid);
      if (IsOnePastTheEnd)
        return true;
      if (!isMostDerivedAnUnsizedArray() && MostDerivedIsArrayElement &&
          Entries[MostDerivedPathLength - 1].getAsArrayIndex() ==
              MostDerivedArraySize)
        return true;
      return false;
    }

    /// Get the range of valid index adjustments in the form
    ///   {maximum value that can be subtracted from this pointer,
    ///    maximum value that can be added to this pointer}
    std::pair<uint64_t, uint64_t> validIndexAdjustments() {
      if (Invalid || isMostDerivedAnUnsizedArray())
        return {0, 0};

      // [expr.add]p4: For the purposes of these operators, a pointer to a
      // nonarray object behaves the same as a pointer to the first element of
      // an array of length one with the type of the object as its element type.
      bool IsArray = MostDerivedPathLength == Entries.size() &&
                     MostDerivedIsArrayElement;
      uint64_t ArrayIndex = IsArray ? Entries.back().getAsArrayIndex()
                                    : (uint64_t)IsOnePastTheEnd;
      uint64_t ArraySize =
          IsArray ? getMostDerivedArraySize() : (uint64_t)1;
      return {ArrayIndex, ArraySize - ArrayIndex};
    }

    /// Check that this refers to a valid subobject.
    bool isValidSubobject() const {
      if (Invalid)
        return false;
      return !isOnePastTheEnd();
    }
    /// Check that this refers to a valid subobject, and if not, produce a
    /// relevant diagnostic and set the designator as invalid.
    bool checkSubobject(EvalInfo &Info, const Expr *E, CheckSubobjectKind CSK);

    /// Get the type of the designated object.
    QualType getType(ASTContext &Ctx) const {
      assert(!Invalid && "invalid designator has no subobject type");
      return MostDerivedPathLength == Entries.size()
                 ? MostDerivedType
                 : Ctx.getRecordType(getAsBaseClass(Entries.back()));
    }

    /// Update this designator to refer to the first element within this array.
    void addArrayUnchecked(const ConstantArrayType *CAT) {
      Entries.push_back(PathEntry::ArrayIndex(0));

      // This is a most-derived object.
      MostDerivedType = CAT->getElementType();
      MostDerivedIsArrayElement = true;
      MostDerivedArraySize = CAT->getSize().getZExtValue();
      MostDerivedPathLength = Entries.size();
    }
    /// Update this designator to refer to the first element within the array of
    /// elements of type T. This is an array of unknown size.
    void addUnsizedArrayUnchecked(QualType ElemTy) {
      Entries.push_back(PathEntry::ArrayIndex(0));

      MostDerivedType = ElemTy;
      MostDerivedIsArrayElement = true;
      // The value in MostDerivedArraySize is undefined in this case. So, set it
      // to an arbitrary value that's likely to loudly break things if it's
      // used.
      MostDerivedArraySize = AssumedSizeForUnsizedArray;
      MostDerivedPathLength = Entries.size();
    }
    /// Update this designator to refer to the given base or member of this
    /// object.
    void addDeclUnchecked(const Decl *D, bool Virtual = false) {
      Entries.push_back(APValue::BaseOrMemberType(D, Virtual));

      // If this isn't a base class, it's a new most-derived object.
      if (const FieldDecl *FD = dyn_cast<FieldDecl>(D)) {
        MostDerivedType = FD->getType();
        MostDerivedIsArrayElement = false;
        MostDerivedArraySize = 0;
        MostDerivedPathLength = Entries.size();
      }
    }
    /// Update this designator to refer to the given complex component.
    void addComplexUnchecked(QualType EltTy, bool Imag) {
      Entries.push_back(PathEntry::ArrayIndex(Imag));

      // This is technically a most-derived object, though in practice this
      // is unlikely to matter.
      MostDerivedType = EltTy;
      MostDerivedIsArrayElement = true;
      MostDerivedArraySize = 2;
      MostDerivedPathLength = Entries.size();
    }
    void diagnoseUnsizedArrayPointerArithmetic(EvalInfo &Info, const Expr *E);
    void diagnosePointerArithmetic(EvalInfo &Info, const Expr *E,
                                   const APSInt &N);
    /// Add N to the address of this subobject.
    void adjustIndex(EvalInfo &Info, const Expr *E, APSInt N) {
      if (Invalid || !N) return;
      uint64_t TruncatedN = N.extOrTrunc(64).getZExtValue();
      if (isMostDerivedAnUnsizedArray()) {
        diagnoseUnsizedArrayPointerArithmetic(Info, E);
        // Can't verify -- trust that the user is doing the right thing (or if
        // not, trust that the caller will catch the bad behavior).
        // FIXME: Should we reject if this overflows, at least?
        Entries.back() = PathEntry::ArrayIndex(
            Entries.back().getAsArrayIndex() + TruncatedN);
        return;
      }

      // [expr.add]p4: For the purposes of these operators, a pointer to a
      // nonarray object behaves the same as a pointer to the first element of
      // an array of length one with the type of the object as its element type.
      bool IsArray = MostDerivedPathLength == Entries.size() &&
                     MostDerivedIsArrayElement;
      uint64_t ArrayIndex = IsArray ? Entries.back().getAsArrayIndex()
                                    : (uint64_t)IsOnePastTheEnd;
      uint64_t ArraySize =
          IsArray ? getMostDerivedArraySize() : (uint64_t)1;

      if (N < -(int64_t)ArrayIndex || N > ArraySize - ArrayIndex) {
        // Calculate the actual index in a wide enough type, so we can include
        // it in the note.
        N = N.extend(std::max<unsigned>(N.getBitWidth() + 1, 65));
        (llvm::APInt&)N += ArrayIndex;
        assert(N.ugt(ArraySize) && "bounds check failed for in-bounds index");
        diagnosePointerArithmetic(Info, E, N);
        setInvalid();
        return;
      }

      ArrayIndex += TruncatedN;
      assert(ArrayIndex <= ArraySize &&
             "bounds check succeeded for out-of-bounds index");

      if (IsArray)
        Entries.back() = PathEntry::ArrayIndex(ArrayIndex);
      else
        IsOnePastTheEnd = (ArrayIndex != 0);
    }
  };

  /// A stack frame in the constexpr call stack.
  struct CallStackFrame {
    EvalInfo &Info;

    /// Parent - The caller of this stack frame.
    CallStackFrame *Caller;

    /// Callee - The function which was called.
    const FunctionDecl *Callee;

    /// This - The binding for the this pointer in this call, if any.
    const LValue *This;

    /// Arguments - Parameter bindings for this function call, indexed by
    /// parameters' function scope indices.
    APValue *Arguments;

    /// Source location information about the default argument or default
    /// initializer expression we're evaluating, if any.
    CurrentSourceLocExprScope CurSourceLocExprScope;

    // Note that we intentionally use std::map here so that references to
    // values are stable.
    typedef std::pair<const void *, unsigned> MapKeyTy;
    typedef std::map<MapKeyTy, APValue> MapTy;
    /// Temporaries - Temporary lvalues materialized within this stack frame.
    MapTy Temporaries;

    /// CallLoc - The location of the call expression for this call.
    SourceLocation CallLoc;

    /// Index - The call index of this call.
    unsigned Index;

    /// The stack of integers for tracking version numbers for temporaries.
    SmallVector<unsigned, 2> TempVersionStack = {1};
    unsigned CurTempVersion = TempVersionStack.back();

    unsigned getTempVersion() const { return TempVersionStack.back(); }

    void pushTempVersion() {
      TempVersionStack.push_back(++CurTempVersion);
    }

    void popTempVersion() {
      TempVersionStack.pop_back();
    }

    // FIXME: Adding this to every 'CallStackFrame' may have a nontrivial impact
    // on the overall stack usage of deeply-recursing constexpr evaluations.
    // (We should cache this map rather than recomputing it repeatedly.)
    // But let's try this and see how it goes; we can look into caching the map
    // as a later change.

    /// LambdaCaptureFields - Mapping from captured variables/this to
    /// corresponding data members in the closure class.
    llvm::DenseMap<const VarDecl *, FieldDecl *> LambdaCaptureFields;
    FieldDecl *LambdaThisCaptureField;

    CallStackFrame(EvalInfo &Info, SourceLocation CallLoc,
                   const FunctionDecl *Callee, const LValue *This,
                   APValue *Arguments);
    ~CallStackFrame();

    // Return the temporary for Key whose version number is Version.
    APValue *getTemporary(const void *Key, unsigned Version) {
      MapKeyTy KV(Key, Version);
      auto LB = Temporaries.lower_bound(KV);
      if (LB != Temporaries.end() && LB->first == KV)
        return &LB->second;
      // Pair (Key,Version) wasn't found in the map. Check that no elements
      // in the map have 'Key' as their key.
      assert((LB == Temporaries.end() || LB->first.first != Key) &&
             (LB == Temporaries.begin() || std::prev(LB)->first.first != Key) &&
             "Element with key 'Key' found in map");
      return nullptr;
    }

    // Return the current temporary for Key in the map.
    APValue *getCurrentTemporary(const void *Key) {
      auto UB = Temporaries.upper_bound(MapKeyTy(Key, UINT_MAX));
      if (UB != Temporaries.begin() && std::prev(UB)->first.first == Key)
        return &std::prev(UB)->second;
      return nullptr;
    }

    // Return the version number of the current temporary for Key.
    unsigned getCurrentTemporaryVersion(const void *Key) const {
      auto UB = Temporaries.upper_bound(MapKeyTy(Key, UINT_MAX));
      if (UB != Temporaries.begin() && std::prev(UB)->first.first == Key)
        return std::prev(UB)->first.second;
      return 0;
    }

    APValue &createTemporary(const void *Key, bool IsLifetimeExtended);
  };

  /// Temporarily override 'this'.
  class ThisOverrideRAII {
  public:
    ThisOverrideRAII(CallStackFrame &Frame, const LValue *NewThis, bool Enable)
        : Frame(Frame), OldThis(Frame.This) {
      if (Enable)
        Frame.This = NewThis;
    }
    ~ThisOverrideRAII() {
      Frame.This = OldThis;
    }
  private:
    CallStackFrame &Frame;
    const LValue *OldThis;
  };

  /// A partial diagnostic which we might know in advance that we are not going
  /// to emit.
  class OptionalDiagnostic {
    PartialDiagnostic *Diag;

  public:
    explicit OptionalDiagnostic(PartialDiagnostic *Diag = nullptr)
      : Diag(Diag) {}

    template<typename T>
    OptionalDiagnostic &operator<<(const T &v) {
      if (Diag)
        *Diag << v;
      return *this;
    }

    OptionalDiagnostic &operator<<(const APSInt &I) {
      if (Diag) {
        SmallVector<char, 32> Buffer;
        I.toString(Buffer);
        *Diag << StringRef(Buffer.data(), Buffer.size());
      }
      return *this;
    }

    OptionalDiagnostic &operator<<(const APFloat &F) {
      if (Diag) {
        // FIXME: Force the precision of the source value down so we don't
        // print digits which are usually useless (we don't really care here if
        // we truncate a digit by accident in edge cases).  Ideally,
        // APFloat::toString would automatically print the shortest
        // representation which rounds to the correct value, but it's a bit
        // tricky to implement.
        unsigned precision =
            llvm::APFloat::semanticsPrecision(F.getSemantics());
        precision = (precision * 59 + 195) / 196;
        SmallVector<char, 32> Buffer;
        F.toString(Buffer, precision);
        *Diag << StringRef(Buffer.data(), Buffer.size());
      }
      return *this;
    }

    OptionalDiagnostic &operator<<(const APFixedPoint &FX) {
      if (Diag) {
        SmallVector<char, 32> Buffer;
        FX.toString(Buffer);
        *Diag << StringRef(Buffer.data(), Buffer.size());
      }
      return *this;
    }
  };

  /// A cleanup, and a flag indicating whether it is lifetime-extended.
  class Cleanup {
    llvm::PointerIntPair<APValue*, 1, bool> Value;

  public:
    Cleanup(APValue *Val, bool IsLifetimeExtended)
        : Value(Val, IsLifetimeExtended) {}

    bool isLifetimeExtended() const { return Value.getInt(); }
    void endLifetime() {
      *Value.getPointer() = APValue();
    }
  };

  /// A reference to an object whose construction we are currently evaluating.
  struct ObjectUnderConstruction {
    APValue::LValueBase Base;
    ArrayRef<APValue::LValuePathEntry> Path;
    friend bool operator==(const ObjectUnderConstruction &LHS,
                           const ObjectUnderConstruction &RHS) {
      return LHS.Base == RHS.Base && LHS.Path == RHS.Path;
    }
    friend llvm::hash_code hash_value(const ObjectUnderConstruction &Obj) {
      return llvm::hash_combine(Obj.Base, Obj.Path);
    }
  };
  enum class ConstructionPhase { None, Bases, AfterBases };
}

namespace llvm {
template<> struct DenseMapInfo<ObjectUnderConstruction> {
  using Base = DenseMapInfo<APValue::LValueBase>;
  static ObjectUnderConstruction getEmptyKey() {
    return {Base::getEmptyKey(), {}}; }
  static ObjectUnderConstruction getTombstoneKey() {
    return {Base::getTombstoneKey(), {}};
  }
  static unsigned getHashValue(const ObjectUnderConstruction &Object) {
    return hash_value(Object);
  }
  static bool isEqual(const ObjectUnderConstruction &LHS,
                      const ObjectUnderConstruction &RHS) {
    return LHS == RHS;
  }
};
}

namespace {
  /// EvalInfo - This is a private struct used by the evaluator to capture
  /// information about a subexpression as it is folded.  It retains information
  /// about the AST context, but also maintains information about the folded
  /// expression.
  ///
  /// If an expression could be evaluated, it is still possible it is not a C
  /// "integer constant expression" or constant expression.  If not, this struct
  /// captures information about how and why not.
  ///
  /// One bit of information passed *into* the request for constant folding
  /// indicates whether the subexpression is "evaluated" or not according to C
  /// rules.  For example, the RHS of (0 && foo()) is not evaluated.  We can
  /// evaluate the expression regardless of what the RHS is, but C only allows
  /// certain things in certain situations.
  struct EvalInfo {
    ASTContext &Ctx;

    /// EvalStatus - Contains information about the evaluation.
    Expr::EvalStatus &EvalStatus;

    /// CurrentCall - The top of the constexpr call stack.
    CallStackFrame *CurrentCall;

    /// CallStackDepth - The number of calls in the call stack right now.
    unsigned CallStackDepth;

    /// NextCallIndex - The next call index to assign.
    unsigned NextCallIndex;

    /// StepsLeft - The remaining number of evaluation steps we're permitted
    /// to perform. This is essentially a limit for the number of statements
    /// we will evaluate.
    unsigned StepsLeft;

    /// BottomFrame - The frame in which evaluation started. This must be
    /// initialized after CurrentCall and CallStackDepth.
    CallStackFrame BottomFrame;

    /// A stack of values whose lifetimes end at the end of some surrounding
    /// evaluation frame.
    llvm::SmallVector<Cleanup, 16> CleanupStack;

    /// EvaluatingDecl - This is the declaration whose initializer is being
    /// evaluated, if any.
    APValue::LValueBase EvaluatingDecl;

    /// EvaluatingDeclValue - This is the value being constructed for the
    /// declaration whose initializer is being evaluated, if any.
    APValue *EvaluatingDeclValue;

    /// Set of objects that are currently being constructed.
    llvm::DenseMap<ObjectUnderConstruction, ConstructionPhase>
        ObjectsUnderConstruction;

    struct EvaluatingConstructorRAII {
      EvalInfo &EI;
      ObjectUnderConstruction Object;
      bool DidInsert;
      EvaluatingConstructorRAII(EvalInfo &EI, ObjectUnderConstruction Object,
                                bool HasBases)
          : EI(EI), Object(Object) {
        DidInsert =
            EI.ObjectsUnderConstruction
                .insert({Object, HasBases ? ConstructionPhase::Bases
                                          : ConstructionPhase::AfterBases})
                .second;
      }
      void finishedConstructingBases() {
        EI.ObjectsUnderConstruction[Object] = ConstructionPhase::AfterBases;
      }
      ~EvaluatingConstructorRAII() {
        if (DidInsert) EI.ObjectsUnderConstruction.erase(Object);
      }
    };

    ConstructionPhase
    isEvaluatingConstructor(APValue::LValueBase Base,
                            ArrayRef<APValue::LValuePathEntry> Path) {
      return ObjectsUnderConstruction.lookup({Base, Path});
    }

    /// If we're currently speculatively evaluating, the outermost call stack
    /// depth at which we can mutate state, otherwise 0.
    unsigned SpeculativeEvaluationDepth = 0;

    /// The current array initialization index, if we're performing array
    /// initialization.
    uint64_t ArrayInitIndex = -1;

    /// HasActiveDiagnostic - Was the previous diagnostic stored? If so, further
    /// notes attached to it will also be stored, otherwise they will not be.
    bool HasActiveDiagnostic;

    /// Have we emitted a diagnostic explaining why we couldn't constant
    /// fold (not just why it's not strictly a constant expression)?
    bool HasFoldFailureDiagnostic;

    /// Whether or not we're in a context where the front end requires a
    /// constant value.
    bool InConstantContext;

    enum EvaluationMode {
      /// Evaluate as a constant expression. Stop if we find that the expression
      /// is not a constant expression.
      EM_ConstantExpression,

      /// Evaluate as a potential constant expression. Keep going if we hit a
      /// construct that we can't evaluate yet (because we don't yet know the
      /// value of something) but stop if we hit something that could never be
      /// a constant expression.
      EM_PotentialConstantExpression,

      /// Fold the expression to a constant. Stop if we hit a side-effect that
      /// we can't model.
      EM_ConstantFold,

      /// Evaluate the expression looking for integer overflow and similar
      /// issues. Don't worry about side-effects, and try to visit all
      /// subexpressions.
      EM_EvaluateForOverflow,

      /// Evaluate in any way we know how. Don't worry about side-effects that
      /// can't be modeled.
      EM_IgnoreSideEffects,

      /// Evaluate as a constant expression. Stop if we find that the expression
      /// is not a constant expression. Some expressions can be retried in the
      /// optimizer if we don't constant fold them here, but in an unevaluated
      /// context we try to fold them immediately since the optimizer never
      /// gets a chance to look at it.
      EM_ConstantExpressionUnevaluated,

      /// Evaluate as a potential constant expression. Keep going if we hit a
      /// construct that we can't evaluate yet (because we don't yet know the
      /// value of something) but stop if we hit something that could never be
      /// a constant expression. Some expressions can be retried in the
      /// optimizer if we don't constant fold them here, but in an unevaluated
      /// context we try to fold them immediately since the optimizer never
      /// gets a chance to look at it.
      EM_PotentialConstantExpressionUnevaluated,
    } EvalMode;

    /// Are we checking whether the expression is a potential constant
    /// expression?
    bool checkingPotentialConstantExpression() const {
      return EvalMode == EM_PotentialConstantExpression ||
             EvalMode == EM_PotentialConstantExpressionUnevaluated;
    }

    /// Are we checking an expression for overflow?
    // FIXME: We should check for any kind of undefined or suspicious behavior
    // in such constructs, not just overflow.
    bool checkingForOverflow() { return EvalMode == EM_EvaluateForOverflow; }

    EvalInfo(const ASTContext &C, Expr::EvalStatus &S, EvaluationMode Mode)
      : Ctx(const_cast<ASTContext &>(C)), EvalStatus(S), CurrentCall(nullptr),
        CallStackDepth(0), NextCallIndex(1),
        StepsLeft(getLangOpts().ConstexprStepLimit),
        BottomFrame(*this, SourceLocation(), nullptr, nullptr, nullptr),
        EvaluatingDecl((const ValueDecl *)nullptr),
        EvaluatingDeclValue(nullptr), HasActiveDiagnostic(false),
        HasFoldFailureDiagnostic(false),
        InConstantContext(false), EvalMode(Mode) {}

    void setEvaluatingDecl(APValue::LValueBase Base, APValue &Value) {
      EvaluatingDecl = Base;
      EvaluatingDeclValue = &Value;
    }

    const LangOptions &getLangOpts() const { return Ctx.getLangOpts(); }

    bool CheckCallLimit(SourceLocation Loc) {
      // Don't perform any constexpr calls (other than the call we're checking)
      // when checking a potential constant expression.
      if (checkingPotentialConstantExpression() && CallStackDepth > 1)
        return false;
      if (NextCallIndex == 0) {
        // NextCallIndex has wrapped around.
        FFDiag(Loc, diag::note_constexpr_call_limit_exceeded);
        return false;
      }
      if (CallStackDepth <= getLangOpts().ConstexprCallDepth)
        return true;
      FFDiag(Loc, diag::note_constexpr_depth_limit_exceeded)
        << getLangOpts().ConstexprCallDepth;
      return false;
    }

    std::pair<CallStackFrame *, unsigned>
    getCallFrameAndDepth(unsigned CallIndex) {
      assert(CallIndex && "no call index in getCallFrameAndDepth");
      // We will eventually hit BottomFrame, which has Index 1, so Frame can't
      // be null in this loop.
      unsigned Depth = CallStackDepth;
      CallStackFrame *Frame = CurrentCall;
      while (Frame->Index > CallIndex) {
        Frame = Frame->Caller;
        --Depth;
      }
      if (Frame->Index == CallIndex)
        return {Frame, Depth};
      return {nullptr, 0};
    }

    bool nextStep(const Stmt *S) {
      if (!StepsLeft) {
        FFDiag(S->getBeginLoc(), diag::note_constexpr_step_limit_exceeded);
        return false;
      }
      --StepsLeft;
      return true;
    }

  private:
    /// Add a diagnostic to the diagnostics list.
    PartialDiagnostic &addDiag(SourceLocation Loc, diag::kind DiagId) {
      PartialDiagnostic PD(DiagId, Ctx.getDiagAllocator());
      EvalStatus.Diag->push_back(std::make_pair(Loc, PD));
      return EvalStatus.Diag->back().second;
    }

    /// Add notes containing a call stack to the current point of evaluation.
    void addCallStack(unsigned Limit);

  private:
    OptionalDiagnostic Diag(SourceLocation Loc, diag::kind DiagId,
                            unsigned ExtraNotes, bool IsCCEDiag) {

      if (EvalStatus.Diag) {
        // If we have a prior diagnostic, it will be noting that the expression
        // isn't a constant expression. This diagnostic is more important,
        // unless we require this evaluation to produce a constant expression.
        //
        // FIXME: We might want to show both diagnostics to the user in
        // EM_ConstantFold mode.
        if (!EvalStatus.Diag->empty()) {
          switch (EvalMode) {
          case EM_ConstantFold:
          case EM_IgnoreSideEffects:
          case EM_EvaluateForOverflow:
            if (!HasFoldFailureDiagnostic)
              break;
            // We've already failed to fold something. Keep that diagnostic.
            LLVM_FALLTHROUGH;
          case EM_ConstantExpression:
          case EM_PotentialConstantExpression:
          case EM_ConstantExpressionUnevaluated:
          case EM_PotentialConstantExpressionUnevaluated:
            HasActiveDiagnostic = false;
            return OptionalDiagnostic();
          }
        }

        unsigned CallStackNotes = CallStackDepth - 1;
        unsigned Limit = Ctx.getDiagnostics().getConstexprBacktraceLimit();
        if (Limit)
          CallStackNotes = std::min(CallStackNotes, Limit + 1);
        if (checkingPotentialConstantExpression())
          CallStackNotes = 0;

        HasActiveDiagnostic = true;
        HasFoldFailureDiagnostic = !IsCCEDiag;
        EvalStatus.Diag->clear();
        EvalStatus.Diag->reserve(1 + ExtraNotes + CallStackNotes);
        addDiag(Loc, DiagId);
        if (!checkingPotentialConstantExpression())
          addCallStack(Limit);
        return OptionalDiagnostic(&(*EvalStatus.Diag)[0].second);
      }
      HasActiveDiagnostic = false;
      return OptionalDiagnostic();
    }
  public:
    // Diagnose that the evaluation could not be folded (FF => FoldFailure)
    OptionalDiagnostic
    FFDiag(SourceLocation Loc,
          diag::kind DiagId = diag::note_invalid_subexpr_in_const_expr,
          unsigned ExtraNotes = 0) {
      return Diag(Loc, DiagId, ExtraNotes, false);
    }

    OptionalDiagnostic FFDiag(const Expr *E, diag::kind DiagId
                              = diag::note_invalid_subexpr_in_const_expr,
                            unsigned ExtraNotes = 0) {
      if (EvalStatus.Diag)
        return Diag(E->getExprLoc(), DiagId, ExtraNotes, /*IsCCEDiag*/false);
      HasActiveDiagnostic = false;
      return OptionalDiagnostic();
    }

    /// Diagnose that the evaluation does not produce a C++11 core constant
    /// expression.
    ///
    /// FIXME: Stop evaluating if we're in EM_ConstantExpression or
    /// EM_PotentialConstantExpression mode and we produce one of these.
    OptionalDiagnostic CCEDiag(SourceLocation Loc, diag::kind DiagId
                                 = diag::note_invalid_subexpr_in_const_expr,
                               unsigned ExtraNotes = 0) {
      // Don't override a previous diagnostic. Don't bother collecting
      // diagnostics if we're evaluating for overflow.
      if (!EvalStatus.Diag || !EvalStatus.Diag->empty()) {
        HasActiveDiagnostic = false;
        return OptionalDiagnostic();
      }
      return Diag(Loc, DiagId, ExtraNotes, true);
    }
    OptionalDiagnostic CCEDiag(const Expr *E, diag::kind DiagId
                                 = diag::note_invalid_subexpr_in_const_expr,
                               unsigned ExtraNotes = 0) {
      return CCEDiag(E->getExprLoc(), DiagId, ExtraNotes);
    }
    /// Add a note to a prior diagnostic.
    OptionalDiagnostic Note(SourceLocation Loc, diag::kind DiagId) {
      if (!HasActiveDiagnostic)
        return OptionalDiagnostic();
      return OptionalDiagnostic(&addDiag(Loc, DiagId));
    }

    /// Add a stack of notes to a prior diagnostic.
    void addNotes(ArrayRef<PartialDiagnosticAt> Diags) {
      if (HasActiveDiagnostic) {
        EvalStatus.Diag->insert(EvalStatus.Diag->end(),
                                Diags.begin(), Diags.end());
      }
    }

    /// Should we continue evaluation after encountering a side-effect that we
    /// couldn't model?
    bool keepEvaluatingAfterSideEffect() {
      switch (EvalMode) {
      case EM_PotentialConstantExpression:
      case EM_PotentialConstantExpressionUnevaluated:
      case EM_EvaluateForOverflow:
      case EM_IgnoreSideEffects:
        return true;

      case EM_ConstantExpression:
      case EM_ConstantExpressionUnevaluated:
      case EM_ConstantFold:
        return false;
      }
      llvm_unreachable("Missed EvalMode case");
    }

    /// Note that we have had a side-effect, and determine whether we should
    /// keep evaluating.
    bool noteSideEffect() {
      EvalStatus.HasSideEffects = true;
      return keepEvaluatingAfterSideEffect();
    }

    /// Should we continue evaluation after encountering undefined behavior?
    bool keepEvaluatingAfterUndefinedBehavior() {
      switch (EvalMode) {
      case EM_EvaluateForOverflow:
      case EM_IgnoreSideEffects:
      case EM_ConstantFold:
        return true;

      case EM_PotentialConstantExpression:
      case EM_PotentialConstantExpressionUnevaluated:
      case EM_ConstantExpression:
      case EM_ConstantExpressionUnevaluated:
        return false;
      }
      llvm_unreachable("Missed EvalMode case");
    }

    /// Note that we hit something that was technically undefined behavior, but
    /// that we can evaluate past it (such as signed overflow or floating-point
    /// division by zero.)
    bool noteUndefinedBehavior() {
      EvalStatus.HasUndefinedBehavior = true;
      return keepEvaluatingAfterUndefinedBehavior();
    }

    /// Should we continue evaluation as much as possible after encountering a
    /// construct which can't be reduced to a value?
    bool keepEvaluatingAfterFailure() {
      if (!StepsLeft)
        return false;

      switch (EvalMode) {
      case EM_PotentialConstantExpression:
      case EM_PotentialConstantExpressionUnevaluated:
      case EM_EvaluateForOverflow:
        return true;

      case EM_ConstantExpression:
      case EM_ConstantExpressionUnevaluated:
      case EM_ConstantFold:
      case EM_IgnoreSideEffects:
        return false;
      }
      llvm_unreachable("Missed EvalMode case");
    }

    /// Notes that we failed to evaluate an expression that other expressions
    /// directly depend on, and determine if we should keep evaluating. This
    /// should only be called if we actually intend to keep evaluating.
    ///
    /// Call noteSideEffect() instead if we may be able to ignore the value that
    /// we failed to evaluate, e.g. if we failed to evaluate Foo() in:
    ///
    /// (Foo(), 1)      // use noteSideEffect
    /// (Foo() || true) // use noteSideEffect
    /// Foo() + 1       // use noteFailure
    LLVM_NODISCARD bool noteFailure() {
      // Failure when evaluating some expression often means there is some
      // subexpression whose evaluation was skipped. Therefore, (because we
      // don't track whether we skipped an expression when unwinding after an
      // evaluation failure) every evaluation failure that bubbles up from a
      // subexpression implies that a side-effect has potentially happened. We
      // skip setting the HasSideEffects flag to true until we decide to
      // continue evaluating after that point, which happens here.
      bool KeepGoing = keepEvaluatingAfterFailure();
      EvalStatus.HasSideEffects |= KeepGoing;
      return KeepGoing;
    }

    class ArrayInitLoopIndex {
      EvalInfo &Info;
      uint64_t OuterIndex;

    public:
      ArrayInitLoopIndex(EvalInfo &Info)
          : Info(Info), OuterIndex(Info.ArrayInitIndex) {
        Info.ArrayInitIndex = 0;
      }
      ~ArrayInitLoopIndex() { Info.ArrayInitIndex = OuterIndex; }

      operator uint64_t&() { return Info.ArrayInitIndex; }
    };
  };

  /// Object used to treat all foldable expressions as constant expressions.
  struct FoldConstant {
    EvalInfo &Info;
    bool Enabled;
    bool HadNoPriorDiags;
    EvalInfo::EvaluationMode OldMode;

    explicit FoldConstant(EvalInfo &Info, bool Enabled)
      : Info(Info),
        Enabled(Enabled),
        HadNoPriorDiags(Info.EvalStatus.Diag &&
                        Info.EvalStatus.Diag->empty() &&
                        !Info.EvalStatus.HasSideEffects),
        OldMode(Info.EvalMode) {
      if (Enabled &&
          (Info.EvalMode == EvalInfo::EM_ConstantExpression ||
           Info.EvalMode == EvalInfo::EM_ConstantExpressionUnevaluated))
        Info.EvalMode = EvalInfo::EM_ConstantFold;
    }
    void keepDiagnostics() { Enabled = false; }
    ~FoldConstant() {
      if (Enabled && HadNoPriorDiags && !Info.EvalStatus.Diag->empty() &&
          !Info.EvalStatus.HasSideEffects)
        Info.EvalStatus.Diag->clear();
      Info.EvalMode = OldMode;
    }
  };

  /// RAII object used to set the current evaluation mode to ignore
  /// side-effects.
  struct IgnoreSideEffectsRAII {
    EvalInfo &Info;
    EvalInfo::EvaluationMode OldMode;
    explicit IgnoreSideEffectsRAII(EvalInfo &Info)
        : Info(Info), OldMode(Info.EvalMode) {
      if (!Info.checkingPotentialConstantExpression())
        Info.EvalMode = EvalInfo::EM_IgnoreSideEffects;
    }

    ~IgnoreSideEffectsRAII() { Info.EvalMode = OldMode; }
  };

  /// RAII object used to optionally suppress diagnostics and side-effects from
  /// a speculative evaluation.
  class SpeculativeEvaluationRAII {
    EvalInfo *Info = nullptr;
    Expr::EvalStatus OldStatus;
    unsigned OldSpeculativeEvaluationDepth;

    void moveFromAndCancel(SpeculativeEvaluationRAII &&Other) {
      Info = Other.Info;
      OldStatus = Other.OldStatus;
      OldSpeculativeEvaluationDepth = Other.OldSpeculativeEvaluationDepth;
      Other.Info = nullptr;
    }

    void maybeRestoreState() {
      if (!Info)
        return;

      Info->EvalStatus = OldStatus;
      Info->SpeculativeEvaluationDepth = OldSpeculativeEvaluationDepth;
    }

  public:
    SpeculativeEvaluationRAII() = default;

    SpeculativeEvaluationRAII(
        EvalInfo &Info, SmallVectorImpl<PartialDiagnosticAt> *NewDiag = nullptr)
        : Info(&Info), OldStatus(Info.EvalStatus),
          OldSpeculativeEvaluationDepth(Info.SpeculativeEvaluationDepth) {
      Info.EvalStatus.Diag = NewDiag;
      Info.SpeculativeEvaluationDepth = Info.CallStackDepth + 1;
    }

    SpeculativeEvaluationRAII(const SpeculativeEvaluationRAII &Other) = delete;
    SpeculativeEvaluationRAII(SpeculativeEvaluationRAII &&Other) {
      moveFromAndCancel(std::move(Other));
    }

    SpeculativeEvaluationRAII &operator=(SpeculativeEvaluationRAII &&Other) {
      maybeRestoreState();
      moveFromAndCancel(std::move(Other));
      return *this;
    }

    ~SpeculativeEvaluationRAII() { maybeRestoreState(); }
  };

  /// RAII object wrapping a full-expression or block scope, and handling
  /// the ending of the lifetime of temporaries created within it.
  template<bool IsFullExpression>
  class ScopeRAII {
    EvalInfo &Info;
    unsigned OldStackSize;
  public:
    ScopeRAII(EvalInfo &Info)
        : Info(Info), OldStackSize(Info.CleanupStack.size()) {
      // Push a new temporary version. This is needed to distinguish between
      // temporaries created in different iterations of a loop.
      Info.CurrentCall->pushTempVersion();
    }
    ~ScopeRAII() {
      // Body moved to a static method to encourage the compiler to inline away
      // instances of this class.
      cleanup(Info, OldStackSize);
      Info.CurrentCall->popTempVersion();
    }
  private:
    static void cleanup(EvalInfo &Info, unsigned OldStackSize) {
      unsigned NewEnd = OldStackSize;
      for (unsigned I = OldStackSize, N = Info.CleanupStack.size();
           I != N; ++I) {
        if (IsFullExpression && Info.CleanupStack[I].isLifetimeExtended()) {
          // Full-expression cleanup of a lifetime-extended temporary: nothing
          // to do, just move this cleanup to the right place in the stack.
          std::swap(Info.CleanupStack[I], Info.CleanupStack[NewEnd]);
          ++NewEnd;
        } else {
          // End the lifetime of the object.
          Info.CleanupStack[I].endLifetime();
        }
      }
      Info.CleanupStack.erase(Info.CleanupStack.begin() + NewEnd,
                              Info.CleanupStack.end());
    }
  };
  typedef ScopeRAII<false> BlockScopeRAII;
  typedef ScopeRAII<true> FullExpressionRAII;
}

bool SubobjectDesignator::checkSubobject(EvalInfo &Info, const Expr *E,
                                         CheckSubobjectKind CSK) {
  if (Invalid)
    return false;
  if (isOnePastTheEnd()) {
    Info.CCEDiag(E, diag::note_constexpr_past_end_subobject)
      << CSK;
    setInvalid();
    return false;
  }
  // Note, we do not diagnose if isMostDerivedAnUnsizedArray(), because there
  // must actually be at least one array element; even a VLA cannot have a
  // bound of zero. And if our index is nonzero, we already had a CCEDiag.
  return true;
}

void SubobjectDesignator::diagnoseUnsizedArrayPointerArithmetic(EvalInfo &Info,
                                                                const Expr *E) {
  Info.CCEDiag(E, diag::note_constexpr_unsized_array_indexed);
  // Do not set the designator as invalid: we can represent this situation,
  // and correct handling of __builtin_object_size requires us to do so.
}

void SubobjectDesignator::diagnosePointerArithmetic(EvalInfo &Info,
                                                    const Expr *E,
                                                    const APSInt &N) {
  // If we're complaining, we must be able to statically determine the size of
  // the most derived array.
  if (MostDerivedPathLength == Entries.size() && MostDerivedIsArrayElement)
    Info.CCEDiag(E, diag::note_constexpr_array_index)
      << N << /*array*/ 0
      << static_cast<unsigned>(getMostDerivedArraySize());
  else
    Info.CCEDiag(E, diag::note_constexpr_array_index)
      << N << /*non-array*/ 1;
  setInvalid();
}

CallStackFrame::CallStackFrame(EvalInfo &Info, SourceLocation CallLoc,
                               const FunctionDecl *Callee, const LValue *This,
                               APValue *Arguments)
    : Info(Info), Caller(Info.CurrentCall), Callee(Callee), This(This),
      Arguments(Arguments), CallLoc(CallLoc), Index(Info.NextCallIndex++) {
  Info.CurrentCall = this;
  ++Info.CallStackDepth;
}

CallStackFrame::~CallStackFrame() {
  assert(Info.CurrentCall == this && "calls retired out of order");
  --Info.CallStackDepth;
  Info.CurrentCall = Caller;
}

APValue &CallStackFrame::createTemporary(const void *Key,
                                         bool IsLifetimeExtended) {
  unsigned Version = Info.CurrentCall->getTempVersion();
  APValue &Result = Temporaries[MapKeyTy(Key, Version)];
  assert(Result.isAbsent() && "temporary created multiple times");
  Info.CleanupStack.push_back(Cleanup(&Result, IsLifetimeExtended));
  return Result;
}

static void describeCall(CallStackFrame *Frame, raw_ostream &Out);

void EvalInfo::addCallStack(unsigned Limit) {
  // Determine which calls to skip, if any.
  unsigned ActiveCalls = CallStackDepth - 1;
  unsigned SkipStart = ActiveCalls, SkipEnd = SkipStart;
  if (Limit && Limit < ActiveCalls) {
    SkipStart = Limit / 2 + Limit % 2;
    SkipEnd = ActiveCalls - Limit / 2;
  }

  // Walk the call stack and add the diagnostics.
  unsigned CallIdx = 0;
  for (CallStackFrame *Frame = CurrentCall; Frame != &BottomFrame;
       Frame = Frame->Caller, ++CallIdx) {
    // Skip this call?
    if (CallIdx >= SkipStart && CallIdx < SkipEnd) {
      if (CallIdx == SkipStart) {
        // Note that we're skipping calls.
        addDiag(Frame->CallLoc, diag::note_constexpr_calls_suppressed)
          << unsigned(ActiveCalls - Limit);
      }
      continue;
    }

    // Use a different note for an inheriting constructor, because from the
    // user's perspective it's not really a function at all.
    if (auto *CD = dyn_cast_or_null<CXXConstructorDecl>(Frame->Callee)) {
      if (CD->isInheritingConstructor()) {
        addDiag(Frame->CallLoc, diag::note_constexpr_inherited_ctor_call_here)
          << CD->getParent();
        continue;
      }
    }

    SmallVector<char, 128> Buffer;
    llvm::raw_svector_ostream Out(Buffer);
    describeCall(Frame, Out);
    addDiag(Frame->CallLoc, diag::note_constexpr_call_here) << Out.str();
  }
}

/// Kinds of access we can perform on an object, for diagnostics. Note that
/// we consider a member function call to be a kind of access, even though
/// it is not formally an access of the object, because it has (largely) the
/// same set of semantic restrictions.
enum AccessKinds {
  AK_Read,
  AK_Assign,
  AK_Increment,
  AK_Decrement,
  AK_MemberCall,
  AK_DynamicCast,
  AK_TypeId,
};

static bool isModification(AccessKinds AK) {
  switch (AK) {
  case AK_Read:
  case AK_MemberCall:
  case AK_DynamicCast:
  case AK_TypeId:
    return false;
  case AK_Assign:
  case AK_Increment:
  case AK_Decrement:
    return true;
  }
  llvm_unreachable("unknown access kind");
}

/// Is this an access per the C++ definition?
static bool isFormalAccess(AccessKinds AK) {
  return AK == AK_Read || isModification(AK);
}

namespace {
  struct ComplexValue {
  private:
    bool IsInt;

  public:
    APSInt IntReal, IntImag;
    APFloat FloatReal, FloatImag;

    ComplexValue() : FloatReal(APFloat::Bogus()), FloatImag(APFloat::Bogus()) {}

    void makeComplexFloat() { IsInt = false; }
    bool isComplexFloat() const { return !IsInt; }
    APFloat &getComplexFloatReal() { return FloatReal; }
    APFloat &getComplexFloatImag() { return FloatImag; }

    void makeComplexInt() { IsInt = true; }
    bool isComplexInt() const { return IsInt; }
    APSInt &getComplexIntReal() { return IntReal; }
    APSInt &getComplexIntImag() { return IntImag; }

    void moveInto(APValue &v) const {
      if (isComplexFloat())
        v = APValue(FloatReal, FloatImag);
      else
        v = APValue(IntReal, IntImag);
    }
    void setFrom(const APValue &v) {
      assert(v.isComplexFloat() || v.isComplexInt());
      if (v.isComplexFloat()) {
        makeComplexFloat();
        FloatReal = v.getComplexFloatReal();
        FloatImag = v.getComplexFloatImag();
      } else {
        makeComplexInt();
        IntReal = v.getComplexIntReal();
        IntImag = v.getComplexIntImag();
      }
    }
  };

  struct LValue {
    APValue::LValueBase Base;
    CharUnits Offset;
    SubobjectDesignator Designator;
    bool IsNullPtr : 1;
    bool InvalidBase : 1;

    const APValue::LValueBase getLValueBase() const { return Base; }
    CharUnits &getLValueOffset() { return Offset; }
    const CharUnits &getLValueOffset() const { return Offset; }
    SubobjectDesignator &getLValueDesignator() { return Designator; }
    const SubobjectDesignator &getLValueDesignator() const { return Designator;}
    bool isNullPointer() const { return IsNullPtr;}

    unsigned getLValueCallIndex() const { return Base.getCallIndex(); }
    unsigned getLValueVersion() const { return Base.getVersion(); }

    void moveInto(APValue &V) const {
      if (Designator.Invalid)
        V = APValue(Base, Offset, APValue::NoLValuePath(), IsNullPtr);
      else {
        assert(!InvalidBase && "APValues can't handle invalid LValue bases");
        V = APValue(Base, Offset, Designator.Entries,
                    Designator.IsOnePastTheEnd, IsNullPtr);
      }
    }
    void setFrom(ASTContext &Ctx, const APValue &V) {
      assert(V.isLValue() && "Setting LValue from a non-LValue?");
      Base = V.getLValueBase();
      Offset = V.getLValueOffset();
      InvalidBase = false;
      Designator = SubobjectDesignator(Ctx, V);
      IsNullPtr = V.isNullPointer();
    }

    void set(APValue::LValueBase B, bool BInvalid = false) {
#ifndef NDEBUG
      // We only allow a few types of invalid bases. Enforce that here.
      if (BInvalid) {
        const auto *E = B.get<const Expr *>();
        assert((isa<MemberExpr>(E) || tryUnwrapAllocSizeCall(E)) &&
               "Unexpected type of invalid base");
      }
#endif

      Base = B;
      Offset = CharUnits::fromQuantity(0);
      InvalidBase = BInvalid;
      Designator = SubobjectDesignator(getType(B));
      IsNullPtr = false;
    }

    void setNull(QualType PointerTy, uint64_t TargetVal) {
      Base = (Expr *)nullptr;
      Offset = CharUnits::fromQuantity(TargetVal);
      InvalidBase = false;
      Designator = SubobjectDesignator(PointerTy->getPointeeType());
      IsNullPtr = true;
    }

    void setInvalid(APValue::LValueBase B, unsigned I = 0) {
      set(B, true);
    }

  private:
    // Check that this LValue is not based on a null pointer. If it is, produce
    // a diagnostic and mark the designator as invalid.
    template <typename GenDiagType>
    bool checkNullPointerDiagnosingWith(const GenDiagType &GenDiag) {
      if (Designator.Invalid)
        return false;
      if (IsNullPtr) {
        GenDiag();
        Designator.setInvalid();
        return false;
      }
      return true;
    }

  public:
    bool checkNullPointer(EvalInfo &Info, const Expr *E,
                          CheckSubobjectKind CSK) {
      return checkNullPointerDiagnosingWith([&Info, E, CSK] {
        Info.CCEDiag(E, diag::note_constexpr_null_subobject) << CSK;
      });
    }

    bool checkNullPointerForFoldAccess(EvalInfo &Info, const Expr *E,
                                       AccessKinds AK) {
      return checkNullPointerDiagnosingWith([&Info, E, AK] {
        Info.FFDiag(E, diag::note_constexpr_access_null) << AK;
      });
    }

    // Check this LValue refers to an object. If not, set the designator to be
    // invalid and emit a diagnostic.
    bool checkSubobject(EvalInfo &Info, const Expr *E, CheckSubobjectKind CSK) {
      return (CSK == CSK_ArrayToPointer || checkNullPointer(Info, E, CSK)) &&
             Designator.checkSubobject(Info, E, CSK);
    }

    void addDecl(EvalInfo &Info, const Expr *E,
                 const Decl *D, bool Virtual = false) {
      if (checkSubobject(Info, E, isa<FieldDecl>(D) ? CSK_Field : CSK_Base))
        Designator.addDeclUnchecked(D, Virtual);
    }
    void addUnsizedArray(EvalInfo &Info, const Expr *E, QualType ElemTy) {
      if (!Designator.Entries.empty()) {
        Info.CCEDiag(E, diag::note_constexpr_unsupported_unsized_array);
        Designator.setInvalid();
        return;
      }
      if (checkSubobject(Info, E, CSK_ArrayToPointer)) {
        assert(getType(Base)->isPointerType() || getType(Base)->isArrayType());
        Designator.FirstEntryIsAnUnsizedArray = true;
        Designator.addUnsizedArrayUnchecked(ElemTy);
      }
    }
    void addArray(EvalInfo &Info, const Expr *E, const ConstantArrayType *CAT) {
      if (checkSubobject(Info, E, CSK_ArrayToPointer))
        Designator.addArrayUnchecked(CAT);
    }
    void addComplex(EvalInfo &Info, const Expr *E, QualType EltTy, bool Imag) {
      if (checkSubobject(Info, E, Imag ? CSK_Imag : CSK_Real))
        Designator.addComplexUnchecked(EltTy, Imag);
    }
    void clearIsNullPointer() {
      IsNullPtr = false;
    }
    void adjustOffsetAndIndex(EvalInfo &Info, const Expr *E,
                              const APSInt &Index, CharUnits ElementSize) {
      // An index of 0 has no effect. (In C, adding 0 to a null pointer is UB,
      // but we're not required to diagnose it and it's valid in C++.)
      if (!Index)
        return;

      // Compute the new offset in the appropriate width, wrapping at 64 bits.
      // FIXME: When compiling for a 32-bit target, we should use 32-bit
      // offsets.
      uint64_t Offset64 = Offset.getQuantity();
      uint64_t ElemSize64 = ElementSize.getQuantity();
      uint64_t Index64 = Index.extOrTrunc(64).getZExtValue();
      Offset = CharUnits::fromQuantity(Offset64 + ElemSize64 * Index64);

      if (checkNullPointer(Info, E, CSK_ArrayIndex))
        Designator.adjustIndex(Info, E, Index);
      clearIsNullPointer();
    }
    void adjustOffset(CharUnits N) {
      Offset += N;
      if (N.getQuantity())
        clearIsNullPointer();
    }
  };

  struct MemberPtr {
    MemberPtr() {}
    explicit MemberPtr(const ValueDecl *Decl) :
      DeclAndIsDerivedMember(Decl, false), Path() {}

    /// The member or (direct or indirect) field referred to by this member
    /// pointer, or 0 if this is a null member pointer.
    const ValueDecl *getDecl() const {
      return DeclAndIsDerivedMember.getPointer();
    }
    /// Is this actually a member of some type derived from the relevant class?
    bool isDerivedMember() const {
      return DeclAndIsDerivedMember.getInt();
    }
    /// Get the class which the declaration actually lives in.
    const CXXRecordDecl *getContainingRecord() const {
      return cast<CXXRecordDecl>(
          DeclAndIsDerivedMember.getPointer()->getDeclContext());
    }

    void moveInto(APValue &V) const {
      V = APValue(getDecl(), isDerivedMember(), Path);
    }
    void setFrom(const APValue &V) {
      assert(V.isMemberPointer());
      DeclAndIsDerivedMember.setPointer(V.getMemberPointerDecl());
      DeclAndIsDerivedMember.setInt(V.isMemberPointerToDerivedMember());
      Path.clear();
      ArrayRef<const CXXRecordDecl*> P = V.getMemberPointerPath();
      Path.insert(Path.end(), P.begin(), P.end());
    }

    /// DeclAndIsDerivedMember - The member declaration, and a flag indicating
    /// whether the member is a member of some class derived from the class type
    /// of the member pointer.
    llvm::PointerIntPair<const ValueDecl*, 1, bool> DeclAndIsDerivedMember;
    /// Path - The path of base/derived classes from the member declaration's
    /// class (exclusive) to the class type of the member pointer (inclusive).
    SmallVector<const CXXRecordDecl*, 4> Path;

    /// Perform a cast towards the class of the Decl (either up or down the
    /// hierarchy).
    bool castBack(const CXXRecordDecl *Class) {
      assert(!Path.empty());
      const CXXRecordDecl *Expected;
      if (Path.size() >= 2)
        Expected = Path[Path.size() - 2];
      else
        Expected = getContainingRecord();
      if (Expected->getCanonicalDecl() != Class->getCanonicalDecl()) {
        // C++11 [expr.static.cast]p12: In a conversion from (D::*) to (B::*),
        // if B does not contain the original member and is not a base or
        // derived class of the class containing the original member, the result
        // of the cast is undefined.
        // C++11 [conv.mem]p2 does not cover this case for a cast from (B::*) to
        // (D::*). We consider that to be a language defect.
        return false;
      }
      Path.pop_back();
      return true;
    }
    /// Perform a base-to-derived member pointer cast.
    bool castToDerived(const CXXRecordDecl *Derived) {
      if (!getDecl())
        return true;
      if (!isDerivedMember()) {
        Path.push_back(Derived);
        return true;
      }
      if (!castBack(Derived))
        return false;
      if (Path.empty())
        DeclAndIsDerivedMember.setInt(false);
      return true;
    }
    /// Perform a derived-to-base member pointer cast.
    bool castToBase(const CXXRecordDecl *Base) {
      if (!getDecl())
        return true;
      if (Path.empty())
        DeclAndIsDerivedMember.setInt(true);
      if (isDerivedMember()) {
        Path.push_back(Base);
        return true;
      }
      return castBack(Base);
    }
  };

  /// Compare two member pointers, which are assumed to be of the same type.
  static bool operator==(const MemberPtr &LHS, const MemberPtr &RHS) {
    if (!LHS.getDecl() || !RHS.getDecl())
      return !LHS.getDecl() && !RHS.getDecl();
    if (LHS.getDecl()->getCanonicalDecl() != RHS.getDecl()->getCanonicalDecl())
      return false;
    return LHS.Path == RHS.Path;
  }
}

static bool Evaluate(APValue &Result, EvalInfo &Info, const Expr *E);
static bool EvaluateInPlace(APValue &Result, EvalInfo &Info,
                            const LValue &This, const Expr *E,
                            bool AllowNonLiteralTypes = false);
static bool EvaluateLValue(const Expr *E, LValue &Result, EvalInfo &Info,
                           bool InvalidBaseOK = false);
static bool EvaluatePointer(const Expr *E, LValue &Result, EvalInfo &Info,
                            bool InvalidBaseOK = false);
static bool EvaluateMemberPointer(const Expr *E, MemberPtr &Result,
                                  EvalInfo &Info);
static bool EvaluateTemporary(const Expr *E, LValue &Result, EvalInfo &Info);
static bool EvaluateInteger(const Expr *E, APSInt &Result, EvalInfo &Info);
static bool EvaluateIntegerOrLValue(const Expr *E, APValue &Result,
                                    EvalInfo &Info);
static bool EvaluateFloat(const Expr *E, APFloat &Result, EvalInfo &Info);
static bool EvaluateComplex(const Expr *E, ComplexValue &Res, EvalInfo &Info);
static bool EvaluateAtomic(const Expr *E, const LValue *This, APValue &Result,
                           EvalInfo &Info);
static bool EvaluateAsRValue(EvalInfo &Info, const Expr *E, APValue &Result);

/// Evaluate an integer or fixed point expression into an APResult.
static bool EvaluateFixedPointOrInteger(const Expr *E, APFixedPoint &Result,
                                        EvalInfo &Info);

/// Evaluate only a fixed point expression into an APResult.
static bool EvaluateFixedPoint(const Expr *E, APFixedPoint &Result,
                               EvalInfo &Info);

//===----------------------------------------------------------------------===//
// Misc utilities
//===----------------------------------------------------------------------===//

/// A helper function to create a temporary and set an LValue.
template <class KeyTy>
static APValue &createTemporary(const KeyTy *Key, bool IsLifetimeExtended,
                                LValue &LV, CallStackFrame &Frame) {
  LV.set({Key, Frame.Info.CurrentCall->Index,
          Frame.Info.CurrentCall->getTempVersion()});
  return Frame.createTemporary(Key, IsLifetimeExtended);
}

/// Negate an APSInt in place, converting it to a signed form if necessary, and
/// preserving its value (by extending by up to one bit as needed).
static void negateAsSigned(APSInt &Int) {
  if (Int.isUnsigned() || Int.isMinSignedValue()) {
    Int = Int.extend(Int.getBitWidth() + 1);
    Int.setIsSigned(true);
  }
  Int = -Int;
}

/// Produce a string describing the given constexpr call.
static void describeCall(CallStackFrame *Frame, raw_ostream &Out) {
  unsigned ArgIndex = 0;
  bool IsMemberCall = isa<CXXMethodDecl>(Frame->Callee) &&
                      !isa<CXXConstructorDecl>(Frame->Callee) &&
                      cast<CXXMethodDecl>(Frame->Callee)->isInstance();

  if (!IsMemberCall)
    Out << *Frame->Callee << '(';

  if (Frame->This && IsMemberCall) {
    APValue Val;
    Frame->This->moveInto(Val);
    Val.printPretty(Out, Frame->Info.Ctx,
                    Frame->This->Designator.MostDerivedType);
    // FIXME: Add parens around Val if needed.
    Out << "->" << *Frame->Callee << '(';
    IsMemberCall = false;
  }

  for (FunctionDecl::param_const_iterator I = Frame->Callee->param_begin(),
       E = Frame->Callee->param_end(); I != E; ++I, ++ArgIndex) {
    if (ArgIndex > (unsigned)IsMemberCall)
      Out << ", ";

    const ParmVarDecl *Param = *I;
    const APValue &Arg = Frame->Arguments[ArgIndex];
    Arg.printPretty(Out, Frame->Info.Ctx, Param->getType());

    if (ArgIndex == 0 && IsMemberCall)
      Out << "->" << *Frame->Callee << '(';
  }

  Out << ')';
}

/// Evaluate an expression to see if it had side-effects, and discard its
/// result.
/// \return \c true if the caller should keep evaluating.
static bool EvaluateIgnoredValue(EvalInfo &Info, const Expr *E) {
  APValue Scratch;
  if (!Evaluate(Scratch, Info, E))
    // We don't need the value, but we might have skipped a side effect here.
    return Info.noteSideEffect();
  return true;
}

/// Should this call expression be treated as a string literal?
static bool IsStringLiteralCall(const CallExpr *E) {
  unsigned Builtin = E->getBuiltinCallee();
  return (Builtin == Builtin::BI__builtin___CFStringMakeConstantString ||
          Builtin == Builtin::BI__builtin___NSStringMakeConstantString);
}

static bool IsGlobalLValue(APValue::LValueBase B) {
  // C++11 [expr.const]p3 An address constant expression is a prvalue core
  // constant expression of pointer type that evaluates to...

  // ... a null pointer value, or a prvalue core constant expression of type
  // std::nullptr_t.
  if (!B) return true;

  if (const ValueDecl *D = B.dyn_cast<const ValueDecl*>()) {
    // ... the address of an object with static storage duration,
    if (const VarDecl *VD = dyn_cast<VarDecl>(D))
      return VD->hasGlobalStorage();
    // ... the address of a function,
    return isa<FunctionDecl>(D);
  }

  if (B.is<TypeInfoLValue>())
    return true;

  const Expr *E = B.get<const Expr*>();
  switch (E->getStmtClass()) {
  default:
    return false;
  case Expr::CompoundLiteralExprClass: {
    const CompoundLiteralExpr *CLE = cast<CompoundLiteralExpr>(E);
    return CLE->isFileScope() && CLE->isLValue();
  }
  case Expr::MaterializeTemporaryExprClass:
    // A materialized temporary might have been lifetime-extended to static
    // storage duration.
    return cast<MaterializeTemporaryExpr>(E)->getStorageDuration() == SD_Static;
  // A string literal has static storage duration.
  case Expr::StringLiteralClass:
  case Expr::PredefinedExprClass:
  case Expr::ObjCStringLiteralClass:
  case Expr::ObjCEncodeExprClass:
  case Expr::CXXUuidofExprClass:
    return true;
  case Expr::ObjCBoxedExprClass:
    return cast<ObjCBoxedExpr>(E)->isExpressibleAsConstantInitializer();
  case Expr::CallExprClass:
    return IsStringLiteralCall(cast<CallExpr>(E));
  // For GCC compatibility, &&label has static storage duration.
  case Expr::AddrLabelExprClass:
    return true;
  // A Block literal expression may be used as the initialization value for
  // Block variables at global or local static scope.
  case Expr::BlockExprClass:
    return !cast<BlockExpr>(E)->getBlockDecl()->hasCaptures();
  case Expr::ImplicitValueInitExprClass:
    // FIXME:
    // We can never form an lvalue with an implicit value initialization as its
    // base through expression evaluation, so these only appear in one case: the
    // implicit variable declaration we invent when checking whether a constexpr
    // constructor can produce a constant expression. We must assume that such
    // an expression might be a global lvalue.
    return true;
  }
}

static const ValueDecl *GetLValueBaseDecl(const LValue &LVal) {
  return LVal.Base.dyn_cast<const ValueDecl*>();
}

static bool IsLiteralLValue(const LValue &Value) {
  if (Value.getLValueCallIndex())
    return false;
  const Expr *E = Value.Base.dyn_cast<const Expr*>();
  return E && !isa<MaterializeTemporaryExpr>(E);
}

static bool IsWeakLValue(const LValue &Value) {
  const ValueDecl *Decl = GetLValueBaseDecl(Value);
  return Decl && Decl->isWeak();
}

static bool isZeroSized(const LValue &Value) {
  const ValueDecl *Decl = GetLValueBaseDecl(Value);
  if (Decl && isa<VarDecl>(Decl)) {
    QualType Ty = Decl->getType();
    if (Ty->isArrayType())
      return Ty->isIncompleteType() ||
             Decl->getASTContext().getTypeSize(Ty) == 0;
  }
  return false;
}

static bool HasSameBase(const LValue &A, const LValue &B) {
  if (!A.getLValueBase())
    return !B.getLValueBase();
  if (!B.getLValueBase())
    return false;

  if (A.getLValueBase().getOpaqueValue() !=
      B.getLValueBase().getOpaqueValue()) {
    const Decl *ADecl = GetLValueBaseDecl(A);
    if (!ADecl)
      return false;
    const Decl *BDecl = GetLValueBaseDecl(B);
    if (!BDecl || ADecl->getCanonicalDecl() != BDecl->getCanonicalDecl())
      return false;
  }

  return IsGlobalLValue(A.getLValueBase()) ||
         (A.getLValueCallIndex() == B.getLValueCallIndex() &&
          A.getLValueVersion() == B.getLValueVersion());
}

static void NoteLValueLocation(EvalInfo &Info, APValue::LValueBase Base) {
  assert(Base && "no location for a null lvalue");
  const ValueDecl *VD = Base.dyn_cast<const ValueDecl*>();
  if (VD)
    Info.Note(VD->getLocation(), diag::note_declared_at);
  else if (const Expr *E = Base.dyn_cast<const Expr*>())
    Info.Note(E->getExprLoc(), diag::note_constexpr_temporary_here);
  // We have no information to show for a typeid(T) object.
}

/// Check that this reference or pointer core constant expression is a valid
/// value for an address or reference constant expression. Return true if we
/// can fold this expression, whether or not it's a constant expression.
static bool CheckLValueConstantExpression(EvalInfo &Info, SourceLocation Loc,
                                          QualType Type, const LValue &LVal,
                                          Expr::ConstExprUsage Usage) {
  bool IsReferenceType = Type->isReferenceType();

  APValue::LValueBase Base = LVal.getLValueBase();
  const SubobjectDesignator &Designator = LVal.getLValueDesignator();

  // Check that the object is a global. Note that the fake 'this' object we
  // manufacture when checking potential constant expressions is conservatively
  // assumed to be global here.
  if (!IsGlobalLValue(Base)) {
    if (Info.getLangOpts().CPlusPlus11) {
      const ValueDecl *VD = Base.dyn_cast<const ValueDecl*>();
      Info.FFDiag(Loc, diag::note_constexpr_non_global, 1)
        << IsReferenceType << !Designator.Entries.empty()
        << !!VD << VD;
      NoteLValueLocation(Info, Base);
    } else {
      Info.FFDiag(Loc);
    }
    // Don't allow references to temporaries to escape.
    return false;
  }
  assert((Info.checkingPotentialConstantExpression() ||
          LVal.getLValueCallIndex() == 0) &&
         "have call index for global lvalue");

  if (const ValueDecl *VD = Base.dyn_cast<const ValueDecl*>()) {
    if (const VarDecl *Var = dyn_cast<const VarDecl>(VD)) {
      // Check if this is a thread-local variable.
      if (Var->getTLSKind())
        return false;

      // A dllimport variable never acts like a constant.
      if (Usage == Expr::EvaluateForCodeGen && Var->hasAttr<DLLImportAttr>())
        return false;
    }
    if (const auto *FD = dyn_cast<const FunctionDecl>(VD)) {
      // __declspec(dllimport) must be handled very carefully:
      // We must never initialize an expression with the thunk in C++.
      // Doing otherwise would allow the same id-expression to yield
      // different addresses for the same function in different translation
      // units.  However, this means that we must dynamically initialize the
      // expression with the contents of the import address table at runtime.
      //
      // The C language has no notion of ODR; furthermore, it has no notion of
      // dynamic initialization.  This means that we are permitted to
      // perform initialization with the address of the thunk.
      if (Info.getLangOpts().CPlusPlus && Usage == Expr::EvaluateForCodeGen &&
          FD->hasAttr<DLLImportAttr>())
        return false;
    }
  }

  // Allow address constant expressions to be past-the-end pointers. This is
  // an extension: the standard requires them to point to an object.
  if (!IsReferenceType)
    return true;

  // A reference constant expression must refer to an object.
  if (!Base) {
    // FIXME: diagnostic
    Info.CCEDiag(Loc);
    return true;
  }

  // Does this refer one past the end of some object?
  if (!Designator.Invalid && Designator.isOnePastTheEnd()) {
    const ValueDecl *VD = Base.dyn_cast<const ValueDecl*>();
    Info.FFDiag(Loc, diag::note_constexpr_past_end, 1)
      << !Designator.Entries.empty() << !!VD << VD;
    NoteLValueLocation(Info, Base);
  }

  return true;
}

/// Member pointers are constant expressions unless they point to a
/// non-virtual dllimport member function.
static bool CheckMemberPointerConstantExpression(EvalInfo &Info,
                                                 SourceLocation Loc,
                                                 QualType Type,
                                                 const APValue &Value,
                                                 Expr::ConstExprUsage Usage) {
  const ValueDecl *Member = Value.getMemberPointerDecl();
  const auto *FD = dyn_cast_or_null<CXXMethodDecl>(Member);
  if (!FD)
    return true;
  return Usage == Expr::EvaluateForMangling || FD->isVirtual() ||
         !FD->hasAttr<DLLImportAttr>();
}

/// Check that this core constant expression is of literal type, and if not,
/// produce an appropriate diagnostic.
static bool CheckLiteralType(EvalInfo &Info, const Expr *E,
                             const LValue *This = nullptr) {
  if (!E->isRValue() || E->getType()->isLiteralType(Info.Ctx))
    return true;

  // C++1y: A constant initializer for an object o [...] may also invoke
  // constexpr constructors for o and its subobjects even if those objects
  // are of non-literal class types.
  //
  // C++11 missed this detail for aggregates, so classes like this:
  //   struct foo_t { union { int i; volatile int j; } u; };
  // are not (obviously) initializable like so:
  //   __attribute__((__require_constant_initialization__))
  //   static const foo_t x = {{0}};
  // because "i" is a subobject with non-literal initialization (due to the
  // volatile member of the union). See:
  //   http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1677
  // Therefore, we use the C++1y behavior.
  if (This && Info.EvaluatingDecl == This->getLValueBase())
    return true;

  // Prvalue constant expressions must be of literal types.
  if (Info.getLangOpts().CPlusPlus11)
    Info.FFDiag(E, diag::note_constexpr_nonliteral)
      << E->getType();
  else
    Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
  return false;
}

/// Check that this core constant expression value is a valid value for a
/// constant expression. If not, report an appropriate diagnostic. Does not
/// check that the expression is of literal type.
static bool
CheckConstantExpression(EvalInfo &Info, SourceLocation DiagLoc, QualType Type,
                        const APValue &Value,
                        Expr::ConstExprUsage Usage = Expr::EvaluateForCodeGen,
                        SourceLocation SubobjectLoc = SourceLocation()) {
  if (!Value.hasValue()) {
    Info.FFDiag(DiagLoc, diag::note_constexpr_uninitialized)
      << true << Type;
    if (SubobjectLoc.isValid())
      Info.Note(SubobjectLoc, diag::note_constexpr_subobject_declared_here);
    return false;
  }

  // We allow _Atomic(T) to be initialized from anything that T can be
  // initialized from.
  if (const AtomicType *AT = Type->getAs<AtomicType>())
    Type = AT->getValueType();

  // Core issue 1454: For a literal constant expression of array or class type,
  // each subobject of its value shall have been initialized by a constant
  // expression.
  if (Value.isArray()) {
    QualType EltTy = Type->castAsArrayTypeUnsafe()->getElementType();
    for (unsigned I = 0, N = Value.getArrayInitializedElts(); I != N; ++I) {
      if (!CheckConstantExpression(Info, DiagLoc, EltTy,
                                   Value.getArrayInitializedElt(I), Usage,
                                   SubobjectLoc))
        return false;
    }
    if (!Value.hasArrayFiller())
      return true;
    return CheckConstantExpression(Info, DiagLoc, EltTy, Value.getArrayFiller(),
                                   Usage, SubobjectLoc);
  }
  if (Value.isUnion() && Value.getUnionField()) {
    return CheckConstantExpression(Info, DiagLoc,
                                   Value.getUnionField()->getType(),
                                   Value.getUnionValue(), Usage,
                                   Value.getUnionField()->getLocation());
  }
  if (Value.isStruct()) {
    RecordDecl *RD = Type->castAs<RecordType>()->getDecl();
    if (const CXXRecordDecl *CD = dyn_cast<CXXRecordDecl>(RD)) {
      unsigned BaseIndex = 0;
      for (const CXXBaseSpecifier &BS : CD->bases()) {
        if (!CheckConstantExpression(Info, DiagLoc, BS.getType(),
                                     Value.getStructBase(BaseIndex), Usage,
                                     BS.getBeginLoc()))
          return false;
        ++BaseIndex;
      }
    }
    for (const auto *I : RD->fields()) {
      if (I->isUnnamedBitfield())
        continue;

      if (!CheckConstantExpression(Info, DiagLoc, I->getType(),
                                   Value.getStructField(I->getFieldIndex()),
                                   Usage, I->getLocation()))
        return false;
    }
  }

  if (Value.isLValue()) {
    LValue LVal;
    LVal.setFrom(Info.Ctx, Value);
    return CheckLValueConstantExpression(Info, DiagLoc, Type, LVal, Usage);
  }

  if (Value.isMemberPointer())
    return CheckMemberPointerConstantExpression(Info, DiagLoc, Type, Value, Usage);

  // Everything else is fine.
  return true;
}

static bool EvalPointerValueAsBool(const APValue &Value, bool &Result) {
  // A null base expression indicates a null pointer.  These are always
  // evaluatable, and they are false unless the offset is zero.
  if (!Value.getLValueBase()) {
    Result = !Value.getLValueOffset().isZero();
    return true;
  }

  // We have a non-null base.  These are generally known to be true, but if it's
  // a weak declaration it can be null at runtime.
  Result = true;
  const ValueDecl *Decl = Value.getLValueBase().dyn_cast<const ValueDecl*>();
  return !Decl || !Decl->isWeak();
}

static bool HandleConversionToBool(const APValue &Val, bool &Result) {
  switch (Val.getKind()) {
  case APValue::None:
  case APValue::Indeterminate:
    return false;
  case APValue::Int:
    Result = Val.getInt().getBoolValue();
    return true;
  case APValue::FixedPoint:
    Result = Val.getFixedPoint().getBoolValue();
    return true;
  case APValue::Float:
    Result = !Val.getFloat().isZero();
    return true;
  case APValue::ComplexInt:
    Result = Val.getComplexIntReal().getBoolValue() ||
             Val.getComplexIntImag().getBoolValue();
    return true;
  case APValue::ComplexFloat:
    Result = !Val.getComplexFloatReal().isZero() ||
             !Val.getComplexFloatImag().isZero();
    return true;
  case APValue::LValue:
    return EvalPointerValueAsBool(Val, Result);
  case APValue::MemberPointer:
    Result = Val.getMemberPointerDecl();
    return true;
  case APValue::Vector:
  case APValue::Array:
  case APValue::Struct:
  case APValue::Union:
  case APValue::AddrLabelDiff:
    return false;
  }

  llvm_unreachable("unknown APValue kind");
}

static bool EvaluateAsBooleanCondition(const Expr *E, bool &Result,
                                       EvalInfo &Info) {
  assert(E->isRValue() && "missing lvalue-to-rvalue conv in bool condition");
  APValue Val;
  if (!Evaluate(Val, Info, E))
    return false;
  return HandleConversionToBool(Val, Result);
}

template<typename T>
static bool HandleOverflow(EvalInfo &Info, const Expr *E,
                           const T &SrcValue, QualType DestType) {
  Info.CCEDiag(E, diag::note_constexpr_overflow)
    << SrcValue << DestType;
  return Info.noteUndefinedBehavior();
}

static bool HandleFloatToIntCast(EvalInfo &Info, const Expr *E,
                                 QualType SrcType, const APFloat &Value,
                                 QualType DestType, APSInt &Result) {
  unsigned DestWidth = Info.Ctx.getIntWidth(DestType);
  // Determine whether we are converting to unsigned or signed.
  bool DestSigned = DestType->isSignedIntegerOrEnumerationType();

  Result = APSInt(DestWidth, !DestSigned);
  bool ignored;
  if (Value.convertToInteger(Result, llvm::APFloat::rmTowardZero, &ignored)
      & APFloat::opInvalidOp)
    return HandleOverflow(Info, E, Value, DestType);
  return true;
}

static bool HandleFloatToFloatCast(EvalInfo &Info, const Expr *E,
                                   QualType SrcType, QualType DestType,
                                   APFloat &Result) {
  APFloat Value = Result;
  bool ignored;
  Result.convert(Info.Ctx.getFloatTypeSemantics(DestType),
                 APFloat::rmNearestTiesToEven, &ignored);
  return true;
}

static APSInt HandleIntToIntCast(EvalInfo &Info, const Expr *E,
                                 QualType DestType, QualType SrcType,
                                 const APSInt &Value) {
  unsigned DestWidth = Info.Ctx.getIntWidth(DestType);
  // Figure out if this is a truncate, extend or noop cast.
  // If the input is signed, do a sign extend, noop, or truncate.
  APSInt Result = Value.extOrTrunc(DestWidth);
  Result.setIsUnsigned(DestType->isUnsignedIntegerOrEnumerationType());
  if (DestType->isBooleanType())
    Result = Value.getBoolValue();
  return Result;
}

static bool HandleIntToFloatCast(EvalInfo &Info, const Expr *E,
                                 QualType SrcType, const APSInt &Value,
                                 QualType DestType, APFloat &Result) {
  Result = APFloat(Info.Ctx.getFloatTypeSemantics(DestType), 1);
  Result.convertFromAPInt(Value, Value.isSigned(),
                          APFloat::rmNearestTiesToEven);
  return true;
}

static bool truncateBitfieldValue(EvalInfo &Info, const Expr *E,
                                  APValue &Value, const FieldDecl *FD) {
  assert(FD->isBitField() && "truncateBitfieldValue on non-bitfield");

  if (!Value.isInt()) {
    // Trying to store a pointer-cast-to-integer into a bitfield.
    // FIXME: In this case, we should provide the diagnostic for casting
    // a pointer to an integer.
    assert(Value.isLValue() && "integral value neither int nor lvalue?");
    Info.FFDiag(E);
    return false;
  }

  APSInt &Int = Value.getInt();
  unsigned OldBitWidth = Int.getBitWidth();
  unsigned NewBitWidth = FD->getBitWidthValue(Info.Ctx);
  if (NewBitWidth < OldBitWidth)
    Int = Int.trunc(NewBitWidth).extend(OldBitWidth);
  return true;
}

static bool EvalAndBitcastToAPInt(EvalInfo &Info, const Expr *E,
                                  llvm::APInt &Res) {
  APValue SVal;
  if (!Evaluate(SVal, Info, E))
    return false;
  if (SVal.isInt()) {
    Res = SVal.getInt();
    return true;
  }
  if (SVal.isFloat()) {
    Res = SVal.getFloat().bitcastToAPInt();
    return true;
  }
  if (SVal.isVector()) {
    QualType VecTy = E->getType();
    unsigned VecSize = Info.Ctx.getTypeSize(VecTy);
    QualType EltTy = VecTy->castAs<VectorType>()->getElementType();
    unsigned EltSize = Info.Ctx.getTypeSize(EltTy);
    bool BigEndian = Info.Ctx.getTargetInfo().isBigEndian();
    Res = llvm::APInt::getNullValue(VecSize);
    for (unsigned i = 0; i < SVal.getVectorLength(); i++) {
      APValue &Elt = SVal.getVectorElt(i);
      llvm::APInt EltAsInt;
      if (Elt.isInt()) {
        EltAsInt = Elt.getInt();
      } else if (Elt.isFloat()) {
        EltAsInt = Elt.getFloat().bitcastToAPInt();
      } else {
        // Don't try to handle vectors of anything other than int or float
        // (not sure if it's possible to hit this case).
        Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
        return false;
      }
      unsigned BaseEltSize = EltAsInt.getBitWidth();
      if (BigEndian)
        Res |= EltAsInt.zextOrTrunc(VecSize).rotr(i*EltSize+BaseEltSize);
      else
        Res |= EltAsInt.zextOrTrunc(VecSize).rotl(i*EltSize);
    }
    return true;
  }
  // Give up if the input isn't an int, float, or vector.  For example, we
  // reject "(v4i16)(intptr_t)&a".
  Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
  return false;
}

/// Perform the given integer operation, which is known to need at most BitWidth
/// bits, and check for overflow in the original type (if that type was not an
/// unsigned type).
template<typename Operation>
static bool CheckedIntArithmetic(EvalInfo &Info, const Expr *E,
                                 const APSInt &LHS, const APSInt &RHS,
                                 unsigned BitWidth, Operation Op,
                                 APSInt &Result) {
  if (LHS.isUnsigned()) {
    Result = Op(LHS, RHS);
    return true;
  }

  APSInt Value(Op(LHS.extend(BitWidth), RHS.extend(BitWidth)), false);
  Result = Value.trunc(LHS.getBitWidth());
  if (Result.extend(BitWidth) != Value) {
    if (Info.checkingForOverflow())
      Info.Ctx.getDiagnostics().Report(E->getExprLoc(),
                                       diag::warn_integer_constant_overflow)
          << Result.toString(10) << E->getType();
    else
      return HandleOverflow(Info, E, Value, E->getType());
  }
  return true;
}

/// Perform the given binary integer operation.
static bool handleIntIntBinOp(EvalInfo &Info, const Expr *E, const APSInt &LHS,
                              BinaryOperatorKind Opcode, APSInt RHS,
                              APSInt &Result) {
  switch (Opcode) {
  default:
    Info.FFDiag(E);
    return false;
  case BO_Mul:
    return CheckedIntArithmetic(Info, E, LHS, RHS, LHS.getBitWidth() * 2,
                                std::multiplies<APSInt>(), Result);
  case BO_Add:
    return CheckedIntArithmetic(Info, E, LHS, RHS, LHS.getBitWidth() + 1,
                                std::plus<APSInt>(), Result);
  case BO_Sub:
    return CheckedIntArithmetic(Info, E, LHS, RHS, LHS.getBitWidth() + 1,
                                std::minus<APSInt>(), Result);
  case BO_And: Result = LHS & RHS; return true;
  case BO_Xor: Result = LHS ^ RHS; return true;
  case BO_Or:  Result = LHS | RHS; return true;
  case BO_Div:
  case BO_Rem:
    if (RHS == 0) {
      Info.FFDiag(E, diag::note_expr_divide_by_zero);
      return false;
    }
    Result = (Opcode == BO_Rem ? LHS % RHS : LHS / RHS);
    // Check for overflow case: INT_MIN / -1 or INT_MIN % -1. APSInt supports
    // this operation and gives the two's complement result.
    if (RHS.isNegative() && RHS.isAllOnesValue() &&
        LHS.isSigned() && LHS.isMinSignedValue())
      return HandleOverflow(Info, E, -LHS.extend(LHS.getBitWidth() + 1),
                            E->getType());
    return true;
  case BO_Shl: {
    if (Info.getLangOpts().OpenCL)
      // OpenCL 6.3j: shift values are effectively % word size of LHS.
      RHS &= APSInt(llvm::APInt(RHS.getBitWidth(),
                    static_cast<uint64_t>(LHS.getBitWidth() - 1)),
                    RHS.isUnsigned());
    else if (RHS.isSigned() && RHS.isNegative()) {
      // During constant-folding, a negative shift is an opposite shift. Such
      // a shift is not a constant expression.
      Info.CCEDiag(E, diag::note_constexpr_negative_shift) << RHS;
      RHS = -RHS;
      goto shift_right;
    }
  shift_left:
    // C++11 [expr.shift]p1: Shift width must be less than the bit width of
    // the shifted type.
    unsigned SA = (unsigned) RHS.getLimitedValue(LHS.getBitWidth()-1);
    if (SA != RHS) {
      Info.CCEDiag(E, diag::note_constexpr_large_shift)
        << RHS << E->getType() << LHS.getBitWidth();
    } else if (LHS.isSigned() && !Info.getLangOpts().CPlusPlus2a) {
      // C++11 [expr.shift]p2: A signed left shift must have a non-negative
      // operand, and must not overflow the corresponding unsigned type.
      // C++2a [expr.shift]p2: E1 << E2 is the unique value congruent to
      // E1 x 2^E2 module 2^N.
      if (LHS.isNegative())
        Info.CCEDiag(E, diag::note_constexpr_lshift_of_negative) << LHS;
      else if (LHS.countLeadingZeros() < SA)
        Info.CCEDiag(E, diag::note_constexpr_lshift_discards);
    }
    Result = LHS << SA;
    return true;
  }
  case BO_Shr: {
    if (Info.getLangOpts().OpenCL)
      // OpenCL 6.3j: shift values are effectively % word size of LHS.
      RHS &= APSInt(llvm::APInt(RHS.getBitWidth(),
                    static_cast<uint64_t>(LHS.getBitWidth() - 1)),
                    RHS.isUnsigned());
    else if (RHS.isSigned() && RHS.isNegative()) {
      // During constant-folding, a negative shift is an opposite shift. Such a
      // shift is not a constant expression.
      Info.CCEDiag(E, diag::note_constexpr_negative_shift) << RHS;
      RHS = -RHS;
      goto shift_left;
    }
  shift_right:
    // C++11 [expr.shift]p1: Shift width must be less than the bit width of the
    // shifted type.
    unsigned SA = (unsigned) RHS.getLimitedValue(LHS.getBitWidth()-1);
    if (SA != RHS)
      Info.CCEDiag(E, diag::note_constexpr_large_shift)
        << RHS << E->getType() << LHS.getBitWidth();
    Result = LHS >> SA;
    return true;
  }

  case BO_LT: Result = LHS < RHS; return true;
  case BO_GT: Result = LHS > RHS; return true;
  case BO_LE: Result = LHS <= RHS; return true;
  case BO_GE: Result = LHS >= RHS; return true;
  case BO_EQ: Result = LHS == RHS; return true;
  case BO_NE: Result = LHS != RHS; return true;
  case BO_Cmp:
    llvm_unreachable("BO_Cmp should be handled elsewhere");
  }
}

/// Perform the given binary floating-point operation, in-place, on LHS.
static bool handleFloatFloatBinOp(EvalInfo &Info, const Expr *E,
                                  APFloat &LHS, BinaryOperatorKind Opcode,
                                  const APFloat &RHS) {
  switch (Opcode) {
  default:
    Info.FFDiag(E);
    return false;
  case BO_Mul:
    LHS.multiply(RHS, APFloat::rmNearestTiesToEven);
    break;
  case BO_Add:
    LHS.add(RHS, APFloat::rmNearestTiesToEven);
    break;
  case BO_Sub:
    LHS.subtract(RHS, APFloat::rmNearestTiesToEven);
    break;
  case BO_Div:
    // [expr.mul]p4:
    //   If the second operand of / or % is zero the behavior is undefined.
    if (RHS.isZero())
      Info.CCEDiag(E, diag::note_expr_divide_by_zero);
    LHS.divide(RHS, APFloat::rmNearestTiesToEven);
    break;
  }

  // [expr.pre]p4:
  //   If during the evaluation of an expression, the result is not
  //   mathematically defined [...], the behavior is undefined.
  // FIXME: C++ rules require us to not conform to IEEE 754 here.
  if (LHS.isNaN()) {
    Info.CCEDiag(E, diag::note_constexpr_float_arithmetic) << LHS.isNaN();
    return Info.noteUndefinedBehavior();
  }
  return true;
}

/// Cast an lvalue referring to a base subobject to a derived class, by
/// truncating the lvalue's path to the given length.
static bool CastToDerivedClass(EvalInfo &Info, const Expr *E, LValue &Result,
                               const RecordDecl *TruncatedType,
                               unsigned TruncatedElements) {
  SubobjectDesignator &D = Result.Designator;

  // Check we actually point to a derived class object.
  if (TruncatedElements == D.Entries.size())
    return true;
  assert(TruncatedElements >= D.MostDerivedPathLength &&
         "not casting to a derived class");
  if (!Result.checkSubobject(Info, E, CSK_Derived))
    return false;

  // Truncate the path to the subobject, and remove any derived-to-base offsets.
  const RecordDecl *RD = TruncatedType;
  for (unsigned I = TruncatedElements, N = D.Entries.size(); I != N; ++I) {
    if (RD->isInvalidDecl()) return false;
    const ASTRecordLayout &Layout = Info.Ctx.getASTRecordLayout(RD);
    const CXXRecordDecl *Base = getAsBaseClass(D.Entries[I]);
    if (isVirtualBaseClass(D.Entries[I]))
      Result.Offset -= Layout.getVBaseClassOffset(Base);
    else
      Result.Offset -= Layout.getBaseClassOffset(Base);
    RD = Base;
  }
  D.Entries.resize(TruncatedElements);
  return true;
}

static bool HandleLValueDirectBase(EvalInfo &Info, const Expr *E, LValue &Obj,
                                   const CXXRecordDecl *Derived,
                                   const CXXRecordDecl *Base,
                                   const ASTRecordLayout *RL = nullptr) {
  if (!RL) {
    if (Derived->isInvalidDecl()) return false;
    RL = &Info.Ctx.getASTRecordLayout(Derived);
  }

  Obj.getLValueOffset() += RL->getBaseClassOffset(Base);
  Obj.addDecl(Info, E, Base, /*Virtual*/ false);
  return true;
}

static bool HandleLValueBase(EvalInfo &Info, const Expr *E, LValue &Obj,
                             const CXXRecordDecl *DerivedDecl,
                             const CXXBaseSpecifier *Base) {
  const CXXRecordDecl *BaseDecl = Base->getType()->getAsCXXRecordDecl();

  if (!Base->isVirtual())
    return HandleLValueDirectBase(Info, E, Obj, DerivedDecl, BaseDecl);

  SubobjectDesignator &D = Obj.Designator;
  if (D.Invalid)
    return false;

  // Extract most-derived object and corresponding type.
  DerivedDecl = D.MostDerivedType->getAsCXXRecordDecl();
  if (!CastToDerivedClass(Info, E, Obj, DerivedDecl, D.MostDerivedPathLength))
    return false;

  // Find the virtual base class.
  if (DerivedDecl->isInvalidDecl()) return false;
  const ASTRecordLayout &Layout = Info.Ctx.getASTRecordLayout(DerivedDecl);
  Obj.getLValueOffset() += Layout.getVBaseClassOffset(BaseDecl);
  Obj.addDecl(Info, E, BaseDecl, /*Virtual*/ true);
  return true;
}

static bool HandleLValueBasePath(EvalInfo &Info, const CastExpr *E,
                                 QualType Type, LValue &Result) {
  for (CastExpr::path_const_iterator PathI = E->path_begin(),
                                     PathE = E->path_end();
       PathI != PathE; ++PathI) {
    if (!HandleLValueBase(Info, E, Result, Type->getAsCXXRecordDecl(),
                          *PathI))
      return false;
    Type = (*PathI)->getType();
  }
  return true;
}

/// Cast an lvalue referring to a derived class to a known base subobject.
static bool CastToBaseClass(EvalInfo &Info, const Expr *E, LValue &Result,
                            const CXXRecordDecl *DerivedRD,
                            const CXXRecordDecl *BaseRD) {
  CXXBasePaths Paths(/*FindAmbiguities=*/false,
                     /*RecordPaths=*/true, /*DetectVirtual=*/false);
  if (!DerivedRD->isDerivedFrom(BaseRD, Paths))
    llvm_unreachable("Class must be derived from the passed in base class!");

  for (CXXBasePathElement &Elem : Paths.front())
    if (!HandleLValueBase(Info, E, Result, Elem.Class, Elem.Base))
      return false;
  return true;
}

/// Update LVal to refer to the given field, which must be a member of the type
/// currently described by LVal.
static bool HandleLValueMember(EvalInfo &Info, const Expr *E, LValue &LVal,
                               const FieldDecl *FD,
                               const ASTRecordLayout *RL = nullptr) {
  if (!RL) {
    if (FD->getParent()->isInvalidDecl()) return false;
    RL = &Info.Ctx.getASTRecordLayout(FD->getParent());
  }

  unsigned I = FD->getFieldIndex();
  LVal.adjustOffset(Info.Ctx.toCharUnitsFromBits(RL->getFieldOffset(I)));
  LVal.addDecl(Info, E, FD);
  return true;
}

/// Update LVal to refer to the given indirect field.
static bool HandleLValueIndirectMember(EvalInfo &Info, const Expr *E,
                                       LValue &LVal,
                                       const IndirectFieldDecl *IFD) {
  for (const auto *C : IFD->chain())
    if (!HandleLValueMember(Info, E, LVal, cast<FieldDecl>(C)))
      return false;
  return true;
}

/// Get the size of the given type in char units.
static bool HandleSizeof(EvalInfo &Info, SourceLocation Loc,
                         QualType Type, CharUnits &Size) {
  // sizeof(void), __alignof__(void), sizeof(function) = 1 as a gcc
  // extension.
  if (Type->isVoidType() || Type->isFunctionType()) {
    Size = CharUnits::One();
    return true;
  }

  if (Type->isDependentType()) {
    Info.FFDiag(Loc);
    return false;
  }

  if (!Type->isConstantSizeType()) {
    // sizeof(vla) is not a constantexpr: C99 6.5.3.4p2.
    // FIXME: Better diagnostic.
    Info.FFDiag(Loc);
    return false;
  }

  Size = Info.Ctx.getTypeSizeInChars(Type);
  return true;
}

/// Update a pointer value to model pointer arithmetic.
/// \param Info - Information about the ongoing evaluation.
/// \param E - The expression being evaluated, for diagnostic purposes.
/// \param LVal - The pointer value to be updated.
/// \param EltTy - The pointee type represented by LVal.
/// \param Adjustment - The adjustment, in objects of type EltTy, to add.
static bool HandleLValueArrayAdjustment(EvalInfo &Info, const Expr *E,
                                        LValue &LVal, QualType EltTy,
                                        APSInt Adjustment) {
  CharUnits SizeOfPointee;
  if (!HandleSizeof(Info, E->getExprLoc(), EltTy, SizeOfPointee))
    return false;

  LVal.adjustOffsetAndIndex(Info, E, Adjustment, SizeOfPointee);
  return true;
}

static bool HandleLValueArrayAdjustment(EvalInfo &Info, const Expr *E,
                                        LValue &LVal, QualType EltTy,
                                        int64_t Adjustment) {
  return HandleLValueArrayAdjustment(Info, E, LVal, EltTy,
                                     APSInt::get(Adjustment));
}

/// Update an lvalue to refer to a component of a complex number.
/// \param Info - Information about the ongoing evaluation.
/// \param LVal - The lvalue to be updated.
/// \param EltTy - The complex number's component type.
/// \param Imag - False for the real component, true for the imaginary.
static bool HandleLValueComplexElement(EvalInfo &Info, const Expr *E,
                                       LValue &LVal, QualType EltTy,
                                       bool Imag) {
  if (Imag) {
    CharUnits SizeOfComponent;
    if (!HandleSizeof(Info, E->getExprLoc(), EltTy, SizeOfComponent))
      return false;
    LVal.Offset += SizeOfComponent;
  }
  LVal.addComplex(Info, E, EltTy, Imag);
  return true;
}

/// Try to evaluate the initializer for a variable declaration.
///
/// \param Info   Information about the ongoing evaluation.
/// \param E      An expression to be used when printing diagnostics.
/// \param VD     The variable whose initializer should be obtained.
/// \param Frame  The frame in which the variable was created. Must be null
///               if this variable is not local to the evaluation.
/// \param Result Filled in with a pointer to the value of the variable.
static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
                                const VarDecl *VD, CallStackFrame *Frame,
                                APValue *&Result, const LValue *LVal) {

  // If this is a parameter to an active constexpr function call, perform
  // argument substitution.
  if (const ParmVarDecl *PVD = dyn_cast<ParmVarDecl>(VD)) {
    // Assume arguments of a potential constant expression are unknown
    // constant expressions.
    if (Info.checkingPotentialConstantExpression())
      return false;
    if (!Frame || !Frame->Arguments) {
      Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
      return false;
    }
    Result = &Frame->Arguments[PVD->getFunctionScopeIndex()];
    return true;
  }

  // If this is a local variable, dig out its value.
  if (Frame) {
    Result = LVal ? Frame->getTemporary(VD, LVal->getLValueVersion())
                  : Frame->getCurrentTemporary(VD);
    if (!Result) {
      // Assume variables referenced within a lambda's call operator that were
      // not declared within the call operator are captures and during checking
      // of a potential constant expression, assume they are unknown constant
      // expressions.
      assert(isLambdaCallOperator(Frame->Callee) &&
             (VD->getDeclContext() != Frame->Callee || VD->isInitCapture()) &&
             "missing value for local variable");
      if (Info.checkingPotentialConstantExpression())
        return false;
      // FIXME: implement capture evaluation during constant expr evaluation.
      Info.FFDiag(E->getBeginLoc(),
                  diag::note_unimplemented_constexpr_lambda_feature_ast)
          << "captures not currently allowed";
      return false;
    }
    return true;
  }

  // Dig out the initializer, and use the declaration which it's attached to.
  const Expr *Init = VD->getAnyInitializer(VD);
  if (!Init || Init->isValueDependent()) {
    // If we're checking a potential constant expression, the variable could be
    // initialized later.
    if (!Info.checkingPotentialConstantExpression())
      Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
    return false;
  }

  // If we're currently evaluating the initializer of this declaration, use that
  // in-flight value.
  if (Info.EvaluatingDecl.dyn_cast<const ValueDecl*>() == VD) {
    Result = Info.EvaluatingDeclValue;
    return true;
  }

  // Never evaluate the initializer of a weak variable. We can't be sure that
  // this is the definition which will be used.
  if (VD->isWeak()) {
    Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
    return false;
  }

  // Check that we can fold the initializer. In C++, we will have already done
  // this in the cases where it matters for conformance.
  SmallVector<PartialDiagnosticAt, 8> Notes;
  if (!VD->evaluateValue(Notes)) {
    Info.FFDiag(E, diag::note_constexpr_var_init_non_constant,
              Notes.size() + 1) << VD;
    Info.Note(VD->getLocation(), diag::note_declared_at);
    Info.addNotes(Notes);
    return false;
  } else if (!VD->checkInitIsICE()) {
    Info.CCEDiag(E, diag::note_constexpr_var_init_non_constant,
                 Notes.size() + 1) << VD;
    Info.Note(VD->getLocation(), diag::note_declared_at);
    Info.addNotes(Notes);
  }

  Result = VD->getEvaluatedValue();
  return true;
}

static bool IsConstNonVolatile(QualType T) {
  Qualifiers Quals = T.getQualifiers();
  return Quals.hasConst() && !Quals.hasVolatile();
}

/// Get the base index of the given base class within an APValue representing
/// the given derived class.
static unsigned getBaseIndex(const CXXRecordDecl *Derived,
                             const CXXRecordDecl *Base) {
  Base = Base->getCanonicalDecl();
  unsigned Index = 0;
  for (CXXRecordDecl::base_class_const_iterator I = Derived->bases_begin(),
         E = Derived->bases_end(); I != E; ++I, ++Index) {
    if (I->getType()->getAsCXXRecordDecl()->getCanonicalDecl() == Base)
      return Index;
  }

  llvm_unreachable("base class missing from derived class's bases list");
}

/// Extract the value of a character from a string literal.
static APSInt extractStringLiteralCharacter(EvalInfo &Info, const Expr *Lit,
                                            uint64_t Index) {
  assert(!isa<SourceLocExpr>(Lit) &&
         "SourceLocExpr should have already been converted to a StringLiteral");

  // FIXME: Support MakeStringConstant
  if (const auto *ObjCEnc = dyn_cast<ObjCEncodeExpr>(Lit)) {
    std::string Str;
    Info.Ctx.getObjCEncodingForType(ObjCEnc->getEncodedType(), Str);
    assert(Index <= Str.size() && "Index too large");
    return APSInt::getUnsigned(Str.c_str()[Index]);
  }

  if (auto PE = dyn_cast<PredefinedExpr>(Lit))
    Lit = PE->getFunctionName();
  const StringLiteral *S = cast<StringLiteral>(Lit);
  const ConstantArrayType *CAT =
      Info.Ctx.getAsConstantArrayType(S->getType());
  assert(CAT && "string literal isn't an array");
  QualType CharType = CAT->getElementType();
  assert(CharType->isIntegerType() && "unexpected character type");

  APSInt Value(S->getCharByteWidth() * Info.Ctx.getCharWidth(),
               CharType->isUnsignedIntegerType());
  if (Index < S->getLength())
    Value = S->getCodeUnit(Index);
  return Value;
}

// Expand a string literal into an array of characters.
//
// FIXME: This is inefficient; we should probably introduce something similar
// to the LLVM ConstantDataArray to make this cheaper.
static void expandStringLiteral(EvalInfo &Info, const StringLiteral *S,
                                APValue &Result) {
  const ConstantArrayType *CAT =
      Info.Ctx.getAsConstantArrayType(S->getType());
  assert(CAT && "string literal isn't an array");
  QualType CharType = CAT->getElementType();
  assert(CharType->isIntegerType() && "unexpected character type");

  unsigned Elts = CAT->getSize().getZExtValue();
  Result = APValue(APValue::UninitArray(),
                   std::min(S->getLength(), Elts), Elts);
  APSInt Value(S->getCharByteWidth() * Info.Ctx.getCharWidth(),
               CharType->isUnsignedIntegerType());
  if (Result.hasArrayFiller())
    Result.getArrayFiller() = APValue(Value);
  for (unsigned I = 0, N = Result.getArrayInitializedElts(); I != N; ++I) {
    Value = S->getCodeUnit(I);
    Result.getArrayInitializedElt(I) = APValue(Value);
  }
}

// Expand an array so that it has more than Index filled elements.
static void expandArray(APValue &Array, unsigned Index) {
  unsigned Size = Array.getArraySize();
  assert(Index < Size);

  // Always at least double the number of elements for which we store a value.
  unsigned OldElts = Array.getArrayInitializedElts();
  unsigned NewElts = std::max(Index+1, OldElts * 2);
  NewElts = std::min(Size, std::max(NewElts, 8u));

  // Copy the data across.
  APValue NewValue(APValue::UninitArray(), NewElts, Size);
  for (unsigned I = 0; I != OldElts; ++I)
    NewValue.getArrayInitializedElt(I).swap(Array.getArrayInitializedElt(I));
  for (unsigned I = OldElts; I != NewElts; ++I)
    NewValue.getArrayInitializedElt(I) = Array.getArrayFiller();
  if (NewValue.hasArrayFiller())
    NewValue.getArrayFiller() = Array.getArrayFiller();
  Array.swap(NewValue);
}

/// Determine whether a type would actually be read by an lvalue-to-rvalue
/// conversion. If it's of class type, we may assume that the copy operation
/// is trivial. Note that this is never true for a union type with fields
/// (because the copy always "reads" the active member) and always true for
/// a non-class type.
static bool isReadByLvalueToRvalueConversion(QualType T) {
  CXXRecordDecl *RD = T->getBaseElementTypeUnsafe()->getAsCXXRecordDecl();
  if (!RD || (RD->isUnion() && !RD->field_empty()))
    return true;
  if (RD->isEmpty())
    return false;

  for (auto *Field : RD->fields())
    if (isReadByLvalueToRvalueConversion(Field->getType()))
      return true;

  for (auto &BaseSpec : RD->bases())
    if (isReadByLvalueToRvalueConversion(BaseSpec.getType()))
      return true;

  return false;
}

/// Diagnose an attempt to read from any unreadable field within the specified
/// type, which might be a class type.
static bool diagnoseUnreadableFields(EvalInfo &Info, const Expr *E,
                                     QualType T) {
  CXXRecordDecl *RD = T->getBaseElementTypeUnsafe()->getAsCXXRecordDecl();
  if (!RD)
    return false;

  if (!RD->hasMutableFields())
    return false;

  for (auto *Field : RD->fields()) {
    // If we're actually going to read this field in some way, then it can't
    // be mutable. If we're in a union, then assigning to a mutable field
    // (even an empty one) can change the active member, so that's not OK.
    // FIXME: Add core issue number for the union case.
    if (Field->isMutable() &&
        (RD->isUnion() || isReadByLvalueToRvalueConversion(Field->getType()))) {
      Info.FFDiag(E, diag::note_constexpr_ltor_mutable, 1) << Field;
      Info.Note(Field->getLocation(), diag::note_declared_at);
      return true;
    }

    if (diagnoseUnreadableFields(Info, E, Field->getType()))
      return true;
  }

  for (auto &BaseSpec : RD->bases())
    if (diagnoseUnreadableFields(Info, E, BaseSpec.getType()))
      return true;

  // All mutable fields were empty, and thus not actually read.
  return false;
}

static bool lifetimeStartedInEvaluation(EvalInfo &Info,
                                        APValue::LValueBase Base) {
  // A temporary we created.
  if (Base.getCallIndex())
    return true;

  auto *Evaluating = Info.EvaluatingDecl.dyn_cast<const ValueDecl*>();
  if (!Evaluating)
    return false;

  // The variable whose initializer we're evaluating.
  if (auto *BaseD = Base.dyn_cast<const ValueDecl*>())
    if (declaresSameEntity(Evaluating, BaseD))
      return true;

  // A temporary lifetime-extended by the variable whose initializer we're
  // evaluating.
  if (auto *BaseE = Base.dyn_cast<const Expr *>())
    if (auto *BaseMTE = dyn_cast<MaterializeTemporaryExpr>(BaseE))
      if (declaresSameEntity(BaseMTE->getExtendingDecl(), Evaluating))
        return true;

  return false;
}

namespace {
/// A handle to a complete object (an object that is not a subobject of
/// another object).
struct CompleteObject {
  /// The identity of the object.
  APValue::LValueBase Base;
  /// The value of the complete object.
  APValue *Value;
  /// The type of the complete object.
  QualType Type;

  CompleteObject() : Value(nullptr) {}
  CompleteObject(APValue::LValueBase Base, APValue *Value, QualType Type)
      : Base(Base), Value(Value), Type(Type) {}

  bool mayReadMutableMembers(EvalInfo &Info) const {
    // In C++14 onwards, it is permitted to read a mutable member whose
    // lifetime began within the evaluation.
    // FIXME: Should we also allow this in C++11?
    if (!Info.getLangOpts().CPlusPlus14)
      return false;
    return lifetimeStartedInEvaluation(Info, Base);
  }

  explicit operator bool() const { return !Type.isNull(); }
};
} // end anonymous namespace

static QualType getSubobjectType(QualType ObjType, QualType SubobjType,
                                 bool IsMutable = false) {
  // C++ [basic.type.qualifier]p1:
  // - A const object is an object of type const T or a non-mutable subobject
  //   of a const object.
  if (ObjType.isConstQualified() && !IsMutable)
    SubobjType.addConst();
  // - A volatile object is an object of type const T or a subobject of a
  //   volatile object.
  if (ObjType.isVolatileQualified())
    SubobjType.addVolatile();
  return SubobjType;
}

/// Find the designated sub-object of an rvalue.
template<typename SubobjectHandler>
typename SubobjectHandler::result_type
findSubobject(EvalInfo &Info, const Expr *E, const CompleteObject &Obj,
              const SubobjectDesignator &Sub, SubobjectHandler &handler) {
  if (Sub.Invalid)
    // A diagnostic will have already been produced.
    return handler.failed();
  if (Sub.isOnePastTheEnd() || Sub.isMostDerivedAnUnsizedArray()) {
    if (Info.getLangOpts().CPlusPlus11)
      Info.FFDiag(E, Sub.isOnePastTheEnd()
                         ? diag::note_constexpr_access_past_end
                         : diag::note_constexpr_access_unsized_array)
          << handler.AccessKind;
    else
      Info.FFDiag(E);
    return handler.failed();
  }

  APValue *O = Obj.Value;
  QualType ObjType = Obj.Type;
  const FieldDecl *LastField = nullptr;
  const FieldDecl *VolatileField = nullptr;

  // Walk the designator's path to find the subobject.
  for (unsigned I = 0, N = Sub.Entries.size(); /**/; ++I) {
    // Reading an indeterminate value is undefined, but assigning over one is OK.
    if (O->isAbsent() || (O->isIndeterminate() && handler.AccessKind != AK_Assign)) {
      if (!Info.checkingPotentialConstantExpression())
        Info.FFDiag(E, diag::note_constexpr_access_uninit)
            << handler.AccessKind << O->isIndeterminate();
      return handler.failed();
    }

    // C++ [class.ctor]p5:
    //    const and volatile semantics are not applied on an object under
    //    construction.
    if ((ObjType.isConstQualified() || ObjType.isVolatileQualified()) &&
        ObjType->isRecordType() &&
        Info.isEvaluatingConstructor(
            Obj.Base, llvm::makeArrayRef(Sub.Entries.begin(),
                                         Sub.Entries.begin() + I)) !=
                          ConstructionPhase::None) {
      ObjType = Info.Ctx.getCanonicalType(ObjType);
      ObjType.removeLocalConst();
      ObjType.removeLocalVolatile();
    }

    // If this is our last pass, check that the final object type is OK.
    if (I == N || (I == N - 1 && ObjType->isAnyComplexType())) {
      // Accesses to volatile objects are prohibited.
      if (ObjType.isVolatileQualified() && isFormalAccess(handler.AccessKind)) {
        if (Info.getLangOpts().CPlusPlus) {
          int DiagKind;
          SourceLocation Loc;
          const NamedDecl *Decl = nullptr;
          if (VolatileField) {
            DiagKind = 2;
            Loc = VolatileField->getLocation();
            Decl = VolatileField;
          } else if (auto *VD = Obj.Base.dyn_cast<const ValueDecl*>()) {
            DiagKind = 1;
            Loc = VD->getLocation();
            Decl = VD;
          } else {
            DiagKind = 0;
            if (auto *E = Obj.Base.dyn_cast<const Expr *>())
              Loc = E->getExprLoc();
          }
          Info.FFDiag(E, diag::note_constexpr_access_volatile_obj, 1)
              << handler.AccessKind << DiagKind << Decl;
          Info.Note(Loc, diag::note_constexpr_volatile_here) << DiagKind;
        } else {
          Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
        }
        return handler.failed();
      }

      // If we are reading an object of class type, there may still be more
      // things we need to check: if there are any mutable subobjects, we
      // cannot perform this read. (This only happens when performing a trivial
      // copy or assignment.)
      if (ObjType->isRecordType() && handler.AccessKind == AK_Read &&
          !Obj.mayReadMutableMembers(Info) &&
          diagnoseUnreadableFields(Info, E, ObjType))
        return handler.failed();
    }

    if (I == N) {
      if (!handler.found(*O, ObjType))
        return false;

      // If we modified a bit-field, truncate it to the right width.
      if (isModification(handler.AccessKind) &&
          LastField && LastField->isBitField() &&
          !truncateBitfieldValue(Info, E, *O, LastField))
        return false;

      return true;
    }

    LastField = nullptr;
    if (ObjType->isArrayType()) {
      // Next subobject is an array element.
      const ConstantArrayType *CAT = Info.Ctx.getAsConstantArrayType(ObjType);
      assert(CAT && "vla in literal type?");
      uint64_t Index = Sub.Entries[I].getAsArrayIndex();
      if (CAT->getSize().ule(Index)) {
        // Note, it should not be possible to form a pointer with a valid
        // designator which points more than one past the end of the array.
        if (Info.getLangOpts().CPlusPlus11)
          Info.FFDiag(E, diag::note_constexpr_access_past_end)
            << handler.AccessKind;
        else
          Info.FFDiag(E);
        return handler.failed();
      }

      ObjType = CAT->getElementType();

      if (O->getArrayInitializedElts() > Index)
        O = &O->getArrayInitializedElt(Index);
      else if (handler.AccessKind != AK_Read) {
        expandArray(*O, Index);
        O = &O->getArrayInitializedElt(Index);
      } else
        O = &O->getArrayFiller();
    } else if (ObjType->isAnyComplexType()) {
      // Next subobject is a complex number.
      uint64_t Index = Sub.Entries[I].getAsArrayIndex();
      if (Index > 1) {
        if (Info.getLangOpts().CPlusPlus11)
          Info.FFDiag(E, diag::note_constexpr_access_past_end)
            << handler.AccessKind;
        else
          Info.FFDiag(E);
        return handler.failed();
      }

      ObjType = getSubobjectType(
          ObjType, ObjType->castAs<ComplexType>()->getElementType());

      assert(I == N - 1 && "extracting subobject of scalar?");
      if (O->isComplexInt()) {
        return handler.found(Index ? O->getComplexIntImag()
                                   : O->getComplexIntReal(), ObjType);
      } else {
        assert(O->isComplexFloat());
        return handler.found(Index ? O->getComplexFloatImag()
                                   : O->getComplexFloatReal(), ObjType);
      }
    } else if (const FieldDecl *Field = getAsField(Sub.Entries[I])) {
      if (Field->isMutable() && handler.AccessKind == AK_Read &&
          !Obj.mayReadMutableMembers(Info)) {
        Info.FFDiag(E, diag::note_constexpr_ltor_mutable, 1)
          << Field;
        Info.Note(Field->getLocation(), diag::note_declared_at);
        return handler.failed();
      }

      // Next subobject is a class, struct or union field.
      RecordDecl *RD = ObjType->castAs<RecordType>()->getDecl();
      if (RD->isUnion()) {
        const FieldDecl *UnionField = O->getUnionField();
        if (!UnionField ||
            UnionField->getCanonicalDecl() != Field->getCanonicalDecl()) {
          Info.FFDiag(E, diag::note_constexpr_access_inactive_union_member)
            << handler.AccessKind << Field << !UnionField << UnionField;
          return handler.failed();
        }
        O = &O->getUnionValue();
      } else
        O = &O->getStructField(Field->getFieldIndex());

      ObjType = getSubobjectType(ObjType, Field->getType(), Field->isMutable());
      LastField = Field;
      if (Field->getType().isVolatileQualified())
        VolatileField = Field;
    } else {
      // Next subobject is a base class.
      const CXXRecordDecl *Derived = ObjType->getAsCXXRecordDecl();
      const CXXRecordDecl *Base = getAsBaseClass(Sub.Entries[I]);
      O = &O->getStructBase(getBaseIndex(Derived, Base));

      ObjType = getSubobjectType(ObjType, Info.Ctx.getRecordType(Base));
    }
  }
}

namespace {
struct ExtractSubobjectHandler {
  EvalInfo &Info;
  APValue &Result;

  static const AccessKinds AccessKind = AK_Read;

  typedef bool result_type;
  bool failed() { return false; }
  bool found(APValue &Subobj, QualType SubobjType) {
    Result = Subobj;
    return true;
  }
  bool found(APSInt &Value, QualType SubobjType) {
    Result = APValue(Value);
    return true;
  }
  bool found(APFloat &Value, QualType SubobjType) {
    Result = APValue(Value);
    return true;
  }
};
} // end anonymous namespace

const AccessKinds ExtractSubobjectHandler::AccessKind;

/// Extract the designated sub-object of an rvalue.
static bool extractSubobject(EvalInfo &Info, const Expr *E,
                             const CompleteObject &Obj,
                             const SubobjectDesignator &Sub,
                             APValue &Result) {
  ExtractSubobjectHandler Handler = { Info, Result };
  return findSubobject(Info, E, Obj, Sub, Handler);
}

namespace {
struct ModifySubobjectHandler {
  EvalInfo &Info;
  APValue &NewVal;
  const Expr *E;

  typedef bool result_type;
  static const AccessKinds AccessKind = AK_Assign;

  bool checkConst(QualType QT) {
    // Assigning to a const object has undefined behavior.
    if (QT.isConstQualified()) {
      Info.FFDiag(E, diag::note_constexpr_modify_const_type) << QT;
      return false;
    }
    return true;
  }

  bool failed() { return false; }
  bool found(APValue &Subobj, QualType SubobjType) {
    if (!checkConst(SubobjType))
      return false;
    // We've been given ownership of NewVal, so just swap it in.
    Subobj.swap(NewVal);
    return true;
  }
  bool found(APSInt &Value, QualType SubobjType) {
    if (!checkConst(SubobjType))
      return false;
    if (!NewVal.isInt()) {
      // Maybe trying to write a cast pointer value into a complex?
      Info.FFDiag(E);
      return false;
    }
    Value = NewVal.getInt();
    return true;
  }
  bool found(APFloat &Value, QualType SubobjType) {
    if (!checkConst(SubobjType))
      return false;
    Value = NewVal.getFloat();
    return true;
  }
};
} // end anonymous namespace

const AccessKinds ModifySubobjectHandler::AccessKind;

/// Update the designated sub-object of an rvalue to the given value.
static bool modifySubobject(EvalInfo &Info, const Expr *E,
                            const CompleteObject &Obj,
                            const SubobjectDesignator &Sub,
                            APValue &NewVal) {
  ModifySubobjectHandler Handler = { Info, NewVal, E };
  return findSubobject(Info, E, Obj, Sub, Handler);
}

/// Find the position where two subobject designators diverge, or equivalently
/// the length of the common initial subsequence.
static unsigned FindDesignatorMismatch(QualType ObjType,
                                       const SubobjectDesignator &A,
                                       const SubobjectDesignator &B,
                                       bool &WasArrayIndex) {
  unsigned I = 0, N = std::min(A.Entries.size(), B.Entries.size());
  for (/**/; I != N; ++I) {
    if (!ObjType.isNull() &&
        (ObjType->isArrayType() || ObjType->isAnyComplexType())) {
      // Next subobject is an array element.
      if (A.Entries[I].getAsArrayIndex() != B.Entries[I].getAsArrayIndex()) {
        WasArrayIndex = true;
        return I;
      }
      if (ObjType->isAnyComplexType())
        ObjType = ObjType->castAs<ComplexType>()->getElementType();
      else
        ObjType = ObjType->castAsArrayTypeUnsafe()->getElementType();
    } else {
      if (A.Entries[I].getAsBaseOrMember() !=
          B.Entries[I].getAsBaseOrMember()) {
        WasArrayIndex = false;
        return I;
      }
      if (const FieldDecl *FD = getAsField(A.Entries[I]))
        // Next subobject is a field.
        ObjType = FD->getType();
      else
        // Next subobject is a base class.
        ObjType = QualType();
    }
  }
  WasArrayIndex = false;
  return I;
}

/// Determine whether the given subobject designators refer to elements of the
/// same array object.
static bool AreElementsOfSameArray(QualType ObjType,
                                   const SubobjectDesignator &A,
                                   const SubobjectDesignator &B) {
  if (A.Entries.size() != B.Entries.size())
    return false;

  bool IsArray = A.MostDerivedIsArrayElement;
  if (IsArray && A.MostDerivedPathLength != A.Entries.size())
    // A is a subobject of the array element.
    return false;

  // If A (and B) designates an array element, the last entry will be the array
  // index. That doesn't have to match. Otherwise, we're in the 'implicit array
  // of length 1' case, and the entire path must match.
  bool WasArrayIndex;
  unsigned CommonLength = FindDesignatorMismatch(ObjType, A, B, WasArrayIndex);
  return CommonLength >= A.Entries.size() - IsArray;
}

/// Find the complete object to which an LValue refers.
static CompleteObject findCompleteObject(EvalInfo &Info, const Expr *E,
                                         AccessKinds AK, const LValue &LVal,
                                         QualType LValType) {
  if (LVal.InvalidBase) {
    Info.FFDiag(E);
    return CompleteObject();
  }

  if (!LVal.Base) {
    Info.FFDiag(E, diag::note_constexpr_access_null) << AK;
    return CompleteObject();
  }

  CallStackFrame *Frame = nullptr;
  unsigned Depth = 0;
  if (LVal.getLValueCallIndex()) {
    std::tie(Frame, Depth) =
        Info.getCallFrameAndDepth(LVal.getLValueCallIndex());
    if (!Frame) {
      Info.FFDiag(E, diag::note_constexpr_lifetime_ended, 1)
        << AK << LVal.Base.is<const ValueDecl*>();
      NoteLValueLocation(Info, LVal.Base);
      return CompleteObject();
    }
  }

  bool IsAccess = isFormalAccess(AK);

  // C++11 DR1311: An lvalue-to-rvalue conversion on a volatile-qualified type
  // is not a constant expression (even if the object is non-volatile). We also
  // apply this rule to C++98, in order to conform to the expected 'volatile'
  // semantics.
  if (IsAccess && LValType.isVolatileQualified()) {
    if (Info.getLangOpts().CPlusPlus)
      Info.FFDiag(E, diag::note_constexpr_access_volatile_type)
        << AK << LValType;
    else
      Info.FFDiag(E);
    return CompleteObject();
  }

  // Compute value storage location and type of base object.
  APValue *BaseVal = nullptr;
  QualType BaseType = getType(LVal.Base);

  if (const ValueDecl *D = LVal.Base.dyn_cast<const ValueDecl*>()) {
    // In C++98, const, non-volatile integers initialized with ICEs are ICEs.
    // In C++11, constexpr, non-volatile variables initialized with constant
    // expressions are constant expressions too. Inside constexpr functions,
    // parameters are constant expressions even if they're non-const.
    // In C++1y, objects local to a constant expression (those with a Frame) are
    // both readable and writable inside constant expressions.
    // In C, such things can also be folded, although they are not ICEs.
    const VarDecl *VD = dyn_cast<VarDecl>(D);
    if (VD) {
      if (const VarDecl *VDef = VD->getDefinition(Info.Ctx))
        VD = VDef;
    }
    if (!VD || VD->isInvalidDecl()) {
      Info.FFDiag(E);
      return CompleteObject();
    }

    // Unless we're looking at a local variable or argument in a constexpr call,
    // the variable we're reading must be const.
    if (!Frame) {
      if (Info.getLangOpts().CPlusPlus14 &&
          declaresSameEntity(
              VD, Info.EvaluatingDecl.dyn_cast<const ValueDecl *>())) {
        // OK, we can read and modify an object if we're in the process of
        // evaluating its initializer, because its lifetime began in this
        // evaluation.
      } else if (isModification(AK)) {
        // All the remaining cases do not permit modification of the object.
        Info.FFDiag(E, diag::note_constexpr_modify_global);
        return CompleteObject();
      } else if (VD->isConstexpr()) {
        // OK, we can read this variable.
      } else if (BaseType->isIntegralOrEnumerationType()) {
        // In OpenCL if a variable is in constant address space it is a const
        // value.
        if (!(BaseType.isConstQualified() ||
              (Info.getLangOpts().OpenCL &&
               BaseType.getAddressSpace() == LangAS::opencl_constant))) {
          if (!IsAccess)
            return CompleteObject(LVal.getLValueBase(), nullptr, BaseType);
          if (Info.getLangOpts().CPlusPlus) {
            Info.FFDiag(E, diag::note_constexpr_ltor_non_const_int, 1) << VD;
            Info.Note(VD->getLocation(), diag::note_declared_at);
          } else {
            Info.FFDiag(E);
          }
          return CompleteObject();
        }
      } else if (!IsAccess) {
        return CompleteObject(LVal.getLValueBase(), nullptr, BaseType);
      } else if (BaseType->isFloatingType() && BaseType.isConstQualified()) {
        // We support folding of const floating-point types, in order to make
        // static const data members of such types (supported as an extension)
        // more useful.
        if (Info.getLangOpts().CPlusPlus11) {
          Info.CCEDiag(E, diag::note_constexpr_ltor_non_constexpr, 1) << VD;
          Info.Note(VD->getLocation(), diag::note_declared_at);
        } else {
          Info.CCEDiag(E);
        }
      } else if (BaseType.isConstQualified() && VD->hasDefinition(Info.Ctx)) {
        Info.CCEDiag(E, diag::note_constexpr_ltor_non_constexpr) << VD;
        // Keep evaluating to see what we can do.
      } else {
        // FIXME: Allow folding of values of any literal type in all languages.
        if (Info.checkingPotentialConstantExpression() &&
            VD->getType().isConstQualified() && !VD->hasDefinition(Info.Ctx)) {
          // The definition of this variable could be constexpr. We can't
          // access it right now, but may be able to in future.
        } else if (Info.getLangOpts().CPlusPlus11) {
          Info.FFDiag(E, diag::note_constexpr_ltor_non_constexpr, 1) << VD;
          Info.Note(VD->getLocation(), diag::note_declared_at);
        } else {
          Info.FFDiag(E);
        }
        return CompleteObject();
      }
    }

    if (!evaluateVarDeclInit(Info, E, VD, Frame, BaseVal, &LVal))
      return CompleteObject();
  } else {
    const Expr *Base = LVal.Base.dyn_cast<const Expr*>();

    if (!Frame) {
      if (const MaterializeTemporaryExpr *MTE =
              dyn_cast_or_null<MaterializeTemporaryExpr>(Base)) {
        assert(MTE->getStorageDuration() == SD_Static &&
               "should have a frame for a non-global materialized temporary");

        // Per C++1y [expr.const]p2:
        //  an lvalue-to-rvalue conversion [is not allowed unless it applies to]
        //   - a [...] glvalue of integral or enumeration type that refers to
        //     a non-volatile const object [...]
        //   [...]
        //   - a [...] glvalue of literal type that refers to a non-volatile
        //     object whose lifetime began within the evaluation of e.
        //
        // C++11 misses the 'began within the evaluation of e' check and
        // instead allows all temporaries, including things like:
        //   int &&r = 1;
        //   int x = ++r;
        //   constexpr int k = r;
        // Therefore we use the C++14 rules in C++11 too.
        const ValueDecl *VD = Info.EvaluatingDecl.dyn_cast<const ValueDecl*>();
        const ValueDecl *ED = MTE->getExtendingDecl();
        if (!(BaseType.isConstQualified() &&
              BaseType->isIntegralOrEnumerationType()) &&
            !(VD && VD->getCanonicalDecl() == ED->getCanonicalDecl())) {
          if (!IsAccess)
            return CompleteObject(LVal.getLValueBase(), nullptr, BaseType);
          Info.FFDiag(E, diag::note_constexpr_access_static_temporary, 1) << AK;
          Info.Note(MTE->getExprLoc(), diag::note_constexpr_temporary_here);
          return CompleteObject();
        }

        BaseVal = Info.Ctx.getMaterializedTemporaryValue(MTE, false);
        assert(BaseVal && "got reference to unevaluated temporary");
      } else {
        if (!IsAccess)
          return CompleteObject(LVal.getLValueBase(), nullptr, BaseType);
        APValue Val;
        LVal.moveInto(Val);
        Info.FFDiag(E, diag::note_constexpr_access_unreadable_object)
            << AK
            << Val.getAsString(Info.Ctx,
                               Info.Ctx.getLValueReferenceType(LValType));
        NoteLValueLocation(Info, LVal.Base);
        return CompleteObject();
      }
    } else {
      BaseVal = Frame->getTemporary(Base, LVal.Base.getVersion());
      assert(BaseVal && "missing value for temporary");
    }
  }

  // In C++14, we can't safely access any mutable state when we might be
  // evaluating after an unmodeled side effect.
  //
  // FIXME: Not all local state is mutable. Allow local constant subobjects
  // to be read here (but take care with 'mutable' fields).
  if ((Frame && Info.getLangOpts().CPlusPlus14 &&
       Info.EvalStatus.HasSideEffects) ||
      (isModification(AK) && Depth < Info.SpeculativeEvaluationDepth))
    return CompleteObject();

  return CompleteObject(LVal.getLValueBase(), BaseVal, BaseType);
}

/// Perform an lvalue-to-rvalue conversion on the given glvalue. This
/// can also be used for 'lvalue-to-lvalue' conversions for looking up the
/// glvalue referred to by an entity of reference type.
///
/// \param Info - Information about the ongoing evaluation.
/// \param Conv - The expression for which we are performing the conversion.
///               Used for diagnostics.
/// \param Type - The type of the glvalue (before stripping cv-qualifiers in the
///               case of a non-class type).
/// \param LVal - The glvalue on which we are attempting to perform this action.
/// \param RVal - The produced value will be placed here.
static bool handleLValueToRValueConversion(EvalInfo &Info, const Expr *Conv,
                                           QualType Type,
                                           const LValue &LVal, APValue &RVal) {
  if (LVal.Designator.Invalid)
    return false;

  // Check for special cases where there is no existing APValue to look at.
  const Expr *Base = LVal.Base.dyn_cast<const Expr*>();

  if (Base && !LVal.getLValueCallIndex() && !Type.isVolatileQualified()) {
    if (const CompoundLiteralExpr *CLE = dyn_cast<CompoundLiteralExpr>(Base)) {
      // In C99, a CompoundLiteralExpr is an lvalue, and we defer evaluating the
      // initializer until now for such expressions. Such an expression can't be
      // an ICE in C, so this only matters for fold.
      if (Type.isVolatileQualified()) {
        Info.FFDiag(Conv);
        return false;
      }
      APValue Lit;
      if (!Evaluate(Lit, Info, CLE->getInitializer()))
        return false;
      CompleteObject LitObj(LVal.Base, &Lit, Base->getType());
      return extractSubobject(Info, Conv, LitObj, LVal.Designator, RVal);
    } else if (isa<StringLiteral>(Base) || isa<PredefinedExpr>(Base)) {
      // Special-case character extraction so we don't have to construct an
      // APValue for the whole string.
      assert(LVal.Designator.Entries.size() <= 1 &&
             "Can only read characters from string literals");
      if (LVal.Designator.Entries.empty()) {
        // Fail for now for LValue to RValue conversion of an array.
        // (This shouldn't show up in C/C++, but it could be triggered by a
        // weird EvaluateAsRValue call from a tool.)
        Info.FFDiag(Conv);
        return false;
      }
      if (LVal.Designator.isOnePastTheEnd()) {
        if (Info.getLangOpts().CPlusPlus11)
          Info.FFDiag(Conv, diag::note_constexpr_access_past_end) << AK_Read;
        else
          Info.FFDiag(Conv);
        return false;
      }
      uint64_t CharIndex = LVal.Designator.Entries[0].getAsArrayIndex();
      RVal = APValue(extractStringLiteralCharacter(Info, Base, CharIndex));
      return true;
    }
  }

  CompleteObject Obj = findCompleteObject(Info, Conv, AK_Read, LVal, Type);
  return Obj && extractSubobject(Info, Conv, Obj, LVal.Designator, RVal);
}

/// Perform an assignment of Val to LVal. Takes ownership of Val.
static bool handleAssignment(EvalInfo &Info, const Expr *E, const LValue &LVal,
                             QualType LValType, APValue &Val) {
  if (LVal.Designator.Invalid)
    return false;

  if (!Info.getLangOpts().CPlusPlus14) {
    Info.FFDiag(E);
    return false;
  }

  CompleteObject Obj = findCompleteObject(Info, E, AK_Assign, LVal, LValType);
  return Obj && modifySubobject(Info, E, Obj, LVal.Designator, Val);
}

namespace {
struct CompoundAssignSubobjectHandler {
  EvalInfo &Info;
  const Expr *E;
  QualType PromotedLHSType;
  BinaryOperatorKind Opcode;
  const APValue &RHS;

  static const AccessKinds AccessKind = AK_Assign;

  typedef bool result_type;

  bool checkConst(QualType QT) {
    // Assigning to a const object has undefined behavior.
    if (QT.isConstQualified()) {
      Info.FFDiag(E, diag::note_constexpr_modify_const_type) << QT;
      return false;
    }
    return true;
  }

  bool failed() { return false; }
  bool found(APValue &Subobj, QualType SubobjType) {
    switch (Subobj.getKind()) {
    case APValue::Int:
      return found(Subobj.getInt(), SubobjType);
    case APValue::Float:
      return found(Subobj.getFloat(), SubobjType);
    case APValue::ComplexInt:
    case APValue::ComplexFloat:
      // FIXME: Implement complex compound assignment.
      Info.FFDiag(E);
      return false;
    case APValue::LValue:
      return foundPointer(Subobj, SubobjType);
    default:
      // FIXME: can this happen?
      Info.FFDiag(E);
      return false;
    }
  }
  bool found(APSInt &Value, QualType SubobjType) {
    if (!checkConst(SubobjType))
      return false;

    if (!SubobjType->isIntegerType()) {
      // We don't support compound assignment on integer-cast-to-pointer
      // values.
      Info.FFDiag(E);
      return false;
    }

    if (RHS.isInt()) {
      APSInt LHS =
          HandleIntToIntCast(Info, E, PromotedLHSType, SubobjType, Value);
      if (!handleIntIntBinOp(Info, E, LHS, Opcode, RHS.getInt(), LHS))
        return false;
      Value = HandleIntToIntCast(Info, E, SubobjType, PromotedLHSType, LHS);
      return true;
    } else if (RHS.isFloat()) {
      APFloat FValue(0.0);
      return HandleIntToFloatCast(Info, E, SubobjType, Value, PromotedLHSType,
                                  FValue) &&
             handleFloatFloatBinOp(Info, E, FValue, Opcode, RHS.getFloat()) &&
             HandleFloatToIntCast(Info, E, PromotedLHSType, FValue, SubobjType,
                                  Value);
    }

    Info.FFDiag(E);
    return false;
  }
  bool found(APFloat &Value, QualType SubobjType) {
    return checkConst(SubobjType) &&
           HandleFloatToFloatCast(Info, E, SubobjType, PromotedLHSType,
                                  Value) &&
           handleFloatFloatBinOp(Info, E, Value, Opcode, RHS.getFloat()) &&
           HandleFloatToFloatCast(Info, E, PromotedLHSType, SubobjType, Value);
  }
  bool foundPointer(APValue &Subobj, QualType SubobjType) {
    if (!checkConst(SubobjType))
      return false;

    QualType PointeeType;
    if (const PointerType *PT = SubobjType->getAs<PointerType>())
      PointeeType = PT->getPointeeType();

    if (PointeeType.isNull() || !RHS.isInt() ||
        (Opcode != BO_Add && Opcode != BO_Sub)) {
      Info.FFDiag(E);
      return false;
    }

    APSInt Offset = RHS.getInt();
    if (Opcode == BO_Sub)
      negateAsSigned(Offset);

    LValue LVal;
    LVal.setFrom(Info.Ctx, Subobj);
    if (!HandleLValueArrayAdjustment(Info, E, LVal, PointeeType, Offset))
      return false;
    LVal.moveInto(Subobj);
    return true;
  }
};
} // end anonymous namespace

const AccessKinds CompoundAssignSubobjectHandler::AccessKind;

/// Perform a compound assignment of LVal <op>= RVal.
static bool handleCompoundAssignment(
    EvalInfo &Info, const Expr *E,
    const LValue &LVal, QualType LValType, QualType PromotedLValType,
    BinaryOperatorKind Opcode, const APValue &RVal) {
  if (LVal.Designator.Invalid)
    return false;

  if (!Info.getLangOpts().CPlusPlus14) {
    Info.FFDiag(E);
    return false;
  }

  CompleteObject Obj = findCompleteObject(Info, E, AK_Assign, LVal, LValType);
  CompoundAssignSubobjectHandler Handler = { Info, E, PromotedLValType, Opcode,
                                             RVal };
  return Obj && findSubobject(Info, E, Obj, LVal.Designator, Handler);
}

namespace {
struct IncDecSubobjectHandler {
  EvalInfo &Info;
  const UnaryOperator *E;
  AccessKinds AccessKind;
  APValue *Old;

  typedef bool result_type;

  bool checkConst(QualType QT) {
    // Assigning to a const object has undefined behavior.
    if (QT.isConstQualified()) {
      Info.FFDiag(E, diag::note_constexpr_modify_const_type) << QT;
      return false;
    }
    return true;
  }

  bool failed() { return false; }
  bool found(APValue &Subobj, QualType SubobjType) {
    // Stash the old value. Also clear Old, so we don't clobber it later
    // if we're post-incrementing a complex.
    if (Old) {
      *Old = Subobj;
      Old = nullptr;
    }

    switch (Subobj.getKind()) {
    case APValue::Int:
      return found(Subobj.getInt(), SubobjType);
    case APValue::Float:
      return found(Subobj.getFloat(), SubobjType);
    case APValue::ComplexInt:
      return found(Subobj.getComplexIntReal(),
                   SubobjType->castAs<ComplexType>()->getElementType()
                     .withCVRQualifiers(SubobjType.getCVRQualifiers()));
    case APValue::ComplexFloat:
      return found(Subobj.getComplexFloatReal(),
                   SubobjType->castAs<ComplexType>()->getElementType()
                     .withCVRQualifiers(SubobjType.getCVRQualifiers()));
    case APValue::LValue:
      return foundPointer(Subobj, SubobjType);
    default:
      // FIXME: can this happen?
      Info.FFDiag(E);
      return false;
    }
  }
  bool found(APSInt &Value, QualType SubobjType) {
    if (!checkConst(SubobjType))
      return false;

    if (!SubobjType->isIntegerType()) {
      // We don't support increment / decrement on integer-cast-to-pointer
      // values.
      Info.FFDiag(E);
      return false;
    }

    if (Old) *Old = APValue(Value);

    // bool arithmetic promotes to int, and the conversion back to bool
    // doesn't reduce mod 2^n, so special-case it.
    if (SubobjType->isBooleanType()) {
      if (AccessKind == AK_Increment)
        Value = 1;
      else
        Value = !Value;
      return true;
    }

    bool WasNegative = Value.isNegative();
    if (AccessKind == AK_Increment) {
      ++Value;

      if (!WasNegative && Value.isNegative() && E->canOverflow()) {
        APSInt ActualValue(Value, /*IsUnsigned*/true);
        return HandleOverflow(Info, E, ActualValue, SubobjType);
      }
    } else {
      --Value;

      if (WasNegative && !Value.isNegative() && E->canOverflow()) {
        unsigned BitWidth = Value.getBitWidth();
        APSInt ActualValue(Value.sext(BitWidth + 1), /*IsUnsigned*/false);
        ActualValue.setBit(BitWidth);
        return HandleOverflow(Info, E, ActualValue, SubobjType);
      }
    }
    return true;
  }
  bool found(APFloat &Value, QualType SubobjType) {
    if (!checkConst(SubobjType))
      return false;

    if (Old) *Old = APValue(Value);

    APFloat One(Value.getSemantics(), 1);
    if (AccessKind == AK_Increment)
      Value.add(One, APFloat::rmNearestTiesToEven);
    else
      Value.subtract(One, APFloat::rmNearestTiesToEven);
    return true;
  }
  bool foundPointer(APValue &Subobj, QualType SubobjType) {
    if (!checkConst(SubobjType))
      return false;

    QualType PointeeType;
    if (const PointerType *PT = SubobjType->getAs<PointerType>())
      PointeeType = PT->getPointeeType();
    else {
      Info.FFDiag(E);
      return false;
    }

    LValue LVal;
    LVal.setFrom(Info.Ctx, Subobj);
    if (!HandleLValueArrayAdjustment(Info, E, LVal, PointeeType,
                                     AccessKind == AK_Increment ? 1 : -1))
      return false;
    LVal.moveInto(Subobj);
    return true;
  }
};
} // end anonymous namespace

/// Perform an increment or decrement on LVal.
static bool handleIncDec(EvalInfo &Info, const Expr *E, const LValue &LVal,
                         QualType LValType, bool IsIncrement, APValue *Old) {
  if (LVal.Designator.Invalid)
    return false;

  if (!Info.getLangOpts().CPlusPlus14) {
    Info.FFDiag(E);
    return false;
  }

  AccessKinds AK = IsIncrement ? AK_Increment : AK_Decrement;
  CompleteObject Obj = findCompleteObject(Info, E, AK, LVal, LValType);
  IncDecSubobjectHandler Handler = {Info, cast<UnaryOperator>(E), AK, Old};
  return Obj && findSubobject(Info, E, Obj, LVal.Designator, Handler);
}

/// Build an lvalue for the object argument of a member function call.
static bool EvaluateObjectArgument(EvalInfo &Info, const Expr *Object,
                                   LValue &This) {
  if (Object->getType()->isPointerType())
    return EvaluatePointer(Object, This, Info);

  if (Object->isGLValue())
    return EvaluateLValue(Object, This, Info);

  if (Object->getType()->isLiteralType(Info.Ctx))
    return EvaluateTemporary(Object, This, Info);

  Info.FFDiag(Object, diag::note_constexpr_nonliteral) << Object->getType();
  return false;
}

/// HandleMemberPointerAccess - Evaluate a member access operation and build an
/// lvalue referring to the result.
///
/// \param Info - Information about the ongoing evaluation.
/// \param LV - An lvalue referring to the base of the member pointer.
/// \param RHS - The member pointer expression.
/// \param IncludeMember - Specifies whether the member itself is included in
///        the resulting LValue subobject designator. This is not possible when
///        creating a bound member function.
/// \return The field or method declaration to which the member pointer refers,
///         or 0 if evaluation fails.
static const ValueDecl *HandleMemberPointerAccess(EvalInfo &Info,
                                                  QualType LVType,
                                                  LValue &LV,
                                                  const Expr *RHS,
                                                  bool IncludeMember = true) {
  MemberPtr MemPtr;
  if (!EvaluateMemberPointer(RHS, MemPtr, Info))
    return nullptr;

  // C++11 [expr.mptr.oper]p6: If the second operand is the null pointer to
  // member value, the behavior is undefined.
  if (!MemPtr.getDecl()) {
    // FIXME: Specific diagnostic.
    Info.FFDiag(RHS);
    return nullptr;
  }

  if (MemPtr.isDerivedMember()) {
    // This is a member of some derived class. Truncate LV appropriately.
    // The end of the derived-to-base path for the base object must match the
    // derived-to-base path for the member pointer.
    if (LV.Designator.MostDerivedPathLength + MemPtr.Path.size() >
        LV.Designator.Entries.size()) {
      Info.FFDiag(RHS);
      return nullptr;
    }
    unsigned PathLengthToMember =
        LV.Designator.Entries.size() - MemPtr.Path.size();
    for (unsigned I = 0, N = MemPtr.Path.size(); I != N; ++I) {
      const CXXRecordDecl *LVDecl = getAsBaseClass(
          LV.Designator.Entries[PathLengthToMember + I]);
      const CXXRecordDecl *MPDecl = MemPtr.Path[I];
      if (LVDecl->getCanonicalDecl() != MPDecl->getCanonicalDecl()) {
        Info.FFDiag(RHS);
        return nullptr;
      }
    }

    // Truncate the lvalue to the appropriate derived class.
    if (!CastToDerivedClass(Info, RHS, LV, MemPtr.getContainingRecord(),
                            PathLengthToMember))
      return nullptr;
  } else if (!MemPtr.Path.empty()) {
    // Extend the LValue path with the member pointer's path.
    LV.Designator.Entries.reserve(LV.Designator.Entries.size() +
                                  MemPtr.Path.size() + IncludeMember);

    // Walk down to the appropriate base class.
    if (const PointerType *PT = LVType->getAs<PointerType>())
      LVType = PT->getPointeeType();
    const CXXRecordDecl *RD = LVType->getAsCXXRecordDecl();
    assert(RD && "member pointer access on non-class-type expression");
    // The first class in the path is that of the lvalue.
    for (unsigned I = 1, N = MemPtr.Path.size(); I != N; ++I) {
      const CXXRecordDecl *Base = MemPtr.Path[N - I - 1];
      if (!HandleLValueDirectBase(Info, RHS, LV, RD, Base))
        return nullptr;
      RD = Base;
    }
    // Finally cast to the class containing the member.
    if (!HandleLValueDirectBase(Info, RHS, LV, RD,
                                MemPtr.getContainingRecord()))
      return nullptr;
  }

  // Add the member. Note that we cannot build bound member functions here.
  if (IncludeMember) {
    if (const FieldDecl *FD = dyn_cast<FieldDecl>(MemPtr.getDecl())) {
      if (!HandleLValueMember(Info, RHS, LV, FD))
        return nullptr;
    } else if (const IndirectFieldDecl *IFD =
                 dyn_cast<IndirectFieldDecl>(MemPtr.getDecl())) {
      if (!HandleLValueIndirectMember(Info, RHS, LV, IFD))
        return nullptr;
    } else {
      llvm_unreachable("can't construct reference to bound member function");
    }
  }

  return MemPtr.getDecl();
}

static const ValueDecl *HandleMemberPointerAccess(EvalInfo &Info,
                                                  const BinaryOperator *BO,
                                                  LValue &LV,
                                                  bool IncludeMember = true) {
  assert(BO->getOpcode() == BO_PtrMemD || BO->getOpcode() == BO_PtrMemI);

  if (!EvaluateObjectArgument(Info, BO->getLHS(), LV)) {
    if (Info.noteFailure()) {
      MemberPtr MemPtr;
      EvaluateMemberPointer(BO->getRHS(), MemPtr, Info);
    }
    return nullptr;
  }

  return HandleMemberPointerAccess(Info, BO->getLHS()->getType(), LV,
                                   BO->getRHS(), IncludeMember);
}

/// HandleBaseToDerivedCast - Apply the given base-to-derived cast operation on
/// the provided lvalue, which currently refers to the base object.
static bool HandleBaseToDerivedCast(EvalInfo &Info, const CastExpr *E,
                                    LValue &Result) {
  SubobjectDesignator &D = Result.Designator;
  if (D.Invalid || !Result.checkNullPointer(Info, E, CSK_Derived))
    return false;

  QualType TargetQT = E->getType();
  if (const PointerType *PT = TargetQT->getAs<PointerType>())
    TargetQT = PT->getPointeeType();

  // Check this cast lands within the final derived-to-base subobject path.
  if (D.MostDerivedPathLength + E->path_size() > D.Entries.size()) {
    Info.CCEDiag(E, diag::note_constexpr_invalid_downcast)
      << D.MostDerivedType << TargetQT;
    return false;
  }

  // Check the type of the final cast. We don't need to check the path,
  // since a cast can only be formed if the path is unique.
  unsigned NewEntriesSize = D.Entries.size() - E->path_size();
  const CXXRecordDecl *TargetType = TargetQT->getAsCXXRecordDecl();
  const CXXRecordDecl *FinalType;
  if (NewEntriesSize == D.MostDerivedPathLength)
    FinalType = D.MostDerivedType->getAsCXXRecordDecl();
  else
    FinalType = getAsBaseClass(D.Entries[NewEntriesSize - 1]);
  if (FinalType->getCanonicalDecl() != TargetType->getCanonicalDecl()) {
    Info.CCEDiag(E, diag::note_constexpr_invalid_downcast)
      << D.MostDerivedType << TargetQT;
    return false;
  }

  // Truncate the lvalue to the appropriate derived class.
  return CastToDerivedClass(Info, E, Result, TargetType, NewEntriesSize);
}

namespace {
enum EvalStmtResult {
  /// Evaluation failed.
  ESR_Failed,
  /// Hit a 'return' statement.
  ESR_Returned,
  /// Evaluation succeeded.
  ESR_Succeeded,
  /// Hit a 'continue' statement.
  ESR_Continue,
  /// Hit a 'break' statement.
  ESR_Break,
  /// Still scanning for 'case' or 'default' statement.
  ESR_CaseNotFound
};
}

static bool EvaluateVarDecl(EvalInfo &Info, const VarDecl *VD) {
  // We don't need to evaluate the initializer for a static local.
  if (!VD->hasLocalStorage())
    return true;

  LValue Result;
  APValue &Val = createTemporary(VD, true, Result, *Info.CurrentCall);

  const Expr *InitE = VD->getInit();
  if (!InitE) {
    Info.FFDiag(VD->getBeginLoc(), diag::note_constexpr_uninitialized)
        << false << VD->getType();
    Val = APValue();
    return false;
  }

  if (InitE->isValueDependent())
    return false;

  if (!EvaluateInPlace(Val, Info, Result, InitE)) {
    // Wipe out any partially-computed value, to allow tracking that this
    // evaluation failed.
    Val = APValue();
    return false;
  }

  return true;
}

static bool EvaluateDecl(EvalInfo &Info, const Decl *D) {
  bool OK = true;

  if (const VarDecl *VD = dyn_cast<VarDecl>(D))
    OK &= EvaluateVarDecl(Info, VD);

  if (const DecompositionDecl *DD = dyn_cast<DecompositionDecl>(D))
    for (auto *BD : DD->bindings())
      if (auto *VD = BD->getHoldingVar())
        OK &= EvaluateDecl(Info, VD);

  return OK;
}


/// Evaluate a condition (either a variable declaration or an expression).
static bool EvaluateCond(EvalInfo &Info, const VarDecl *CondDecl,
                         const Expr *Cond, bool &Result) {
  FullExpressionRAII Scope(Info);
  if (CondDecl && !EvaluateDecl(Info, CondDecl))
    return false;
  return EvaluateAsBooleanCondition(Cond, Result, Info);
}

namespace {
/// A location where the result (returned value) of evaluating a
/// statement should be stored.
struct StmtResult {
  /// The APValue that should be filled in with the returned value.
  APValue &Value;
  /// The location containing the result, if any (used to support RVO).
  const LValue *Slot;
};

struct TempVersionRAII {
  CallStackFrame &Frame;

  TempVersionRAII(CallStackFrame &Frame) : Frame(Frame) {
    Frame.pushTempVersion();
  }

  ~TempVersionRAII() {
    Frame.popTempVersion();
  }
};

}

static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
                                   const Stmt *S,
                                   const SwitchCase *SC = nullptr);

/// Evaluate the body of a loop, and translate the result as appropriate.
static EvalStmtResult EvaluateLoopBody(StmtResult &Result, EvalInfo &Info,
                                       const Stmt *Body,
                                       const SwitchCase *Case = nullptr) {
  BlockScopeRAII Scope(Info);
  switch (EvalStmtResult ESR = EvaluateStmt(Result, Info, Body, Case)) {
  case ESR_Break:
    return ESR_Succeeded;
  case ESR_Succeeded:
  case ESR_Continue:
    return ESR_Continue;
  case ESR_Failed:
  case ESR_Returned:
  case ESR_CaseNotFound:
    return ESR;
  }
  llvm_unreachable("Invalid EvalStmtResult!");
}

/// Evaluate a switch statement.
static EvalStmtResult EvaluateSwitch(StmtResult &Result, EvalInfo &Info,
                                     const SwitchStmt *SS) {
  BlockScopeRAII Scope(Info);

  // Evaluate the switch condition.
  APSInt Value;
  {
    FullExpressionRAII Scope(Info);
    if (const Stmt *Init = SS->getInit()) {
      EvalStmtResult ESR = EvaluateStmt(Result, Info, Init);
      if (ESR != ESR_Succeeded)
        return ESR;
    }
    if (SS->getConditionVariable() &&
        !EvaluateDecl(Info, SS->getConditionVariable()))
      return ESR_Failed;
    if (!EvaluateInteger(SS->getCond(), Value, Info))
      return ESR_Failed;
  }

  // Find the switch case corresponding to the value of the condition.
  // FIXME: Cache this lookup.
  const SwitchCase *Found = nullptr;
  for (const SwitchCase *SC = SS->getSwitchCaseList(); SC;
       SC = SC->getNextSwitchCase()) {
    if (isa<DefaultStmt>(SC)) {
      Found = SC;
      continue;
    }

    const CaseStmt *CS = cast<CaseStmt>(SC);
    APSInt LHS = CS->getLHS()->EvaluateKnownConstInt(Info.Ctx);
    APSInt RHS = CS->getRHS() ? CS->getRHS()->EvaluateKnownConstInt(Info.Ctx)
                              : LHS;
    if (LHS <= Value && Value <= RHS) {
      Found = SC;
      break;
    }
  }

  if (!Found)
    return ESR_Succeeded;

  // Search the switch body for the switch case and evaluate it from there.
  switch (EvalStmtResult ESR = EvaluateStmt(Result, Info, SS->getBody(), Found)) {
  case ESR_Break:
    return ESR_Succeeded;
  case ESR_Succeeded:
  case ESR_Continue:
  case ESR_Failed:
  case ESR_Returned:
    return ESR;
  case ESR_CaseNotFound:
    // This can only happen if the switch case is nested within a statement
    // expression. We have no intention of supporting that.
    Info.FFDiag(Found->getBeginLoc(),
                diag::note_constexpr_stmt_expr_unsupported);
    return ESR_Failed;
  }
  llvm_unreachable("Invalid EvalStmtResult!");
}

// Evaluate a statement.
static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
                                   const Stmt *S, const SwitchCase *Case) {
  if (!Info.nextStep(S))
    return ESR_Failed;

  // If we're hunting down a 'case' or 'default' label, recurse through
  // substatements until we hit the label.
  if (Case) {
    // FIXME: We don't start the lifetime of objects whose initialization we
    // jump over. However, such objects must be of class type with a trivial
    // default constructor that initialize all subobjects, so must be empty,
    // so this almost never matters.
    switch (S->getStmtClass()) {
    case Stmt::CompoundStmtClass:
      // FIXME: Precompute which substatement of a compound statement we
      // would jump to, and go straight there rather than performing a
      // linear scan each time.
    case Stmt::LabelStmtClass:
    case Stmt::AttributedStmtClass:
    case Stmt::DoStmtClass:
      break;

    case Stmt::CaseStmtClass:
    case Stmt::DefaultStmtClass:
      if (Case == S)
        Case = nullptr;
      break;

    case Stmt::IfStmtClass: {
      // FIXME: Precompute which side of an 'if' we would jump to, and go
      // straight there rather than scanning both sides.
      const IfStmt *IS = cast<IfStmt>(S);

      // Wrap the evaluation in a block scope, in case it's a DeclStmt
      // preceded by our switch label.
      BlockScopeRAII Scope(Info);

      EvalStmtResult ESR = EvaluateStmt(Result, Info, IS->getThen(), Case);
      if (ESR != ESR_CaseNotFound || !IS->getElse())
        return ESR;
      return EvaluateStmt(Result, Info, IS->getElse(), Case);
    }

    case Stmt::WhileStmtClass: {
      EvalStmtResult ESR =
          EvaluateLoopBody(Result, Info, cast<WhileStmt>(S)->getBody(), Case);
      if (ESR != ESR_Continue)
        return ESR;
      break;
    }

    case Stmt::ForStmtClass: {
      const ForStmt *FS = cast<ForStmt>(S);
      EvalStmtResult ESR =
          EvaluateLoopBody(Result, Info, FS->getBody(), Case);
      if (ESR != ESR_Continue)
        return ESR;
      if (FS->getInc()) {
        FullExpressionRAII IncScope(Info);
        if (!EvaluateIgnoredValue(Info, FS->getInc()))
          return ESR_Failed;
      }
      break;
    }

    case Stmt::DeclStmtClass:
      // FIXME: If the variable has initialization that can't be jumped over,
      // bail out of any immediately-surrounding compound-statement too.
    default:
      return ESR_CaseNotFound;
    }
  }

  switch (S->getStmtClass()) {
  default:
    if (const Expr *E = dyn_cast<Expr>(S)) {
      // Don't bother evaluating beyond an expression-statement which couldn't
      // be evaluated.
      FullExpressionRAII Scope(Info);
      if (!EvaluateIgnoredValue(Info, E))
        return ESR_Failed;
      return ESR_Succeeded;
    }

    Info.FFDiag(S->getBeginLoc());
    return ESR_Failed;

  case Stmt::NullStmtClass:
    return ESR_Succeeded;

  case Stmt::DeclStmtClass: {
    const DeclStmt *DS = cast<DeclStmt>(S);
    for (const auto *DclIt : DS->decls()) {
      // Each declaration initialization is its own full-expression.
      // FIXME: This isn't quite right; if we're performing aggregate
      // initialization, each braced subexpression is its own full-expression.
      FullExpressionRAII Scope(Info);
      if (!EvaluateDecl(Info, DclIt) && !Info.noteFailure())
        return ESR_Failed;
    }
    return ESR_Succeeded;
  }

  case Stmt::ReturnStmtClass: {
    const Expr *RetExpr = cast<ReturnStmt>(S)->getRetValue();
    FullExpressionRAII Scope(Info);
    if (RetExpr &&
        !(Result.Slot
              ? EvaluateInPlace(Result.Value, Info, *Result.Slot, RetExpr)
              : Evaluate(Result.Value, Info, RetExpr)))
      return ESR_Failed;
    return ESR_Returned;
  }

  case Stmt::CompoundStmtClass: {
    BlockScopeRAII Scope(Info);

    const CompoundStmt *CS = cast<CompoundStmt>(S);
    for (const auto *BI : CS->body()) {
      EvalStmtResult ESR = EvaluateStmt(Result, Info, BI, Case);
      if (ESR == ESR_Succeeded)
        Case = nullptr;
      else if (ESR != ESR_CaseNotFound)
        return ESR;
    }
    return Case ? ESR_CaseNotFound : ESR_Succeeded;
  }

  case Stmt::IfStmtClass: {
    const IfStmt *IS = cast<IfStmt>(S);

    // Evaluate the condition, as either a var decl or as an expression.
    BlockScopeRAII Scope(Info);
    if (const Stmt *Init = IS->getInit()) {
      EvalStmtResult ESR = EvaluateStmt(Result, Info, Init);
      if (ESR != ESR_Succeeded)
        return ESR;
    }
    bool Cond;
    if (!EvaluateCond(Info, IS->getConditionVariable(), IS->getCond(), Cond))
      return ESR_Failed;

    if (const Stmt *SubStmt = Cond ? IS->getThen() : IS->getElse()) {
      EvalStmtResult ESR = EvaluateStmt(Result, Info, SubStmt);
      if (ESR != ESR_Succeeded)
        return ESR;
    }
    return ESR_Succeeded;
  }

  case Stmt::WhileStmtClass: {
    const WhileStmt *WS = cast<WhileStmt>(S);
    while (true) {
      BlockScopeRAII Scope(Info);
      bool Continue;
      if (!EvaluateCond(Info, WS->getConditionVariable(), WS->getCond(),
                        Continue))
        return ESR_Failed;
      if (!Continue)
        break;

      EvalStmtResult ESR = EvaluateLoopBody(Result, Info, WS->getBody());
      if (ESR != ESR_Continue)
        return ESR;
    }
    return ESR_Succeeded;
  }

  case Stmt::DoStmtClass: {
    const DoStmt *DS = cast<DoStmt>(S);
    bool Continue;
    do {
      EvalStmtResult ESR = EvaluateLoopBody(Result, Info, DS->getBody(), Case);
      if (ESR != ESR_Continue)
        return ESR;
      Case = nullptr;

      FullExpressionRAII CondScope(Info);
      if (!EvaluateAsBooleanCondition(DS->getCond(), Continue, Info))
        return ESR_Failed;
    } while (Continue);
    return ESR_Succeeded;
  }

  case Stmt::ForStmtClass: {
    const ForStmt *FS = cast<ForStmt>(S);
    BlockScopeRAII Scope(Info);
    if (FS->getInit()) {
      EvalStmtResult ESR = EvaluateStmt(Result, Info, FS->getInit());
      if (ESR != ESR_Succeeded)
        return ESR;
    }
    while (true) {
      BlockScopeRAII Scope(Info);
      bool Continue = true;
      if (FS->getCond() && !EvaluateCond(Info, FS->getConditionVariable(),
                                         FS->getCond(), Continue))
        return ESR_Failed;
      if (!Continue)
        break;

      EvalStmtResult ESR = EvaluateLoopBody(Result, Info, FS->getBody());
      if (ESR != ESR_Continue)
        return ESR;

      if (FS->getInc()) {
        FullExpressionRAII IncScope(Info);
        if (!EvaluateIgnoredValue(Info, FS->getInc()))
          return ESR_Failed;
      }
    }
    return ESR_Succeeded;
  }

  case Stmt::CXXForRangeStmtClass: {
    const CXXForRangeStmt *FS = cast<CXXForRangeStmt>(S);
    BlockScopeRAII Scope(Info);

    // Evaluate the init-statement if present.
    if (FS->getInit()) {
      EvalStmtResult ESR = EvaluateStmt(Result, Info, FS->getInit());
      if (ESR != ESR_Succeeded)
        return ESR;
    }

    // Initialize the __range variable.
    EvalStmtResult ESR = EvaluateStmt(Result, Info, FS->getRangeStmt());
    if (ESR != ESR_Succeeded)
      return ESR;

    // Create the __begin and __end iterators.
    ESR = EvaluateStmt(Result, Info, FS->getBeginStmt());
    if (ESR != ESR_Succeeded)
      return ESR;
    ESR = EvaluateStmt(Result, Info, FS->getEndStmt());
    if (ESR != ESR_Succeeded)
      return ESR;

    while (true) {
      // Condition: __begin != __end.
      {
        bool Continue = true;
        FullExpressionRAII CondExpr(Info);
        if (!EvaluateAsBooleanCondition(FS->getCond(), Continue, Info))
          return ESR_Failed;
        if (!Continue)
          break;
      }

      // User's variable declaration, initialized by *__begin.
      BlockScopeRAII InnerScope(Info);
      ESR = EvaluateStmt(Result, Info, FS->getLoopVarStmt());
      if (ESR != ESR_Succeeded)
        return ESR;

      // Loop body.
      ESR = EvaluateLoopBody(Result, Info, FS->getBody());
      if (ESR != ESR_Continue)
        return ESR;

      // Increment: ++__begin
      if (!EvaluateIgnoredValue(Info, FS->getInc()))
        return ESR_Failed;
    }

    return ESR_Succeeded;
  }

  case Stmt::SwitchStmtClass:
    return EvaluateSwitch(Result, Info, cast<SwitchStmt>(S));

  case Stmt::ContinueStmtClass:
    return ESR_Continue;

  case Stmt::BreakStmtClass:
    return ESR_Break;

  case Stmt::LabelStmtClass:
    return EvaluateStmt(Result, Info, cast<LabelStmt>(S)->getSubStmt(), Case);

  case Stmt::AttributedStmtClass:
    // As a general principle, C++11 attributes can be ignored without
    // any semantic impact.
    return EvaluateStmt(Result, Info, cast<AttributedStmt>(S)->getSubStmt(),
                        Case);

  case Stmt::CaseStmtClass:
  case Stmt::DefaultStmtClass:
    return EvaluateStmt(Result, Info, cast<SwitchCase>(S)->getSubStmt(), Case);
  case Stmt::CXXTryStmtClass:
    // Evaluate try blocks by evaluating all sub statements.
    return EvaluateStmt(Result, Info, cast<CXXTryStmt>(S)->getTryBlock(), Case);
  }
}

/// CheckTrivialDefaultConstructor - Check whether a constructor is a trivial
/// default constructor. If so, we'll fold it whether or not it's marked as
/// constexpr. If it is marked as constexpr, we will never implicitly define it,
/// so we need special handling.
static bool CheckTrivialDefaultConstructor(EvalInfo &Info, SourceLocation Loc,
                                           const CXXConstructorDecl *CD,
                                           bool IsValueInitialization) {
  if (!CD->isTrivial() || !CD->isDefaultConstructor())
    return false;

  // Value-initialization does not call a trivial default constructor, so such a
  // call is a core constant expression whether or not the constructor is
  // constexpr.
  if (!CD->isConstexpr() && !IsValueInitialization) {
    if (Info.getLangOpts().CPlusPlus11) {
      // FIXME: If DiagDecl is an implicitly-declared special member function,
      // we should be much more explicit about why it's not constexpr.
      Info.CCEDiag(Loc, diag::note_constexpr_invalid_function, 1)
        << /*IsConstexpr*/0 << /*IsConstructor*/1 << CD;
      Info.Note(CD->getLocation(), diag::note_declared_at);
    } else {
      Info.CCEDiag(Loc, diag::note_invalid_subexpr_in_const_expr);
    }
  }
  return true;
}

/// CheckConstexprFunction - Check that a function can be called in a constant
/// expression.
static bool CheckConstexprFunction(EvalInfo &Info, SourceLocation CallLoc,
                                   const FunctionDecl *Declaration,
                                   const FunctionDecl *Definition,
                                   const Stmt *Body) {
  // Potential constant expressions can contain calls to declared, but not yet
  // defined, constexpr functions.
  if (Info.checkingPotentialConstantExpression() && !Definition &&
      Declaration->isConstexpr())
    return false;

  // Bail out if the function declaration itself is invalid.  We will
  // have produced a relevant diagnostic while parsing it, so just
  // note the problematic sub-expression.
  if (Declaration->isInvalidDecl()) {
    Info.FFDiag(CallLoc, diag::note_invalid_subexpr_in_const_expr);
    return false;
  }

  // DR1872: An instantiated virtual constexpr function can't be called in a
  // constant expression (prior to C++20). We can still constant-fold such a
  // call.
  if (!Info.Ctx.getLangOpts().CPlusPlus2a && isa<CXXMethodDecl>(Declaration) &&
      cast<CXXMethodDecl>(Declaration)->isVirtual())
    Info.CCEDiag(CallLoc, diag::note_constexpr_virtual_call);

  if (Definition && Definition->isInvalidDecl()) {
    Info.FFDiag(CallLoc, diag::note_invalid_subexpr_in_const_expr);
    return false;
  }

  // Can we evaluate this function call?
  if (Definition && Definition->isConstexpr() && Body)
    return true;

  if (Info.getLangOpts().CPlusPlus11) {
    const FunctionDecl *DiagDecl = Definition ? Definition : Declaration;

    // If this function is not constexpr because it is an inherited
    // non-constexpr constructor, diagnose that directly.
    auto *CD = dyn_cast<CXXConstructorDecl>(DiagDecl);
    if (CD && CD->isInheritingConstructor()) {
      auto *Inherited = CD->getInheritedConstructor().getConstructor();
      if (!Inherited->isConstexpr())
        DiagDecl = CD = Inherited;
    }

    // FIXME: If DiagDecl is an implicitly-declared special member function
    // or an inheriting constructor, we should be much more explicit about why
    // it's not constexpr.
    if (CD && CD->isInheritingConstructor())
      Info.FFDiag(CallLoc, diag::note_constexpr_invalid_inhctor, 1)
        << CD->getInheritedConstructor().getConstructor()->getParent();
    else
      Info.FFDiag(CallLoc, diag::note_constexpr_invalid_function, 1)
        << DiagDecl->isConstexpr() << (bool)CD << DiagDecl;
    Info.Note(DiagDecl->getLocation(), diag::note_declared_at);
  } else {
    Info.FFDiag(CallLoc, diag::note_invalid_subexpr_in_const_expr);
  }
  return false;
}

namespace {
struct CheckDynamicTypeHandler {
  AccessKinds AccessKind;
  typedef bool result_type;
  bool failed() { return false; }
  bool found(APValue &Subobj, QualType SubobjType) { return true; }
  bool found(APSInt &Value, QualType SubobjType) { return true; }
  bool found(APFloat &Value, QualType SubobjType) { return true; }
};
} // end anonymous namespace

/// Check that we can access the notional vptr of an object / determine its
/// dynamic type.
static bool checkDynamicType(EvalInfo &Info, const Expr *E, const LValue &This,
                             AccessKinds AK, bool Polymorphic) {
  if (This.Designator.Invalid)
    return false;

  CompleteObject Obj = findCompleteObject(Info, E, AK, This, QualType());

  if (!Obj)
    return false;

  if (!Obj.Value) {
    // The object is not usable in constant expressions, so we can't inspect
    // its value to see if it's in-lifetime or what the active union members
    // are. We can still check for a one-past-the-end lvalue.
    if (This.Designator.isOnePastTheEnd() ||
        This.Designator.isMostDerivedAnUnsizedArray()) {
      Info.FFDiag(E, This.Designator.isOnePastTheEnd()
                         ? diag::note_constexpr_access_past_end
                         : diag::note_constexpr_access_unsized_array)
          << AK;
      return false;
    } else if (Polymorphic) {
      // Conservatively refuse to perform a polymorphic operation if we would
      // not be able to read a notional 'vptr' value.
      APValue Val;
      This.moveInto(Val);
      QualType StarThisType =
          Info.Ctx.getLValueReferenceType(This.Designator.getType(Info.Ctx));
      Info.FFDiag(E, diag::note_constexpr_polymorphic_unknown_dynamic_type)
          << AK << Val.getAsString(Info.Ctx, StarThisType);
      return false;
    }
    return true;
  }

  CheckDynamicTypeHandler Handler{AK};
  return Obj && findSubobject(Info, E, Obj, This.Designator, Handler);
}

/// Check that the pointee of the 'this' pointer in a member function call is
/// either within its lifetime or in its period of construction or destruction.
static bool checkNonVirtualMemberCallThisPointer(EvalInfo &Info, const Expr *E,
                                                 const LValue &This) {
  return checkDynamicType(Info, E, This, AK_MemberCall, false);
}

struct DynamicType {
  /// The dynamic class type of the object.
  const CXXRecordDecl *Type;
  /// The corresponding path length in the lvalue.
  unsigned PathLength;
};

static const CXXRecordDecl *getBaseClassType(SubobjectDesignator &Designator,
                                             unsigned PathLength) {
  assert(PathLength >= Designator.MostDerivedPathLength && PathLength <=
      Designator.Entries.size() && "invalid path length");
  return (PathLength == Designator.MostDerivedPathLength)
             ? Designator.MostDerivedType->getAsCXXRecordDecl()
             : getAsBaseClass(Designator.Entries[PathLength - 1]);
}

/// Determine the dynamic type of an object.
static Optional<DynamicType> ComputeDynamicType(EvalInfo &Info, const Expr *E,
                                                LValue &This, AccessKinds AK) {
  // If we don't have an lvalue denoting an object of class type, there is no
  // meaningful dynamic type. (We consider objects of non-class type to have no
  // dynamic type.)
  if (!checkDynamicType(Info, E, This, AK, true))
    return None;

  // Refuse to compute a dynamic type in the presence of virtual bases. This
  // shouldn't happen other than in constant-folding situations, since literal
  // types can't have virtual bases.
  //
  // Note that consumers of DynamicType assume that the type has no virtual
  // bases, and will need modifications if this restriction is relaxed.
  const CXXRecordDecl *Class =
      This.Designator.MostDerivedType->getAsCXXRecordDecl();
  if (!Class || Class->getNumVBases()) {
    Info.FFDiag(E);
    return None;
  }

  // FIXME: For very deep class hierarchies, it might be beneficial to use a
  // binary search here instead. But the overwhelmingly common case is that
  // we're not in the middle of a constructor, so it probably doesn't matter
  // in practice.
  ArrayRef<APValue::LValuePathEntry> Path = This.Designator.Entries;
  for (unsigned PathLength = This.Designator.MostDerivedPathLength;
       PathLength <= Path.size(); ++PathLength) {
    switch (Info.isEvaluatingConstructor(This.getLValueBase(),
                                         Path.slice(0, PathLength))) {
    case ConstructionPhase::Bases:
      // We're constructing a base class. This is not the dynamic type.
      break;

    case ConstructionPhase::None:
    case ConstructionPhase::AfterBases:
      // We've finished constructing the base classes, so this is the dynamic
      // type.
      return DynamicType{getBaseClassType(This.Designator, PathLength),
                         PathLength};
    }
  }

  // CWG issue 1517: we're constructing a base class of the object described by
  // 'This', so that object has not yet begun its period of construction and
  // any polymorphic operation on it results in undefined behavior.
  Info.FFDiag(E);
  return None;
}

/// Perform virtual dispatch.
static const CXXMethodDecl *HandleVirtualDispatch(
    EvalInfo &Info, const Expr *E, LValue &This, const CXXMethodDecl *Found,
    llvm::SmallVectorImpl<QualType> &CovariantAdjustmentPath) {
  Optional<DynamicType> DynType =
      ComputeDynamicType(Info, E, This, AK_MemberCall);
  if (!DynType)
    return nullptr;

  // Find the final overrider. It must be declared in one of the classes on the
  // path from the dynamic type to the static type.
  // FIXME: If we ever allow literal types to have virtual base classes, that
  // won't be true.
  const CXXMethodDecl *Callee = Found;
  unsigned PathLength = DynType->PathLength;
  for (/**/; PathLength <= This.Designator.Entries.size(); ++PathLength) {
    const CXXRecordDecl *Class = getBaseClassType(This.Designator, PathLength);
    const CXXMethodDecl *Overrider =
        Found->getCorrespondingMethodDeclaredInClass(Class, false);
    if (Overrider) {
      Callee = Overrider;
      break;
    }
  }

  // C++2a [class.abstract]p6:
  //   the effect of making a virtual call to a pure virtual function [...] is
  //   undefined
  if (Callee->isPure()) {
    Info.FFDiag(E, diag::note_constexpr_pure_virtual_call, 1) << Callee;
    Info.Note(Callee->getLocation(), diag::note_declared_at);
    return nullptr;
  }

  // If necessary, walk the rest of the path to determine the sequence of
  // covariant adjustment steps to apply.
  if (!Info.Ctx.hasSameUnqualifiedType(Callee->getReturnType(),
                                       Found->getReturnType())) {
    CovariantAdjustmentPath.push_back(Callee->getReturnType());
    for (unsigned CovariantPathLength = PathLength + 1;
         CovariantPathLength != This.Designator.Entries.size();
         ++CovariantPathLength) {
      const CXXRecordDecl *NextClass =
          getBaseClassType(This.Designator, CovariantPathLength);
      const CXXMethodDecl *Next =
          Found->getCorrespondingMethodDeclaredInClass(NextClass, false);
      if (Next && !Info.Ctx.hasSameUnqualifiedType(
                      Next->getReturnType(), CovariantAdjustmentPath.back()))
        CovariantAdjustmentPath.push_back(Next->getReturnType());
    }
    if (!Info.Ctx.hasSameUnqualifiedType(Found->getReturnType(),
                                         CovariantAdjustmentPath.back()))
      CovariantAdjustmentPath.push_back(Found->getReturnType());
  }

  // Perform 'this' adjustment.
  if (!CastToDerivedClass(Info, E, This, Callee->getParent(), PathLength))
    return nullptr;

  return Callee;
}

/// Perform the adjustment from a value returned by a virtual function to
/// a value of the statically expected type, which may be a pointer or
/// reference to a base class of the returned type.
static bool HandleCovariantReturnAdjustment(EvalInfo &Info, const Expr *E,
                                            APValue &Result,
                                            ArrayRef<QualType> Path) {
  assert(Result.isLValue() &&
         "unexpected kind of APValue for covariant return");
  if (Result.isNullPointer())
    return true;

  LValue LVal;
  LVal.setFrom(Info.Ctx, Result);

  const CXXRecordDecl *OldClass = Path[0]->getPointeeCXXRecordDecl();
  for (unsigned I = 1; I != Path.size(); ++I) {
    const CXXRecordDecl *NewClass = Path[I]->getPointeeCXXRecordDecl();
    assert(OldClass && NewClass && "unexpected kind of covariant return");
    if (OldClass != NewClass &&
        !CastToBaseClass(Info, E, LVal, OldClass, NewClass))
      return false;
    OldClass = NewClass;
  }

  LVal.moveInto(Result);
  return true;
}

/// Determine whether \p Base, which is known to be a direct base class of
/// \p Derived, is a public base class.
static bool isBaseClassPublic(const CXXRecordDecl *Derived,
                              const CXXRecordDecl *Base) {
  for (const CXXBaseSpecifier &BaseSpec : Derived->bases()) {
    auto *BaseClass = BaseSpec.getType()->getAsCXXRecordDecl();
    if (BaseClass && declaresSameEntity(BaseClass, Base))
      return BaseSpec.getAccessSpecifier() == AS_public;
  }
  llvm_unreachable("Base is not a direct base of Derived");
}

/// Apply the given dynamic cast operation on the provided lvalue.
///
/// This implements the hard case of dynamic_cast, requiring a "runtime check"
/// to find a suitable target subobject.
static bool HandleDynamicCast(EvalInfo &Info, const ExplicitCastExpr *E,
                              LValue &Ptr) {
  // We can't do anything with a non-symbolic pointer value.
  SubobjectDesignator &D = Ptr.Designator;
  if (D.Invalid)
    return false;

  // C++ [expr.dynamic.cast]p6:
  //   If v is a null pointer value, the result is a null pointer value.
  if (Ptr.isNullPointer() && !E->isGLValue())
    return true;

  // For all the other cases, we need the pointer to point to an object within
  // its lifetime / period of construction / destruction, and we need to know
  // its dynamic type.
  Optional<DynamicType> DynType =
      ComputeDynamicType(Info, E, Ptr, AK_DynamicCast);
  if (!DynType)
    return false;

  // C++ [expr.dynamic.cast]p7:
  //   If T is "pointer to cv void", then the result is a pointer to the most
  //   derived object
  if (E->getType()->isVoidPointerType())
    return CastToDerivedClass(Info, E, Ptr, DynType->Type, DynType->PathLength);

  const CXXRecordDecl *C = E->getTypeAsWritten()->getPointeeCXXRecordDecl();
  assert(C && "dynamic_cast target is not void pointer nor class");
  CanQualType CQT = Info.Ctx.getCanonicalType(Info.Ctx.getRecordType(C));

  auto RuntimeCheckFailed = [&] (CXXBasePaths *Paths) {
    // C++ [expr.dynamic.cast]p9:
    if (!E->isGLValue()) {
      //   The value of a failed cast to pointer type is the null pointer value
      //   of the required result type.
      auto TargetVal = Info.Ctx.getTargetNullPointerValue(E->getType());
      Ptr.setNull(E->getType(), TargetVal);
      return true;
    }

    //   A failed cast to reference type throws [...] std::bad_cast.
    unsigned DiagKind;
    if (!Paths && (declaresSameEntity(DynType->Type, C) ||
                   DynType->Type->isDerivedFrom(C)))
      DiagKind = 0;
    else if (!Paths || Paths->begin() == Paths->end())
      DiagKind = 1;
    else if (Paths->isAmbiguous(CQT))
      DiagKind = 2;
    else {
      assert(Paths->front().Access != AS_public && "why did the cast fail?");
      DiagKind = 3;
    }
    Info.FFDiag(E, diag::note_constexpr_dynamic_cast_to_reference_failed)
        << DiagKind << Ptr.Designator.getType(Info.Ctx)
        << Info.Ctx.getRecordType(DynType->Type)
        << E->getType().getUnqualifiedType();
    return false;
  };

  // Runtime check, phase 1:
  //   Walk from the base subobject towards the derived object looking for the
  //   target type.
  for (int PathLength = Ptr.Designator.Entries.size();
       PathLength >= (int)DynType->PathLength; --PathLength) {
    const CXXRecordDecl *Class = getBaseClassType(Ptr.Designator, PathLength);
    if (declaresSameEntity(Class, C))
      return CastToDerivedClass(Info, E, Ptr, Class, PathLength);
    // We can only walk across public inheritance edges.
    if (PathLength > (int)DynType->PathLength &&
        !isBaseClassPublic(getBaseClassType(Ptr.Designator, PathLength - 1),
                           Class))
      return RuntimeCheckFailed(nullptr);
  }

  // Runtime check, phase 2:
  //   Search the dynamic type for an unambiguous public base of type C.
  CXXBasePaths Paths(/*FindAmbiguities=*/true,
                     /*RecordPaths=*/true, /*DetectVirtual=*/false);
  if (DynType->Type->isDerivedFrom(C, Paths) && !Paths.isAmbiguous(CQT) &&
      Paths.front().Access == AS_public) {
    // Downcast to the dynamic type...
    if (!CastToDerivedClass(Info, E, Ptr, DynType->Type, DynType->PathLength))
      return false;
    // ... then upcast to the chosen base class subobject.
    for (CXXBasePathElement &Elem : Paths.front())
      if (!HandleLValueBase(Info, E, Ptr, Elem.Class, Elem.Base))
        return false;
    return true;
  }

  // Otherwise, the runtime check fails.
  return RuntimeCheckFailed(&Paths);
}

namespace {
struct StartLifetimeOfUnionMemberHandler {
  const FieldDecl *Field;

  static const AccessKinds AccessKind = AK_Assign;

  APValue getDefaultInitValue(QualType SubobjType) {
    if (auto *RD = SubobjType->getAsCXXRecordDecl()) {
      if (RD->isUnion())
        return APValue((const FieldDecl*)nullptr);

      APValue Struct(APValue::UninitStruct(), RD->getNumBases(),
                     std::distance(RD->field_begin(), RD->field_end()));

      unsigned Index = 0;
      for (CXXRecordDecl::base_class_const_iterator I = RD->bases_begin(),
             End = RD->bases_end(); I != End; ++I, ++Index)
        Struct.getStructBase(Index) = getDefaultInitValue(I->getType());

      for (const auto *I : RD->fields()) {
        if (I->isUnnamedBitfield())
          continue;
        Struct.getStructField(I->getFieldIndex()) =
            getDefaultInitValue(I->getType());
      }
      return Struct;
    }

    if (auto *AT = dyn_cast_or_null<ConstantArrayType>(
            SubobjType->getAsArrayTypeUnsafe())) {
      APValue Array(APValue::UninitArray(), 0, AT->getSize().getZExtValue());
      if (Array.hasArrayFiller())
        Array.getArrayFiller() = getDefaultInitValue(AT->getElementType());
      return Array;
    }

    return APValue::IndeterminateValue();
  }

  typedef bool result_type;
  bool failed() { return false; }
  bool found(APValue &Subobj, QualType SubobjType) {
    // We are supposed to perform no initialization but begin the lifetime of
    // the object. We interpret that as meaning to do what default
    // initialization of the object would do if all constructors involved were
    // trivial:
    //  * All base, non-variant member, and array element subobjects' lifetimes
    //    begin
    //  * No variant members' lifetimes begin
    //  * All scalar subobjects whose lifetimes begin have indeterminate values
    assert(SubobjType->isUnionType());
    if (!declaresSameEntity(Subobj.getUnionField(), Field))
      Subobj.setUnion(Field, getDefaultInitValue(Field->getType()));
    return true;
  }
  bool found(APSInt &Value, QualType SubobjType) {
    llvm_unreachable("wrong value kind for union object");
  }
  bool found(APFloat &Value, QualType SubobjType) {
    llvm_unreachable("wrong value kind for union object");
  }
};
} // end anonymous namespace

const AccessKinds StartLifetimeOfUnionMemberHandler::AccessKind;

/// Handle a builtin simple-assignment or a call to a trivial assignment
/// operator whose left-hand side might involve a union member access. If it
/// does, implicitly start the lifetime of any accessed union elements per
/// C++20 [class.union]5.
static bool HandleUnionActiveMemberChange(EvalInfo &Info, const Expr *LHSExpr,
                                          const LValue &LHS) {
  if (LHS.InvalidBase || LHS.Designator.Invalid)
    return false;

  llvm::SmallVector<std::pair<unsigned, const FieldDecl*>, 4> UnionPathLengths;
  // C++ [class.union]p5:
  //   define the set S(E) of subexpressions of E as follows:
  unsigned PathLength = LHS.Designator.Entries.size();
  for (const Expr *E = LHSExpr; E != nullptr;) {
    //   -- If E is of the form A.B, S(E) contains the elements of S(A)...
    if (auto *ME = dyn_cast<MemberExpr>(E)) {
      auto *FD = dyn_cast<FieldDecl>(ME->getMemberDecl());
      if (!FD)
        break;

      //    ... and also contains A.B if B names a union member
      if (FD->getParent()->isUnion())
        UnionPathLengths.push_back({PathLength - 1, FD});

      E = ME->getBase();
      --PathLength;
      assert(declaresSameEntity(FD,
                                LHS.Designator.Entries[PathLength]
                                    .getAsBaseOrMember().getPointer()));

      //   -- If E is of the form A[B] and is interpreted as a built-in array
      //      subscripting operator, S(E) is [S(the array operand, if any)].
    } else if (auto *ASE = dyn_cast<ArraySubscriptExpr>(E)) {
      // Step over an ArrayToPointerDecay implicit cast.
      auto *Base = ASE->getBase()->IgnoreImplicit();
      if (!Base->getType()->isArrayType())
        break;

      E = Base;
      --PathLength;

    } else if (auto *ICE = dyn_cast<ImplicitCastExpr>(E)) {
      // Step over a derived-to-base conversion.
      E = ICE->getSubExpr();
      if (ICE->getCastKind() == CK_NoOp)
        continue;
      if (ICE->getCastKind() != CK_DerivedToBase &&
          ICE->getCastKind() != CK_UncheckedDerivedToBase)
        break;
      // Walk path backwards as we walk up from the base to the derived class.
      for (const CXXBaseSpecifier *Elt : llvm::reverse(ICE->path())) {
        --PathLength;
        (void)Elt;
        assert(declaresSameEntity(Elt->getType()->getAsCXXRecordDecl(),
                                  LHS.Designator.Entries[PathLength]
                                      .getAsBaseOrMember().getPointer()));
      }

    //   -- Otherwise, S(E) is empty.
    } else {
      break;
    }
  }

  // Common case: no unions' lifetimes are started.
  if (UnionPathLengths.empty())
    return true;

  //   if modification of X [would access an inactive union member], an object
  //   of the type of X is implicitly created
  CompleteObject Obj =
      findCompleteObject(Info, LHSExpr, AK_Assign, LHS, LHSExpr->getType());
  if (!Obj)
    return false;
  for (std::pair<unsigned, const FieldDecl *> LengthAndField :
           llvm::reverse(UnionPathLengths)) {
    // Form a designator for the union object.
    SubobjectDesignator D = LHS.Designator;
    D.truncate(Info.Ctx, LHS.Base, LengthAndField.first);

    StartLifetimeOfUnionMemberHandler StartLifetime{LengthAndField.second};
    if (!findSubobject(Info, LHSExpr, Obj, D, StartLifetime))
      return false;
  }

  return true;
}

/// Determine if a class has any fields that might need to be copied by a
/// trivial copy or move operation.
static bool hasFields(const CXXRecordDecl *RD) {
  if (!RD || RD->isEmpty())
    return false;
  for (auto *FD : RD->fields()) {
    if (FD->isUnnamedBitfield())
      continue;
    return true;
  }
  for (auto &Base : RD->bases())
    if (hasFields(Base.getType()->getAsCXXRecordDecl()))
      return true;
  return false;
}

namespace {
typedef SmallVector<APValue, 8> ArgVector;
}

/// EvaluateArgs - Evaluate the arguments to a function call.
static bool EvaluateArgs(ArrayRef<const Expr *> Args, ArgVector &ArgValues,
                         EvalInfo &Info, const FunctionDecl *Callee) {
  bool Success = true;
  llvm::SmallBitVector ForbiddenNullArgs;
  if (Callee->hasAttr<NonNullAttr>()) {
    ForbiddenNullArgs.resize(Args.size());
    for (const auto *Attr : Callee->specific_attrs<NonNullAttr>()) {
      if (!Attr->args_size()) {
        ForbiddenNullArgs.set();
        break;
      } else
        for (auto Idx : Attr->args()) {
          unsigned ASTIdx = Idx.getASTIndex();
          if (ASTIdx >= Args.size())
            continue;
          ForbiddenNullArgs[ASTIdx] = 1;
        }
    }
  }
  for (ArrayRef<const Expr*>::iterator I = Args.begin(), E = Args.end();
       I != E; ++I) {
    if (!Evaluate(ArgValues[I - Args.begin()], Info, *I)) {
      // If we're checking for a potential constant expression, evaluate all
      // initializers even if some of them fail.
      if (!Info.noteFailure())
        return false;
      Success = false;
    } else if (!ForbiddenNullArgs.empty() &&
               ForbiddenNullArgs[I - Args.begin()] &&
               ArgValues[I - Args.begin()].isNullPointer()) {
      Info.CCEDiag(*I, diag::note_non_null_attribute_failed);
      if (!Info.noteFailure())
        return false;
      Success = false;
    }
  }
  return Success;
}

/// Evaluate a function call.
static bool HandleFunctionCall(SourceLocation CallLoc,
                               const FunctionDecl *Callee, const LValue *This,
                               ArrayRef<const Expr*> Args, const Stmt *Body,
                               EvalInfo &Info, APValue &Result,
                               const LValue *ResultSlot) {
  ArgVector ArgValues(Args.size());
  if (!EvaluateArgs(Args, ArgValues, Info, Callee))
    return false;

  if (!Info.CheckCallLimit(CallLoc))
    return false;

  CallStackFrame Frame(Info, CallLoc, Callee, This, ArgValues.data());

  // For a trivial copy or move assignment, perform an APValue copy. This is
  // essential for unions, where the operations performed by the assignment
  // operator cannot be represented as statements.
  //
  // Skip this for non-union classes with no fields; in that case, the defaulted
  // copy/move does not actually read the object.
  const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(Callee);
  if (MD && MD->isDefaulted() &&
      (MD->getParent()->isUnion() ||
       (MD->isTrivial() && hasFields(MD->getParent())))) {
    assert(This &&
           (MD->isCopyAssignmentOperator() || MD->isMoveAssignmentOperator()));
    LValue RHS;
    RHS.setFrom(Info.Ctx, ArgValues[0]);
    APValue RHSValue;
    if (!handleLValueToRValueConversion(Info, Args[0], Args[0]->getType(),
                                        RHS, RHSValue))
      return false;
    if (Info.getLangOpts().CPlusPlus2a && MD->isTrivial() &&
        !HandleUnionActiveMemberChange(Info, Args[0], *This))
      return false;
    if (!handleAssignment(Info, Args[0], *This, MD->getThisType(),
                          RHSValue))
      return false;
    This->moveInto(Result);
    return true;
  } else if (MD && isLambdaCallOperator(MD)) {
    // We're in a lambda; determine the lambda capture field maps unless we're
    // just constexpr checking a lambda's call operator. constexpr checking is
    // done before the captures have been added to the closure object (unless
    // we're inferring constexpr-ness), so we don't have access to them in this
    // case. But since we don't need the captures to constexpr check, we can
    // just ignore them.
    if (!Info.checkingPotentialConstantExpression())
      MD->getParent()->getCaptureFields(Frame.LambdaCaptureFields,
                                        Frame.LambdaThisCaptureField);
  }

  StmtResult Ret = {Result, ResultSlot};
  EvalStmtResult ESR = EvaluateStmt(Ret, Info, Body);
  if (ESR == ESR_Succeeded) {
    if (Callee->getReturnType()->isVoidType())
      return true;
    Info.FFDiag(Callee->getEndLoc(), diag::note_constexpr_no_return);
  }
  return ESR == ESR_Returned;
}

/// Evaluate a constructor call.
static bool HandleConstructorCall(const Expr *E, const LValue &This,
                                  APValue *ArgValues,
                                  const CXXConstructorDecl *Definition,
                                  EvalInfo &Info, APValue &Result) {
  SourceLocation CallLoc = E->getExprLoc();
  if (!Info.CheckCallLimit(CallLoc))
    return false;

  const CXXRecordDecl *RD = Definition->getParent();
  if (RD->getNumVBases()) {
    Info.FFDiag(CallLoc, diag::note_constexpr_virtual_base) << RD;
    return false;
  }

  EvalInfo::EvaluatingConstructorRAII EvalObj(
      Info,
      ObjectUnderConstruction{This.getLValueBase(), This.Designator.Entries},
      RD->getNumBases());
  CallStackFrame Frame(Info, CallLoc, Definition, &This, ArgValues);

  // FIXME: Creating an APValue just to hold a nonexistent return value is
  // wasteful.
  APValue RetVal;
  StmtResult Ret = {RetVal, nullptr};

  // If it's a delegating constructor, delegate.
  if (Definition->isDelegatingConstructor()) {
    CXXConstructorDecl::init_const_iterator I = Definition->init_begin();
    {
      FullExpressionRAII InitScope(Info);
      if (!EvaluateInPlace(Result, Info, This, (*I)->getInit()))
        return false;
    }
    return EvaluateStmt(Ret, Info, Definition->getBody()) != ESR_Failed;
  }

  // For a trivial copy or move constructor, perform an APValue copy. This is
  // essential for unions (or classes with anonymous union members), where the
  // operations performed by the constructor cannot be represented by
  // ctor-initializers.
  //
  // Skip this for empty non-union classes; we should not perform an
  // lvalue-to-rvalue conversion on them because their copy constructor does not
  // actually read them.
  if (Definition->isDefaulted() && Definition->isCopyOrMoveConstructor() &&
      (Definition->getParent()->isUnion() ||
       (Definition->isTrivial() && hasFields(Definition->getParent())))) {
    LValue RHS;
    RHS.setFrom(Info.Ctx, ArgValues[0]);
    return handleLValueToRValueConversion(
        Info, E, Definition->getParamDecl(0)->getType().getNonReferenceType(),
        RHS, Result);
  }

  // Reserve space for the struct members.
  if (!RD->isUnion() && !Result.hasValue())
    Result = APValue(APValue::UninitStruct(), RD->getNumBases(),
                     std::distance(RD->field_begin(), RD->field_end()));

  if (RD->isInvalidDecl()) return false;
  const ASTRecordLayout &Layout = Info.Ctx.getASTRecordLayout(RD);

  // A scope for temporaries lifetime-extended by reference members.
  BlockScopeRAII LifetimeExtendedScope(Info);

  bool Success = true;
  unsigned BasesSeen = 0;
#ifndef NDEBUG
  CXXRecordDecl::base_class_const_iterator BaseIt = RD->bases_begin();
#endif
  for (const auto *I : Definition->inits()) {
    LValue Subobject = This;
    LValue SubobjectParent = This;
    APValue *Value = &Result;

    // Determine the subobject to initialize.
    FieldDecl *FD = nullptr;
    if (I->isBaseInitializer()) {
      QualType BaseType(I->getBaseClass(), 0);
#ifndef NDEBUG
      // Non-virtual base classes are initialized in the order in the class
      // definition. We have already checked for virtual base classes.
      assert(!BaseIt->isVirtual() && "virtual base for literal type");
      assert(Info.Ctx.hasSameType(BaseIt->getType(), BaseType) &&
             "base class initializers not in expected order");
      ++BaseIt;
#endif
      if (!HandleLValueDirectBase(Info, I->getInit(), Subobject, RD,
                                  BaseType->getAsCXXRecordDecl(), &Layout))
        return false;
      Value = &Result.getStructBase(BasesSeen++);
    } else if ((FD = I->getMember())) {
      if (!HandleLValueMember(Info, I->getInit(), Subobject, FD, &Layout))
        return false;
      if (RD->isUnion()) {
        Result = APValue(FD);
        Value = &Result.getUnionValue();
      } else {
        Value = &Result.getStructField(FD->getFieldIndex());
      }
    } else if (IndirectFieldDecl *IFD = I->getIndirectMember()) {
      // Walk the indirect field decl's chain to find the object to initialize,
      // and make sure we've initialized every step along it.
      auto IndirectFieldChain = IFD->chain();
      for (auto *C : IndirectFieldChain) {
        FD = cast<FieldDecl>(C);
        CXXRecordDecl *CD = cast<CXXRecordDecl>(FD->getParent());
        // Switch the union field if it differs. This happens if we had
        // preceding zero-initialization, and we're now initializing a union
        // subobject other than the first.
        // FIXME: In this case, the values of the other subobjects are
        // specified, since zero-initialization sets all padding bits to zero.
        if (!Value->hasValue() ||
            (Value->isUnion() && Value->getUnionField() != FD)) {
          if (CD->isUnion())
            *Value = APValue(FD);
          else
            *Value = APValue(APValue::UninitStruct(), CD->getNumBases(),
                             std::distance(CD->field_begin(), CD->field_end()));
        }
        // Store Subobject as its parent before updating it for the last element
        // in the chain.
        if (C == IndirectFieldChain.back())
          SubobjectParent = Subobject;
        if (!HandleLValueMember(Info, I->getInit(), Subobject, FD))
          return false;
        if (CD->isUnion())
          Value = &Value->getUnionValue();
        else
          Value = &Value->getStructField(FD->getFieldIndex());
      }
    } else {
      llvm_unreachable("unknown base initializer kind");
    }

    // Need to override This for implicit field initializers as in this case
    // This refers to innermost anonymous struct/union containing initializer,
    // not to currently constructed class.
    const Expr *Init = I->getInit();
    ThisOverrideRAII ThisOverride(*Info.CurrentCall, &SubobjectParent,
                                  isa<CXXDefaultInitExpr>(Init));
    FullExpressionRAII InitScope(Info);
    if (!EvaluateInPlace(*Value, Info, Subobject, Init) ||
        (FD && FD->isBitField() &&
         !truncateBitfieldValue(Info, Init, *Value, FD))) {
      // If we're checking for a potential constant expression, evaluate all
      // initializers even if some of them fail.
      if (!Info.noteFailure())
        return false;
      Success = false;
    }

    // This is the point at which the dynamic type of the object becomes this
    // class type.
    if (I->isBaseInitializer() && BasesSeen == RD->getNumBases())
      EvalObj.finishedConstructingBases();
  }

  return Success &&
         EvaluateStmt(Ret, Info, Definition->getBody()) != ESR_Failed;
}

static bool HandleConstructorCall(const Expr *E, const LValue &This,
                                  ArrayRef<const Expr*> Args,
                                  const CXXConstructorDecl *Definition,
                                  EvalInfo &Info, APValue &Result) {
  ArgVector ArgValues(Args.size());
  if (!EvaluateArgs(Args, ArgValues, Info, Definition))
    return false;

  return HandleConstructorCall(E, This, ArgValues.data(), Definition,
                               Info, Result);
}

//===----------------------------------------------------------------------===//
// Generic Evaluation
//===----------------------------------------------------------------------===//
namespace {

class BitCastBuffer {
  // FIXME: We're going to need bit-level granularity when we support
  // bit-fields.
  // FIXME: Its possible under the C++ standard for 'char' to not be 8 bits, but
  // we don't support a host or target where that is the case. Still, we should
  // use a more generic type in case we ever do.
  SmallVector<Optional<unsigned char>, 32> Bytes;

  static_assert(std::numeric_limits<unsigned char>::digits >= 8,
                "Need at least 8 bit unsigned char");

  bool TargetIsLittleEndian;

public:
  BitCastBuffer(CharUnits Width, bool TargetIsLittleEndian)
      : Bytes(Width.getQuantity()),
        TargetIsLittleEndian(TargetIsLittleEndian) {}

  LLVM_NODISCARD
  bool readObject(CharUnits Offset, CharUnits Width,
                  SmallVectorImpl<unsigned char> &Output) const {
    for (CharUnits I = Offset, E = Offset + Width; I != E; ++I) {
      // If a byte of an integer is uninitialized, then the whole integer is
      // uninitalized.
      if (!Bytes[I.getQuantity()])
        return false;
      Output.push_back(*Bytes[I.getQuantity()]);
    }
    if (llvm::sys::IsLittleEndianHost != TargetIsLittleEndian)
      std::reverse(Output.begin(), Output.end());
    return true;
  }

  void writeObject(CharUnits Offset, SmallVectorImpl<unsigned char> &Input) {
    if (llvm::sys::IsLittleEndianHost != TargetIsLittleEndian)
      std::reverse(Input.begin(), Input.end());

    size_t Index = 0;
    for (unsigned char Byte : Input) {
      assert(!Bytes[Offset.getQuantity() + Index] && "overwriting a byte?");
      Bytes[Offset.getQuantity() + Index] = Byte;
      ++Index;
    }
  }

  size_t size() { return Bytes.size(); }
};

/// Traverse an APValue to produce an BitCastBuffer, emulating how the current
/// target would represent the value at runtime.
class APValueToBufferConverter {
  EvalInfo &Info;
  BitCastBuffer Buffer;
  const CastExpr *BCE;

  APValueToBufferConverter(EvalInfo &Info, CharUnits ObjectWidth,
                           const CastExpr *BCE)
      : Info(Info),
        Buffer(ObjectWidth, Info.Ctx.getTargetInfo().isLittleEndian()),
        BCE(BCE) {}

  bool visit(const APValue &Val, QualType Ty) {
    return visit(Val, Ty, CharUnits::fromQuantity(0));
  }

  // Write out Val with type Ty into Buffer starting at Offset.
  bool visit(const APValue &Val, QualType Ty, CharUnits Offset) {
    assert((size_t)Offset.getQuantity() <= Buffer.size());

    // As a special case, nullptr_t has an indeterminate value.
    if (Ty->isNullPtrType())
      return true;

    // Dig through Src to find the byte at SrcOffset.
    switch (Val.getKind()) {
    case APValue::Indeterminate:
    case APValue::None:
      return true;

    case APValue::Int:
      return visitInt(Val.getInt(), Ty, Offset);
    case APValue::Float:
      return visitFloat(Val.getFloat(), Ty, Offset);
    case APValue::Array:
      return visitArray(Val, Ty, Offset);
    case APValue::Struct:
      return visitRecord(Val, Ty, Offset);

    case APValue::ComplexInt:
    case APValue::ComplexFloat:
    case APValue::Vector:
    case APValue::FixedPoint:
      // FIXME: We should support these.

    case APValue::Union:
    case APValue::MemberPointer:
    case APValue::AddrLabelDiff: {
      Info.FFDiag(BCE->getBeginLoc(),
                  diag::note_constexpr_bit_cast_unsupported_type)
          << Ty;
      return false;
    }

    case APValue::LValue:
      llvm_unreachable("LValue subobject in bit_cast?");
    }
    llvm_unreachable("Unhandled APValue::ValueKind");
  }

  bool visitRecord(const APValue &Val, QualType Ty, CharUnits Offset) {
    const RecordDecl *RD = Ty->getAsRecordDecl();
    const ASTRecordLayout &Layout = Info.Ctx.getASTRecordLayout(RD);

    // Visit the base classes.
    if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
      for (size_t I = 0, E = CXXRD->getNumBases(); I != E; ++I) {
        const CXXBaseSpecifier &BS = CXXRD->bases_begin()[I];
        CXXRecordDecl *BaseDecl = BS.getType()->getAsCXXRecordDecl();

        if (!visitRecord(Val.getStructBase(I), BS.getType(),
                         Layout.getBaseClassOffset(BaseDecl) + Offset))
          return false;
      }
    }

    // Visit the fields.
    unsigned FieldIdx = 0;
    for (FieldDecl *FD : RD->fields()) {
      if (FD->isBitField()) {
        Info.FFDiag(BCE->getBeginLoc(),
                    diag::note_constexpr_bit_cast_unsupported_bitfield);
        return false;
      }

      uint64_t FieldOffsetBits = Layout.getFieldOffset(FieldIdx);

      assert(FieldOffsetBits % Info.Ctx.getCharWidth() == 0 &&
             "only bit-fields can have sub-char alignment");
      CharUnits FieldOffset =
          Info.Ctx.toCharUnitsFromBits(FieldOffsetBits) + Offset;
      QualType FieldTy = FD->getType();
      if (!visit(Val.getStructField(FieldIdx), FieldTy, FieldOffset))
        return false;
      ++FieldIdx;
    }

    return true;
  }

  bool visitArray(const APValue &Val, QualType Ty, CharUnits Offset) {
    const auto *CAT =
        dyn_cast_or_null<ConstantArrayType>(Ty->getAsArrayTypeUnsafe());
    if (!CAT)
      return false;

    CharUnits ElemWidth = Info.Ctx.getTypeSizeInChars(CAT->getElementType());
    unsigned NumInitializedElts = Val.getArrayInitializedElts();
    unsigned ArraySize = Val.getArraySize();
    // First, initialize the initialized elements.
    for (unsigned I = 0; I != NumInitializedElts; ++I) {
      const APValue &SubObj = Val.getArrayInitializedElt(I);
      if (!visit(SubObj, CAT->getElementType(), Offset + I * ElemWidth))
        return false;
    }

    // Next, initialize the rest of the array using the filler.
    if (Val.hasArrayFiller()) {
      const APValue &Filler = Val.getArrayFiller();
      for (unsigned I = NumInitializedElts; I != ArraySize; ++I) {
        if (!visit(Filler, CAT->getElementType(), Offset + I * ElemWidth))
          return false;
      }
    }

    return true;
  }

  bool visitInt(const APSInt &Val, QualType Ty, CharUnits Offset) {
    CharUnits Width = Info.Ctx.getTypeSizeInChars(Ty);
    SmallVector<unsigned char, 8> Bytes(Width.getQuantity());
    llvm::StoreIntToMemory(Val, &*Bytes.begin(), Width.getQuantity());
    Buffer.writeObject(Offset, Bytes);
    return true;
  }

  bool visitFloat(const APFloat &Val, QualType Ty, CharUnits Offset) {
    APSInt AsInt(Val.bitcastToAPInt());
    return visitInt(AsInt, Ty, Offset);
  }

public:
  static Optional<BitCastBuffer> convert(EvalInfo &Info, const APValue &Src,
                                         const CastExpr *BCE) {
    CharUnits DstSize = Info.Ctx.getTypeSizeInChars(BCE->getType());
    APValueToBufferConverter Converter(Info, DstSize, BCE);
    if (!Converter.visit(Src, BCE->getSubExpr()->getType()))
      return None;
    return Converter.Buffer;
  }
};

/// Write an BitCastBuffer into an APValue.
class BufferToAPValueConverter {
  EvalInfo &Info;
  const BitCastBuffer &Buffer;
  const CastExpr *BCE;

  BufferToAPValueConverter(EvalInfo &Info, const BitCastBuffer &Buffer,
                           const CastExpr *BCE)
      : Info(Info), Buffer(Buffer), BCE(BCE) {}

  // Emit an unsupported bit_cast type error. Sema refuses to build a bit_cast
  // with an invalid type, so anything left is a deficiency on our part (FIXME).
  // Ideally this will be unreachable.
  llvm::NoneType unsupportedType(QualType Ty) {
    Info.FFDiag(BCE->getBeginLoc(),
                diag::note_constexpr_bit_cast_unsupported_type)
        << Ty;
    return None;
  }

  Optional<APValue> visit(const BuiltinType *T, CharUnits Offset,
                          const EnumType *EnumSugar = nullptr) {
    if (T->isNullPtrType()) {
      uint64_t NullValue = Info.Ctx.getTargetNullPointerValue(QualType(T, 0));
      return APValue((Expr *)nullptr,
                     /*Offset=*/CharUnits::fromQuantity(NullValue),
                     APValue::NoLValuePath{}, /*IsNullPtr=*/true);
    }

    CharUnits SizeOf = Info.Ctx.getTypeSizeInChars(T);
    SmallVector<uint8_t, 8> Bytes;
    if (!Buffer.readObject(Offset, SizeOf, Bytes)) {
      // If this is std::byte or unsigned char, then its okay to store an
      // indeterminate value.
      bool IsStdByte = EnumSugar && EnumSugar->isStdByteType();
      bool IsUChar =
          !EnumSugar && (T->isSpecificBuiltinType(BuiltinType::UChar) ||
                         T->isSpecificBuiltinType(BuiltinType::Char_U));
      if (!IsStdByte && !IsUChar) {
        QualType DisplayType(EnumSugar ? (const Type *)EnumSugar : T, 0);
        Info.FFDiag(BCE->getExprLoc(),
                    diag::note_constexpr_bit_cast_indet_dest)
            << DisplayType << Info.Ctx.getLangOpts().CharIsSigned;
        return None;
      }

      return APValue::IndeterminateValue();
    }

    APSInt Val(SizeOf.getQuantity() * Info.Ctx.getCharWidth(), true);
    llvm::LoadIntFromMemory(Val, &*Bytes.begin(), Bytes.size());

    if (T->isIntegralOrEnumerationType()) {
      Val.setIsSigned(T->isSignedIntegerOrEnumerationType());
      return APValue(Val);
    }

    if (T->isRealFloatingType()) {
      const llvm::fltSemantics &Semantics =
          Info.Ctx.getFloatTypeSemantics(QualType(T, 0));
      return APValue(APFloat(Semantics, Val));
    }

    return unsupportedType(QualType(T, 0));
  }

  Optional<APValue> visit(const RecordType *RTy, CharUnits Offset) {
    const RecordDecl *RD = RTy->getAsRecordDecl();
    const ASTRecordLayout &Layout = Info.Ctx.getASTRecordLayout(RD);

    unsigned NumBases = 0;
    if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD))
      NumBases = CXXRD->getNumBases();

    APValue ResultVal(APValue::UninitStruct(), NumBases,
                      std::distance(RD->field_begin(), RD->field_end()));

    // Visit the base classes.
    if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
      for (size_t I = 0, E = CXXRD->getNumBases(); I != E; ++I) {
        const CXXBaseSpecifier &BS = CXXRD->bases_begin()[I];
        CXXRecordDecl *BaseDecl = BS.getType()->getAsCXXRecordDecl();
        if (BaseDecl->isEmpty() ||
            Info.Ctx.getASTRecordLayout(BaseDecl).getNonVirtualSize().isZero())
          continue;

        Optional<APValue> SubObj = visitType(
            BS.getType(), Layout.getBaseClassOffset(BaseDecl) + Offset);
        if (!SubObj)
          return None;
        ResultVal.getStructBase(I) = *SubObj;
      }
    }

    // Visit the fields.
    unsigned FieldIdx = 0;
    for (FieldDecl *FD : RD->fields()) {
      // FIXME: We don't currently support bit-fields. A lot of the logic for
      // this is in CodeGen, so we need to factor it around.
      if (FD->isBitField()) {
        Info.FFDiag(BCE->getBeginLoc(),
                    diag::note_constexpr_bit_cast_unsupported_bitfield);
        return None;
      }

      uint64_t FieldOffsetBits = Layout.getFieldOffset(FieldIdx);
      assert(FieldOffsetBits % Info.Ctx.getCharWidth() == 0);

      CharUnits FieldOffset =
          CharUnits::fromQuantity(FieldOffsetBits / Info.Ctx.getCharWidth()) +
          Offset;
      QualType FieldTy = FD->getType();
      Optional<APValue> SubObj = visitType(FieldTy, FieldOffset);
      if (!SubObj)
        return None;
      ResultVal.getStructField(FieldIdx) = *SubObj;
      ++FieldIdx;
    }

    return ResultVal;
  }

  Optional<APValue> visit(const EnumType *Ty, CharUnits Offset) {
    QualType RepresentationType = Ty->getDecl()->getIntegerType();
    assert(!RepresentationType.isNull() &&
           "enum forward decl should be caught by Sema");
    const BuiltinType *AsBuiltin =
        RepresentationType.getCanonicalType()->getAs<BuiltinType>();
    assert(AsBuiltin && "non-integral enum underlying type?");
    // Recurse into the underlying type. Treat std::byte transparently as
    // unsigned char.
    return visit(AsBuiltin, Offset, /*EnumTy=*/Ty);
  }

  Optional<APValue> visit(const ConstantArrayType *Ty, CharUnits Offset) {
    size_t Size = Ty->getSize().getLimitedValue();
    CharUnits ElementWidth = Info.Ctx.getTypeSizeInChars(Ty->getElementType());

    APValue ArrayValue(APValue::UninitArray(), Size, Size);
    for (size_t I = 0; I != Size; ++I) {
      Optional<APValue> ElementValue =
          visitType(Ty->getElementType(), Offset + I * ElementWidth);
      if (!ElementValue)
        return None;
      ArrayValue.getArrayInitializedElt(I) = std::move(*ElementValue);
    }

    return ArrayValue;
  }

  Optional<APValue> visit(const Type *Ty, CharUnits Offset) {
    return unsupportedType(QualType(Ty, 0));
  }

  Optional<APValue> visitType(QualType Ty, CharUnits Offset) {
    QualType Can = Ty.getCanonicalType();

    switch (Can->getTypeClass()) {
#define TYPE(Class, Base)                                                      \
  case Type::Class:                                                            \
    return visit(cast<Class##Type>(Can.getTypePtr()), Offset);
#define ABSTRACT_TYPE(Class, Base)
#define NON_CANONICAL_TYPE(Class, Base)                                        \
  case Type::Class:                                                            \
    llvm_unreachable("non-canonical type should be impossible!");
#define DEPENDENT_TYPE(Class, Base)                                            \
  case Type::Class:                                                            \
    llvm_unreachable(                                                          \
        "dependent types aren't supported in the constant evaluator!");
#define NON_CANONICAL_UNLESS_DEPENDENT(Class, Base)                            \
  case Type::Class:                                                            \
    llvm_unreachable("either dependent or not canonical!");
#include "clang/AST/TypeNodes.def"
    }
    llvm_unreachable("Unhandled Type::TypeClass");
  }

public:
  // Pull out a full value of type DstType.
  static Optional<APValue> convert(EvalInfo &Info, BitCastBuffer &Buffer,
                                   const CastExpr *BCE) {
    BufferToAPValueConverter Converter(Info, Buffer, BCE);
    return Converter.visitType(BCE->getType(), CharUnits::fromQuantity(0));
  }
};

static bool checkBitCastConstexprEligibilityType(SourceLocation Loc,
                                                 QualType Ty, EvalInfo *Info,
                                                 const ASTContext &Ctx,
                                                 bool CheckingDest) {
  Ty = Ty.getCanonicalType();

  auto diag = [&](int Reason) {
    if (Info)
      Info->FFDiag(Loc, diag::note_constexpr_bit_cast_invalid_type)
          << CheckingDest << (Reason == 4) << Reason;
    return false;
  };
  auto note = [&](int Construct, QualType NoteTy, SourceLocation NoteLoc) {
    if (Info)
      Info->Note(NoteLoc, diag::note_constexpr_bit_cast_invalid_subtype)
          << NoteTy << Construct << Ty;
    return false;
  };

  if (Ty->isUnionType())
    return diag(0);
  if (Ty->isPointerType())
    return diag(1);
  if (Ty->isMemberPointerType())
    return diag(2);
  if (Ty.isVolatileQualified())
    return diag(3);

  if (RecordDecl *Record = Ty->getAsRecordDecl()) {
    if (auto *CXXRD = dyn_cast<CXXRecordDecl>(Record)) {
      for (CXXBaseSpecifier &BS : CXXRD->bases())
        if (!checkBitCastConstexprEligibilityType(Loc, BS.getType(), Info, Ctx,
                                                  CheckingDest))
          return note(1, BS.getType(), BS.getBeginLoc());
    }
    for (FieldDecl *FD : Record->fields()) {
      if (FD->getType()->isReferenceType())
        return diag(4);
      if (!checkBitCastConstexprEligibilityType(Loc, FD->getType(), Info, Ctx,
                                                CheckingDest))
        return note(0, FD->getType(), FD->getBeginLoc());
    }
  }

  if (Ty->isArrayType() &&
      !checkBitCastConstexprEligibilityType(Loc, Ctx.getBaseElementType(Ty),
                                            Info, Ctx, CheckingDest))
    return false;

  return true;
}

static bool checkBitCastConstexprEligibility(EvalInfo *Info,
                                             const ASTContext &Ctx,
                                             const CastExpr *BCE) {
  bool DestOK = checkBitCastConstexprEligibilityType(
      BCE->getBeginLoc(), BCE->getType(), Info, Ctx, true);
  bool SourceOK = DestOK && checkBitCastConstexprEligibilityType(
                                BCE->getBeginLoc(),
                                BCE->getSubExpr()->getType(), Info, Ctx, false);
  return SourceOK;
}

static bool handleLValueToRValueBitCast(EvalInfo &Info, APValue &DestValue,
                                        APValue &SourceValue,
                                        const CastExpr *BCE) {
  assert(CHAR_BIT == 8 && Info.Ctx.getTargetInfo().getCharWidth() == 8 &&
         "no host or target supports non 8-bit chars");
  assert(SourceValue.isLValue() &&
         "LValueToRValueBitcast requires an lvalue operand!");

  if (!checkBitCastConstexprEligibility(&Info, Info.Ctx, BCE))
    return false;

  LValue SourceLValue;
  APValue SourceRValue;
  SourceLValue.setFrom(Info.Ctx, SourceValue);
  if (!handleLValueToRValueConversion(Info, BCE,
                                      BCE->getSubExpr()->getType().withConst(),
                                      SourceLValue, SourceRValue))
    return false;

  // Read out SourceValue into a char buffer.
  Optional<BitCastBuffer> Buffer =
      APValueToBufferConverter::convert(Info, SourceRValue, BCE);
  if (!Buffer)
    return false;

  // Write out the buffer into a new APValue.
  Optional<APValue> MaybeDestValue =
      BufferToAPValueConverter::convert(Info, *Buffer, BCE);
  if (!MaybeDestValue)
    return false;

  DestValue = std::move(*MaybeDestValue);
  return true;
}

template <class Derived>
class ExprEvaluatorBase
  : public ConstStmtVisitor<Derived, bool> {
private:
  Derived &getDerived() { return static_cast<Derived&>(*this); }
  bool DerivedSuccess(const APValue &V, const Expr *E) {
    return getDerived().Success(V, E);
  }
  bool DerivedZeroInitialization(const Expr *E) {
    return getDerived().ZeroInitialization(E);
  }

  // Check whether a conditional operator with a non-constant condition is a
  // potential constant expression. If neither arm is a potential constant
  // expression, then the conditional operator is not either.
  template<typename ConditionalOperator>
  void CheckPotentialConstantConditional(const ConditionalOperator *E) {
    assert(Info.checkingPotentialConstantExpression());

    // Speculatively evaluate both arms.
    SmallVector<PartialDiagnosticAt, 8> Diag;
    {
      SpeculativeEvaluationRAII Speculate(Info, &Diag);
      StmtVisitorTy::Visit(E->getFalseExpr());
      if (Diag.empty())
        return;
    }

    {
      SpeculativeEvaluationRAII Speculate(Info, &Diag);
      Diag.clear();
      StmtVisitorTy::Visit(E->getTrueExpr());
      if (Diag.empty())
        return;
    }

    Error(E, diag::note_constexpr_conditional_never_const);
  }


  template<typename ConditionalOperator>
  bool HandleConditionalOperator(const ConditionalOperator *E) {
    bool BoolResult;
    if (!EvaluateAsBooleanCondition(E->getCond(), BoolResult, Info)) {
      if (Info.checkingPotentialConstantExpression() && Info.noteFailure()) {
        CheckPotentialConstantConditional(E);
        return false;
      }
      if (Info.noteFailure()) {
        StmtVisitorTy::Visit(E->getTrueExpr());
        StmtVisitorTy::Visit(E->getFalseExpr());
      }
      return false;
    }

    Expr *EvalExpr = BoolResult ? E->getTrueExpr() : E->getFalseExpr();
    return StmtVisitorTy::Visit(EvalExpr);
  }

protected:
  EvalInfo &Info;
  typedef ConstStmtVisitor<Derived, bool> StmtVisitorTy;
  typedef ExprEvaluatorBase ExprEvaluatorBaseTy;

  OptionalDiagnostic CCEDiag(const Expr *E, diag::kind D) {
    return Info.CCEDiag(E, D);
  }

  bool ZeroInitialization(const Expr *E) { return Error(E); }

public:
  ExprEvaluatorBase(EvalInfo &Info) : Info(Info) {}

  EvalInfo &getEvalInfo() { return Info; }

  /// Report an evaluation error. This should only be called when an error is
  /// first discovered. When propagating an error, just return false.
  bool Error(const Expr *E, diag::kind D) {
    Info.FFDiag(E, D);
    return false;
  }
  bool Error(const Expr *E) {
    return Error(E, diag::note_invalid_subexpr_in_const_expr);
  }

  bool VisitStmt(const Stmt *) {
    llvm_unreachable("Expression evaluator should not be called on stmts");
  }
  bool VisitExpr(const Expr *E) {
    return Error(E);
  }

  bool VisitConstantExpr(const ConstantExpr *E)
    { return StmtVisitorTy::Visit(E->getSubExpr()); }
  bool VisitParenExpr(const ParenExpr *E)
    { return StmtVisitorTy::Visit(E->getSubExpr()); }
  bool VisitUnaryExtension(const UnaryOperator *E)
    { return StmtVisitorTy::Visit(E->getSubExpr()); }
  bool VisitUnaryPlus(const UnaryOperator *E)
    { return StmtVisitorTy::Visit(E->getSubExpr()); }
  bool VisitChooseExpr(const ChooseExpr *E)
    { return StmtVisitorTy::Visit(E->getChosenSubExpr()); }
  bool VisitGenericSelectionExpr(const GenericSelectionExpr *E)
    { return StmtVisitorTy::Visit(E->getResultExpr()); }
  bool VisitSubstNonTypeTemplateParmExpr(const SubstNonTypeTemplateParmExpr *E)
    { return StmtVisitorTy::Visit(E->getReplacement()); }
  bool VisitCXXDefaultArgExpr(const CXXDefaultArgExpr *E) {
    TempVersionRAII RAII(*Info.CurrentCall);
    SourceLocExprScopeGuard Guard(E, Info.CurrentCall->CurSourceLocExprScope);
    return StmtVisitorTy::Visit(E->getExpr());
  }
  bool VisitCXXDefaultInitExpr(const CXXDefaultInitExpr *E) {
    TempVersionRAII RAII(*Info.CurrentCall);
    // The initializer may not have been parsed yet, or might be erroneous.
    if (!E->getExpr())
      return Error(E);
    SourceLocExprScopeGuard Guard(E, Info.CurrentCall->CurSourceLocExprScope);
    return StmtVisitorTy::Visit(E->getExpr());
  }

  // We cannot create any objects for which cleanups are required, so there is
  // nothing to do here; all cleanups must come from unevaluated subexpressions.