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InterpolatingCompilationDatabase.cpp
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1 //===- InterpolatingCompilationDatabase.cpp ---------------------*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // InterpolatingCompilationDatabase wraps another CompilationDatabase and
10 // attempts to heuristically determine appropriate compile commands for files
11 // that are not included, such as headers or newly created files.
12 //
13 // Motivating cases include:
14 // Header files that live next to their implementation files. These typically
15 // share a base filename. (libclang/CXString.h, libclang/CXString.cpp).
16 // Some projects separate headers from includes. Filenames still typically
17 // match, maybe other path segments too. (include/llvm/IR/Use.h, lib/IR/Use.cc).
18 // Matches are sometimes only approximate (Sema.h, SemaDecl.cpp). This goes
19 // for directories too (Support/Unix/Process.inc, lib/Support/Process.cpp).
20 // Even if we can't find a "right" compile command, even a random one from
21 // the project will tend to get important flags like -I and -x right.
22 //
23 // We "borrow" the compile command for the closest available file:
24 // - points are awarded if the filename matches (ignoring extension)
25 // - points are awarded if the directory structure matches
26 // - ties are broken by length of path prefix match
27 //
28 // The compile command is adjusted, replacing the filename and removing output
29 // file arguments. The -x and -std flags may be affected too.
30 //
31 // Source language is a tricky issue: is it OK to use a .c file's command
32 // for building a .cc file? What language is a .h file in?
33 // - We only consider compile commands for c-family languages as candidates.
34 // - For files whose language is implied by the filename (e.g. .m, .hpp)
35 // we prefer candidates from the same language.
36 // If we must cross languages, we drop any -x and -std flags.
37 // - For .h files, candidates from any c-family language are acceptable.
38 // We use the candidate's language, inserting e.g. -x c++-header.
39 //
40 // This class is only useful when wrapping databases that can enumerate all
41 // their compile commands. If getAllFilenames() is empty, no inference occurs.
42 //
43 //===----------------------------------------------------------------------===//
44 
46 #include "clang/Driver/Options.h"
47 #include "clang/Driver/Types.h"
49 #include "llvm/ADT/DenseMap.h"
50 #include "llvm/ADT/Optional.h"
51 #include "llvm/ADT/StringExtras.h"
52 #include "llvm/ADT/StringSwitch.h"
53 #include "llvm/Option/ArgList.h"
54 #include "llvm/Option/OptTable.h"
55 #include "llvm/Support/Debug.h"
56 #include "llvm/Support/Path.h"
57 #include "llvm/Support/StringSaver.h"
58 #include "llvm/Support/raw_ostream.h"
59 #include <memory>
60 
61 namespace clang {
62 namespace tooling {
63 namespace {
64 using namespace llvm;
65 namespace types = clang::driver::types;
66 namespace path = llvm::sys::path;
67 
68 // The length of the prefix these two strings have in common.
69 size_t matchingPrefix(StringRef L, StringRef R) {
70  size_t Limit = std::min(L.size(), R.size());
71  for (size_t I = 0; I < Limit; ++I)
72  if (L[I] != R[I])
73  return I;
74  return Limit;
75 }
76 
77 // A comparator for searching SubstringWithIndexes with std::equal_range etc.
78 // Optionaly prefix semantics: compares equal if the key is a prefix.
79 template <bool Prefix> struct Less {
80  bool operator()(StringRef Key, std::pair<StringRef, size_t> Value) const {
81  StringRef V = Prefix ? Value.first.substr(0, Key.size()) : Value.first;
82  return Key < V;
83  }
84  bool operator()(std::pair<StringRef, size_t> Value, StringRef Key) const {
85  StringRef V = Prefix ? Value.first.substr(0, Key.size()) : Value.first;
86  return V < Key;
87  }
88 };
89 
90 // Infer type from filename. If we might have gotten it wrong, set *Certain.
91 // *.h will be inferred as a C header, but not certain.
92 types::ID guessType(StringRef Filename, bool *Certain = nullptr) {
93  // path::extension is ".cpp", lookupTypeForExtension wants "cpp".
94  auto Lang =
95  types::lookupTypeForExtension(path::extension(Filename).substr(1));
96  if (Certain)
97  *Certain = Lang != types::TY_CHeader && Lang != types::TY_INVALID;
98  return Lang;
99 }
100 
101 // Return Lang as one of the canonical supported types.
102 // e.g. c-header --> c; fortran --> TY_INVALID
103 static types::ID foldType(types::ID Lang) {
104  switch (Lang) {
105  case types::TY_C:
106  case types::TY_CHeader:
107  return types::TY_C;
108  case types::TY_ObjC:
109  case types::TY_ObjCHeader:
110  return types::TY_ObjC;
111  case types::TY_CXX:
112  case types::TY_CXXHeader:
113  return types::TY_CXX;
114  case types::TY_ObjCXX:
115  case types::TY_ObjCXXHeader:
116  return types::TY_ObjCXX;
117  default:
118  return types::TY_INVALID;
119  }
120 }
121 
122 // A CompileCommand that can be applied to another file.
123 struct TransferableCommand {
124  // Flags that should not apply to all files are stripped from CommandLine.
125  CompileCommand Cmd;
126  // Language detected from -x or the filename. Never TY_INVALID.
128  // Standard specified by -std.
130  // Whether the command line is for the cl-compatible driver.
132 
133  TransferableCommand(CompileCommand C)
134  : Cmd(std::move(C)), Type(guessType(Cmd.Filename)),
135  ClangCLMode(checkIsCLMode(Cmd.CommandLine)) {
136  std::vector<std::string> OldArgs = std::move(Cmd.CommandLine);
137  Cmd.CommandLine.clear();
138 
139  // Wrap the old arguments in an InputArgList.
140  llvm::opt::InputArgList ArgList;
141  {
143  for (const std::string &S : OldArgs)
144  TmpArgv.push_back(S.c_str());
145  ArgList = {TmpArgv.begin(), TmpArgv.end()};
146  }
147 
148  // Parse the old args in order to strip out and record unwanted flags.
149  // We parse each argument individually so that we can retain the exact
150  // spelling of each argument; re-rendering is lossy for aliased flags.
151  // E.g. in CL mode, /W4 maps to -Wall.
152  auto &OptTable = clang::driver::getDriverOptTable();
153  if (!OldArgs.empty())
154  Cmd.CommandLine.emplace_back(OldArgs.front());
155  for (unsigned Pos = 1; Pos < OldArgs.size();) {
156  using namespace driver::options;
157 
158  const unsigned OldPos = Pos;
159  std::unique_ptr<llvm::opt::Arg> Arg(OptTable.ParseOneArg(
160  ArgList, Pos,
161  /* Include */ ClangCLMode ? CoreOption | CLOption : 0,
162  /* Exclude */ ClangCLMode ? 0 : CLOption));
163 
164  if (!Arg)
165  continue;
166 
167  const llvm::opt::Option &Opt = Arg->getOption();
168 
169  // Strip input and output files.
170  if (Opt.matches(OPT_INPUT) || Opt.matches(OPT_o) ||
171  (ClangCLMode && (Opt.matches(OPT__SLASH_Fa) ||
172  Opt.matches(OPT__SLASH_Fe) ||
173  Opt.matches(OPT__SLASH_Fi) ||
174  Opt.matches(OPT__SLASH_Fo))))
175  continue;
176 
177  // Strip -x, but record the overridden language.
178  if (const auto GivenType = tryParseTypeArg(*Arg)) {
179  Type = *GivenType;
180  continue;
181  }
182 
183  // Strip -std, but record the value.
184  if (const auto GivenStd = tryParseStdArg(*Arg)) {
185  if (*GivenStd != LangStandard::lang_unspecified)
186  Std = *GivenStd;
187  continue;
188  }
189 
190  Cmd.CommandLine.insert(Cmd.CommandLine.end(),
191  OldArgs.data() + OldPos, OldArgs.data() + Pos);
192  }
193 
194  if (Std != LangStandard::lang_unspecified) // -std take precedence over -x
195  Type = toType(LangStandard::getLangStandardForKind(Std).getLanguage());
196  Type = foldType(*Type);
197  // The contract is to store None instead of TY_INVALID.
198  if (Type == types::TY_INVALID)
199  Type = llvm::None;
200  }
201 
202  // Produce a CompileCommand for \p filename, based on this one.
203  CompileCommand transferTo(StringRef Filename) const {
204  CompileCommand Result = Cmd;
205  Result.Filename = Filename;
206  bool TypeCertain;
207  auto TargetType = guessType(Filename, &TypeCertain);
208  // If the filename doesn't determine the language (.h), transfer with -x.
209  if ((!TargetType || !TypeCertain) && Type) {
210  // Use *Type, or its header variant if the file is a header.
211  // Treat no/invalid extension as header (e.g. C++ standard library).
212  TargetType =
213  (!TargetType || types::onlyPrecompileType(TargetType)) // header?
215  : *Type;
216  if (ClangCLMode) {
217  const StringRef Flag = toCLFlag(TargetType);
218  if (!Flag.empty())
219  Result.CommandLine.push_back(Flag);
220  } else {
221  Result.CommandLine.push_back("-x");
222  Result.CommandLine.push_back(types::getTypeName(TargetType));
223  }
224  }
225  // --std flag may only be transferred if the language is the same.
226  // We may consider "translating" these, e.g. c++11 -> c11.
227  if (Std != LangStandard::lang_unspecified && foldType(TargetType) == Type) {
228  Result.CommandLine.emplace_back((
229  llvm::Twine(ClangCLMode ? "/std:" : "-std=") +
231  }
232  Result.CommandLine.push_back(Filename);
233  Result.Heuristic = "inferred from " + Cmd.Filename;
234  return Result;
235  }
236 
237 private:
238  // Determine whether the given command line is intended for the CL driver.
239  static bool checkIsCLMode(ArrayRef<std::string> CmdLine) {
240  // First look for --driver-mode.
241  for (StringRef S : llvm::reverse(CmdLine)) {
242  if (S.consume_front("--driver-mode="))
243  return S == "cl";
244  }
245 
246  // Otherwise just check the clang executable file name.
247  return !CmdLine.empty() &&
248  llvm::sys::path::stem(CmdLine.front()).endswith_lower("cl");
249  }
250 
251  // Map the language from the --std flag to that of the -x flag.
252  static types::ID toType(Language Lang) {
253  switch (Lang) {
254  case Language::C:
255  return types::TY_C;
256  case Language::CXX:
257  return types::TY_CXX;
258  case Language::ObjC:
259  return types::TY_ObjC;
260  case Language::ObjCXX:
261  return types::TY_ObjCXX;
262  default:
263  return types::TY_INVALID;
264  }
265  }
266 
267  // Convert a file type to the matching CL-style type flag.
268  static StringRef toCLFlag(types::ID Type) {
269  switch (Type) {
270  case types::TY_C:
271  case types::TY_CHeader:
272  return "/TC";
273  case types::TY_CXX:
274  case types::TY_CXXHeader:
275  return "/TP";
276  default:
277  return StringRef();
278  }
279  }
280 
281  // Try to interpret the argument as a type specifier, e.g. '-x'.
282  Optional<types::ID> tryParseTypeArg(const llvm::opt::Arg &Arg) {
283  const llvm::opt::Option &Opt = Arg.getOption();
284  using namespace driver::options;
285  if (ClangCLMode) {
286  if (Opt.matches(OPT__SLASH_TC) || Opt.matches(OPT__SLASH_Tc))
287  return types::TY_C;
288  if (Opt.matches(OPT__SLASH_TP) || Opt.matches(OPT__SLASH_Tp))
289  return types::TY_CXX;
290  } else {
291  if (Opt.matches(driver::options::OPT_x))
292  return types::lookupTypeForTypeSpecifier(Arg.getValue());
293  }
294  return None;
295  }
296 
297  // Try to interpret the argument as '-std='.
298  Optional<LangStandard::Kind> tryParseStdArg(const llvm::opt::Arg &Arg) {
299  using namespace driver::options;
300  if (Arg.getOption().matches(ClangCLMode ? OPT__SLASH_std : OPT_std_EQ))
301  return LangStandard::getLangKind(Arg.getValue());
302  return None;
303  }
304 };
305 
306 // Given a filename, FileIndex picks the best matching file from the underlying
307 // DB. This is the proxy file whose CompileCommand will be reused. The
308 // heuristics incorporate file name, extension, and directory structure.
309 // Strategy:
310 // - Build indexes of each of the substrings we want to look up by.
311 // These indexes are just sorted lists of the substrings.
312 // - Each criterion corresponds to a range lookup into the index, so we only
313 // need O(log N) string comparisons to determine scores.
314 //
315 // Apart from path proximity signals, also takes file extensions into account
316 // when scoring the candidates.
317 class FileIndex {
318 public:
319  FileIndex(std::vector<std::string> Files)
320  : OriginalPaths(std::move(Files)), Strings(Arena) {
321  // Sort commands by filename for determinism (index is a tiebreaker later).
322  llvm::sort(OriginalPaths);
323  Paths.reserve(OriginalPaths.size());
324  Types.reserve(OriginalPaths.size());
325  Stems.reserve(OriginalPaths.size());
326  for (size_t I = 0; I < OriginalPaths.size(); ++I) {
327  StringRef Path = Strings.save(StringRef(OriginalPaths[I]).lower());
328 
329  Paths.emplace_back(Path, I);
330  Types.push_back(foldType(guessType(Path)));
331  Stems.emplace_back(sys::path::stem(Path), I);
332  auto Dir = ++sys::path::rbegin(Path), DirEnd = sys::path::rend(Path);
333  for (int J = 0; J < DirectorySegmentsIndexed && Dir != DirEnd; ++J, ++Dir)
334  if (Dir->size() > ShortDirectorySegment) // not trivial ones
335  Components.emplace_back(*Dir, I);
336  }
337  llvm::sort(Paths);
338  llvm::sort(Stems);
339  llvm::sort(Components);
340  }
341 
342  bool empty() const { return Paths.empty(); }
343 
344  // Returns the path for the file that best fits OriginalFilename.
345  // Candidates with extensions matching PreferLanguage will be chosen over
346  // others (unless it's TY_INVALID, or all candidates are bad).
347  StringRef chooseProxy(StringRef OriginalFilename,
348  types::ID PreferLanguage) const {
349  assert(!empty() && "need at least one candidate!");
350  std::string Filename = OriginalFilename.lower();
351  auto Candidates = scoreCandidates(Filename);
352  std::pair<size_t, int> Best =
353  pickWinner(Candidates, Filename, PreferLanguage);
354 
355  DEBUG_WITH_TYPE(
356  "interpolate",
357  llvm::dbgs() << "interpolate: chose " << OriginalPaths[Best.first]
358  << " as proxy for " << OriginalFilename << " preferring "
359  << (PreferLanguage == types::TY_INVALID
360  ? "none"
361  : types::getTypeName(PreferLanguage))
362  << " score=" << Best.second << "\n");
363  return OriginalPaths[Best.first];
364  }
365 
366 private:
367  using SubstringAndIndex = std::pair<StringRef, size_t>;
368  // Directory matching parameters: we look at the last two segments of the
369  // parent directory (usually the semantically significant ones in practice).
370  // We search only the last four of each candidate (for efficiency).
371  constexpr static int DirectorySegmentsIndexed = 4;
372  constexpr static int DirectorySegmentsQueried = 2;
373  constexpr static int ShortDirectorySegment = 1; // Only look at longer names.
374 
375  // Award points to candidate entries that should be considered for the file.
376  // Returned keys are indexes into paths, and the values are (nonzero) scores.
377  DenseMap<size_t, int> scoreCandidates(StringRef Filename) const {
378  // Decompose Filename into the parts we care about.
379  // /some/path/complicated/project/Interesting.h
380  // [-prefix--][---dir---] [-dir-] [--stem---]
381  StringRef Stem = sys::path::stem(Filename);
383  llvm::StringRef Prefix;
384  auto Dir = ++sys::path::rbegin(Filename),
385  DirEnd = sys::path::rend(Filename);
386  for (int I = 0; I < DirectorySegmentsQueried && Dir != DirEnd; ++I, ++Dir) {
387  if (Dir->size() > ShortDirectorySegment)
388  Dirs.push_back(*Dir);
389  Prefix = Filename.substr(0, Dir - DirEnd);
390  }
391 
392  // Now award points based on lookups into our various indexes.
393  DenseMap<size_t, int> Candidates; // Index -> score.
394  auto Award = [&](int Points, ArrayRef<SubstringAndIndex> Range) {
395  for (const auto &Entry : Range)
396  Candidates[Entry.second] += Points;
397  };
398  // Award one point if the file's basename is a prefix of the candidate,
399  // and another if it's an exact match (so exact matches get two points).
400  Award(1, indexLookup</*Prefix=*/true>(Stem, Stems));
401  Award(1, indexLookup</*Prefix=*/false>(Stem, Stems));
402  // For each of the last few directories in the Filename, award a point
403  // if it's present in the candidate.
404  for (StringRef Dir : Dirs)
405  Award(1, indexLookup</*Prefix=*/false>(Dir, Components));
406  // Award one more point if the whole rest of the path matches.
407  if (sys::path::root_directory(Prefix) != Prefix)
408  Award(1, indexLookup</*Prefix=*/true>(Prefix, Paths));
409  return Candidates;
410  }
411 
412  // Pick a single winner from the set of scored candidates.
413  // Returns (index, score).
414  std::pair<size_t, int> pickWinner(const DenseMap<size_t, int> &Candidates,
415  StringRef Filename,
416  types::ID PreferredLanguage) const {
417  struct ScoredCandidate {
418  size_t Index;
419  bool Preferred;
420  int Points;
421  size_t PrefixLength;
422  };
423  // Choose the best candidate by (preferred, points, prefix length, alpha).
424  ScoredCandidate Best = {size_t(-1), false, 0, 0};
425  for (const auto &Candidate : Candidates) {
426  ScoredCandidate S;
427  S.Index = Candidate.first;
428  S.Preferred = PreferredLanguage == types::TY_INVALID ||
429  PreferredLanguage == Types[S.Index];
430  S.Points = Candidate.second;
431  if (!S.Preferred && Best.Preferred)
432  continue;
433  if (S.Preferred == Best.Preferred) {
434  if (S.Points < Best.Points)
435  continue;
436  if (S.Points == Best.Points) {
437  S.PrefixLength = matchingPrefix(Filename, Paths[S.Index].first);
438  if (S.PrefixLength < Best.PrefixLength)
439  continue;
440  // hidden heuristics should at least be deterministic!
441  if (S.PrefixLength == Best.PrefixLength)
442  if (S.Index > Best.Index)
443  continue;
444  }
445  }
446  // PrefixLength was only set above if actually needed for a tiebreak.
447  // But it definitely needs to be set to break ties in the future.
448  S.PrefixLength = matchingPrefix(Filename, Paths[S.Index].first);
449  Best = S;
450  }
451  // Edge case: no candidate got any points.
452  // We ignore PreferredLanguage at this point (not ideal).
453  if (Best.Index == size_t(-1))
454  return {longestMatch(Filename, Paths).second, 0};
455  return {Best.Index, Best.Points};
456  }
457 
458  // Returns the range within a sorted index that compares equal to Key.
459  // If Prefix is true, it's instead the range starting with Key.
460  template <bool Prefix>
462  indexLookup(StringRef Key, ArrayRef<SubstringAndIndex> Idx) const {
463  // Use pointers as iteratiors to ease conversion of result to ArrayRef.
464  auto Range = std::equal_range(Idx.data(), Idx.data() + Idx.size(), Key,
465  Less<Prefix>());
466  return {Range.first, Range.second};
467  }
468 
469  // Performs a point lookup into a nonempty index, returning a longest match.
470  SubstringAndIndex longestMatch(StringRef Key,
471  ArrayRef<SubstringAndIndex> Idx) const {
472  assert(!Idx.empty());
473  // Longest substring match will be adjacent to a direct lookup.
474  auto It = llvm::lower_bound(Idx, SubstringAndIndex{Key, 0});
475  if (It == Idx.begin())
476  return *It;
477  if (It == Idx.end())
478  return *--It;
479  // Have to choose between It and It-1
480  size_t Prefix = matchingPrefix(Key, It->first);
481  size_t PrevPrefix = matchingPrefix(Key, (It - 1)->first);
482  return Prefix > PrevPrefix ? *It : *--It;
483  }
484 
485  // Original paths, everything else is in lowercase.
486  std::vector<std::string> OriginalPaths;
487  BumpPtrAllocator Arena;
488  StringSaver Strings;
489  // Indexes of candidates by certain substrings.
490  // String is lowercase and sorted, index points into OriginalPaths.
491  std::vector<SubstringAndIndex> Paths; // Full path.
492  // Lang types obtained by guessing on the corresponding path. I-th element is
493  // a type for the I-th path.
494  std::vector<types::ID> Types;
495  std::vector<SubstringAndIndex> Stems; // Basename, without extension.
496  std::vector<SubstringAndIndex> Components; // Last path components.
497 };
498 
499 // The actual CompilationDatabase wrapper delegates to its inner database.
500 // If no match, looks up a proxy file in FileIndex and transfers its
501 // command to the requested file.
502 class InterpolatingCompilationDatabase : public CompilationDatabase {
503 public:
504  InterpolatingCompilationDatabase(std::unique_ptr<CompilationDatabase> Inner)
505  : Inner(std::move(Inner)), Index(this->Inner->getAllFiles()) {}
506 
507  std::vector<CompileCommand>
508  getCompileCommands(StringRef Filename) const override {
509  auto Known = Inner->getCompileCommands(Filename);
510  if (Index.empty() || !Known.empty())
511  return Known;
512  bool TypeCertain;
513  auto Lang = guessType(Filename, &TypeCertain);
514  if (!TypeCertain)
515  Lang = types::TY_INVALID;
516  auto ProxyCommands =
517  Inner->getCompileCommands(Index.chooseProxy(Filename, foldType(Lang)));
518  if (ProxyCommands.empty())
519  return {};
520  return {TransferableCommand(ProxyCommands[0]).transferTo(Filename)};
521  }
522 
523  std::vector<std::string> getAllFiles() const override {
524  return Inner->getAllFiles();
525  }
526 
527  std::vector<CompileCommand> getAllCompileCommands() const override {
528  return Inner->getAllCompileCommands();
529  }
530 
531 private:
532  std::unique_ptr<CompilationDatabase> Inner;
533  FileIndex Index;
534 };
535 
536 } // namespace
537 
538 std::unique_ptr<CompilationDatabase>
539 inferMissingCompileCommands(std::unique_ptr<CompilationDatabase> Inner) {
540  return std::make_unique<InterpolatingCompilationDatabase>(std::move(Inner));
541 }
542 
543 } // namespace tooling
544 } // namespace clang
Specialize PointerLikeTypeTraits to allow LazyGenerationalUpdatePtr to be placed into a PointerUnion...
Definition: Dominators.h:30
static const LangStandard & getLangStandardForKind(Kind K)
static Kind getLangKind(StringRef Name)
Definition: Format.h:2392
__SIZE_TYPE__ size_t
The unsigned integer type of the result of the sizeof operator.
Definition: opencl-c-base.h:40
CompileCommand Cmd
Languages that the frontend can parse and compile.
Language
The language for the input, used to select and validate the language standard and possible actions...
Definition: LangStandard.h:19
const llvm::opt::OptTable & getDriverOptTable()
StringRef Filename
Definition: Format.cpp:1807
ID lookupHeaderTypeForSourceType(ID Id)
Lookup header file input type that corresponds to given source file type (used for clang-cl emulation...
Definition: Types.cpp:382
const char * getTypeName(ID Id)
getTypeName - Return the name of the type for Id.
Definition: Types.cpp:43
#define V(N, I)
Definition: ASTContext.h:2921
std::unique_ptr< CompilationDatabase > inferMissingCompileCommands(std::unique_ptr< CompilationDatabase >)
Returns a wrapped CompilationDatabase that defers to the provided one, but getCompileCommands() will ...
Optional< types::ID > Type
Dataflow Directional Tag Classes.
static std::string getName(const CallEvent &Call)
ID lookupTypeForExtension(llvm::StringRef Ext)
lookupTypeForExtension - Lookup the type to use for the file extension Ext.
Definition: Types.cpp:219
bool onlyPrecompileType(ID Id)
onlyPrecompileType - Should this type only be precompiled.
Definition: Types.cpp:90
__DEVICE__ int min(int __a, int __b)
ID lookupTypeForTypeSpecifier(const char *Name)
lookupTypeForTypSpecifier - Lookup the type to use for a user specified type name.
Definition: Types.cpp:281
A node that&#39;s not selected.
LangStandard::Kind Std