Clang 20.0.0git (In-Progress) Release Notes

Written by the LLVM Team

Warning

These are in-progress notes for the upcoming Clang 20 release. Release notes for previous releases can be found on the Releases Page.

Introduction

This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 20.0.0git. Here we describe the status of Clang in some detail, including major improvements from the previous release and new feature work. For the general LLVM release notes, see the LLVM documentation. For the libc++ release notes, see this page. All LLVM releases may be downloaded from the LLVM releases web site.

For more information about Clang or LLVM, including information about the latest release, please see the Clang Web Site or the LLVM Web Site.

Potentially Breaking Changes

These changes are ones which we think may surprise users when upgrading to Clang 20.0.0git because of the opportunity they pose for disruption to existing code bases.

  • The le32 and le64 targets have been removed.

  • clang -m32 defaults to -mcpu=v9 on SPARC Linux now. Distros still supporting SPARC V8 CPUs need to specify -mcpu=v8 with a config file.

  • The clang-rename tool has been removed.

  • Removed support for RenderScript targets. This technology is officially deprecated and users are encouraged to migrate to Vulkan or other options.

  • Clang now emits distinct type-based alias analysis tags for incompatible pointers by default, enabling more powerful alias analysis when accessing pointer types. This change may silently change code behavior for code containing strict-aliasing violations. The new default behavior can be disabled using -fno-pointer-tbaa.

C/C++ Language Potentially Breaking Changes

  • Clang now rejects _Complex _BitInt types.

C++ Specific Potentially Breaking Changes

  • The type trait builtin __is_nullptr has been removed, since it has very few users and can be written as __is_same(__remove_cv(T), decltype(nullptr)), which GCC supports as well.

  • Clang will now correctly diagnose as ill-formed a constant expression where an enum without a fixed underlying type is set to a value outside the range of the enumeration’s values.

    enum E { Zero, One, Two, Three, Four };
constexpr E Val1 = (E)3;  // Ok
constexpr E Val2 = (E)7;  // Ok
constexpr E Val3 = (E)8;  // Now ill-formed, out of the range [0, 7]
constexpr E Val4 = (E)-1; // Now ill-formed, out of the range [0, 7]

    Since Clang 16, it has been possible to suppress the diagnostic via -Wno-enum-constexpr-conversion, to allow for a transition period for users. Now, in Clang 20, it is no longer possible to suppress the diagnostic.

  • Extraneous template headers are now ill-formed by default. This error can be disable with -Wno-error=extraneous-template-head.

    template <> // error: extraneous template head
template <typename T>
void f();

  • During constant evaluation, comparisons between different evaluations of the same string literal are now correctly treated as non-constant, and comparisons between string literals that cannot possibly overlap in memory are now treated as constant. This updates Clang to match the anticipated direction of open core issue CWG2765 <http://wg21.link/CWG2765>, but is subject to change once that issue is resolved.

    constexpr const char *f() { return "hello"; }
constexpr const char *g() { return "world"; }
// Used to evaluate to false, now error: non-constant comparison.
constexpr bool a = f() == f();
// Might evaluate to true or false, as before.
bool at_runtime() { return f() == f(); }
// Was error, now evaluates to false.
constexpr bool b = f() == g();

  • Clang will now correctly not consider pointers to non classes for covariance and disallow changing return type to a type that doesn’t have the same or less cv-qualifications.

    struct A {
  virtual const int *f() const;
  virtual const std::string *g() const;
};
struct B : A {
  // Return type has less cv-qualification but doesn't point to a class.
  // Error will be generated.
  int *f() const override;

  // Return type doesn't have more cv-qualification also not the same or
  // less cv-qualification.
  // Error will be generated.
  volatile std::string *g() const override;
};

  • The warning -Wdeprecated-literal-operator is now on by default, as this is something that WG21 has shown interest in removing from the language. The result is that anyone who is compiling with -Werror should see this diagnostic. To fix this diagnostic, simply removing the space character from between the operator"" and the user defined literal name will make the source no longer deprecated. This is consistent with CWG2521 <https://cplusplus.github.io/CWG/issues/2521.html>_.

    // Now diagnoses by default.
unsigned operator"" _udl_name(unsigned long long);
// Fixed version:
unsigned operator""_udl_name(unsigned long long);

  • Clang will now produce an error diagnostic when [[clang::lifetimebound]] is applied on a parameter or an implicit object parameter of a function that returns void. This was previously ignored and had no effect. (#107556)

    // Now diagnoses with an error.
void f(int& i [[clang::lifetimebound]]);

  • Clang now rejects all field accesses on null pointers in constant expressions. The following code used to work but will now be rejected:

    struct S { int a; int b; };
constexpr const int *p = &((S*)nullptr)->b;

    Previously, this code was erroneously accepted.

  • Clang will now consider the implicitly deleted destructor of a union or a non-union class without virtual base class to be constexpr in C++20 mode (Clang 19 only did so in C++23 mode but the standard specification for this changed in C++20). (#85550)

    struct NonLiteral {
  NonLiteral() {}
  ~NonLiteral() {}
};

template <class T>
struct Opt {
  union {
    char c;
    T data;
  };
  bool engaged = false;

  constexpr Opt() {}
  constexpr ~Opt() {
    if (engaged)
      data.~T();
  }
};

// Previously only accepted in C++23 and later, now also accepted in C++20.
consteval void foo() { Opt<NonLiteral>{}; }

ABI Changes in This Version

  • Fixed Microsoft name mangling of placeholder, auto and decltype(auto), return types for MSVC 1920+. This change resolves incompatibilities with code compiled by MSVC 1920+ but will introduce incompatibilities with code compiled by earlier versions of Clang unless such code is built with the compiler option -fms-compatibility-version=19.14 to imitate the MSVC 1914 mangling behavior.

  • Fixed the Itanium mangling of the construction vtable name. This change will introduce incompatibilities with code compiled by Clang 19 and earlier versions, unless the -fclang-abi-compat=19 option is used. (#108015)

  • Mangle member-like friend function templates as members of the enclosing class. (#110247, #110503)

AST Dumping Potentially Breaking Changes

Clang Frontend Potentially Breaking Changes

Clang Python Bindings Potentially Breaking Changes

  • Parts of the interface returning string results will now return the empty string "" when no result is available, instead of None.

  • Calling a property on the CompletionChunk or CompletionString class statically now leads to an error, instead of returning a CachedProperty object that is used internally. Properties are only available on instances.

  • For a single-line SourceRange and a SourceLocation in the same line, but after the end of the SourceRange, SourceRange.__contains__ used to incorrectly return True. (#22617), (#52827)

What’s New in Clang 20.0.0git?

Some of the major new features and improvements to Clang are listed here. Generic improvements to Clang as a whole or to its underlying infrastructure are described first, followed by language-specific sections with improvements to Clang’s support for those languages.

C++ Language Changes

  • Allow single element access of GCC vector/ext_vector_type object to be constant expression. Supports the V.xyzw syntax and other tidbits as seen in OpenCL. Selecting multiple elements is left as a future work.

  • Implement CWG1815. Support lifetime extension of temporary created by aggregate initialization using a default member initializer.

  • Accept C++26 user-defined static_assert messages in C++11 as an extension.

  • Add __builtin_elementwise_popcount builtin for integer types only.

  • Add __builtin_elementwise_fmod builtin for floating point types only.

  • Add __builtin_elementwise_minimum and __builtin_elementwise_maximum builtin for floating point types only.

  • The builtin type alias __builtin_common_type has been added to improve the performance of std::common_type.

C++2c Feature Support

C++23 Feature Support

C++20 Feature Support

Resolutions to C++ Defect Reports

  • Allow calling initializer list constructors from initializer lists with a single element of the same type instead of always copying. (CWG2137: List-initialization from object of same type <https://cplusplus.github.io/CWG/issues/2137.html>)

  • Speculative resolution for CWG2311 implemented so that the implementation of CWG2137 doesn’t remove previous cases where guaranteed copy elision was done. Given a prvalue e of class type T, T{e} will try to resolve an initializer list constructor and will use it if successful. Otherwise, if there is no initializer list constructor, the copy will be elided as if it was T(e). (CWG2311: Missed case for guaranteed copy elision <https://cplusplus.github.io/CWG/issues/2311.html>)

  • Casts from a bit-field to an integral type is now not considered narrowing if the width of the bit-field means that all potential values are in the range of the target type, even if the type of the bit-field is larger. (CWG2627: Bit-fields and narrowing conversions)

  • nullptr is now promoted to void* when passed to a C-style variadic function. (CWG722: Can nullptr be passed to an ellipsis?)

  • Allow void{} as a prvalue of type void. (CWG2351: void{}).

  • Clang now has improved resolution to CWG2398, allowing class templates to have default arguments deduced when partial ordering.

  • Clang now allows comparing unequal object pointers that have been cast to void * in constant expressions. These comparisons always worked in non-constant expressions. (CWG2749: Treatment of “pointer to void” for relational comparisons).

  • Reject explicit object parameters with type void (this void). (CWG2915: Explicit object parameters of type void).

  • Clang now allows trailing requires clause on explicit deduction guides. (CWG2707: Deduction guides cannot have a trailing requires-clause).

  • Clang now diagnoses a space in the first production of a literal-operator-id by default. (CWG2521: User-defined literals and reserved identifiers).

  • Fix name lookup for a dependent base class that is the current instantiation. (CWG591: When a dependent base class is the current instantiation).

  • Clang now allows calling explicit object member functions directly with prvalues instead of always materializing a temporary, meaning by-value explicit object parameters do not need to move from a temporary. (CWG2813: Class member access with prvalues).

C Language Changes

  • Extend clang’s <limits.h> to define LONG_LONG_* macros for Android’s bionic.

C2y Feature Support

  • Updated conformance for N3298 which adds the i and j suffixes for the creation of a _Complex constant value. Clang has always supported these suffixes as a GNU extension, so -Wgnu-imaginary-constant no longer has effect in C modes, as this is now a C2y extension in C. -Wgnu-imaginary-constant still applies in C++ modes.

  • Clang updated conformance for N3370 case range expressions. This feature was previously supported by Clang as a GNU extension, so -Wgnu-case-range no longer has effect in C modes, as this is now a C2y extension in C. -Wgnu-case-range still applies in C++ modes.

  • Clang implemented support for N3344 which disallows a void parameter from having a qualifier or storage class specifier. Note that register void was previously accepted in all C language modes but is now rejected (all of the other qualifiers and storage class specifiers were previously rejected).

  • Updated conformance for N3364 on floating-point translation-time initialization with signaling NaN. This paper adopts Clang’s existing practice, so there were no changes to compiler behavior.

  • Implemented support for N3341 which makes empty structure and union objects implementation-defined in C. -Wgnu-empty-struct will be emitted in C23 and earlier modes because the behavior is a conforming GNU extension in those modes, but will no longer have an effect in C2y mode.

  • Updated conformance for N3342 which made qualified function types implementation-defined rather than undefined. Clang has always accepted const and volatile qualified function types by ignoring the qualifiers.

  • Updated conformance for N3346 which changes some undefined behavior around initialization to instead be constraint violations. This paper adopts Clang’s existing practice, so there were no changes to compiler behavior.

C23 Feature Support

  • Clang now supports N3029 Improved Normal Enumerations.

  • Clang now officially supports N3030 Enhancements to Enumerations. Clang already supported it as an extension, so there were no changes to compiler behavior.

  • Fixed the value of BOOL_WIDTH in <limits.h> to return 1 explicitly, as mandated by the standard. Fixes #117348

Non-comprehensive list of changes in this release

  • The floating point comparison builtins (__builtin_isgreater, __builtin_isgreaterequal, __builtin_isless, etc.) and __builtin_signbit can now be used in constant expressions.

  • Plugins can now define custom attributes that apply to statements as well as declarations.

  • __builtin_abs function can now be used in constant expressions.

  • The new builtin __builtin_counted_by_ref was added. In contexts where the programmer needs access to the counted_by attribute’s field, but it’s not available — e.g. in macros. For instance, it can be used to automatically set the counter during allocation in the Linux kernel:

    /* A simplified version of Linux allocation macros */
#define alloc(PTR, FAM, COUNT) ({ \
    sizeof_t __ignored_assignment;                             \
    typeof(P) __p;                                             \
    size_t __size = sizeof(*P) + sizeof(*P->FAM) * COUNT;      \
    __p = malloc(__size);                                      \
    *_Generic(                                                 \
      __builtin_counted_by_ref(__p->FAM),                      \
        void *: &__ignored_assignment,                         \
        default: __builtin_counted_by_ref(__p->FAM)) = COUNT;  \
    __p;                                                       \
})

    The flexible array member (FAM) can now be accessed immediately without causing issues with the sanitizer because the counter is automatically set.

  • The following builtins can now be used in constant expressions: __builtin_reduce_add, __builtin_reduce_mul, __builtin_reduce_and, __builtin_reduce_or, __builtin_reduce_xor, __builtin_elementwise_popcount, __builtin_elementwise_bitreverse, __builtin_elementwise_add_sat, __builtin_elementwise_sub_sat.

  • Clang now rejects _BitInt matrix element types if the bit width is less than CHAR_WIDTH or not a power of two, matching preexisting behaviour for vector types.

  • Matrix types (a Clang extension) can now be used in pseudo-destructor expressions, which allows them to be stored in STL containers.

New Compiler Flags

  • The -fc++-static-destructors={all,thread-local,none} flag was added to control which C++ variables have static destructors registered: all (the default) does so for all variables, thread-local only for thread-local variables, and none (which corresponds to the existing -fno-c++-static-destructors flag) skips all static destructors registration.

  • The -Warray-compare warning has been added to warn about array comparison on versions older than C++20.

  • The -Warray-compare-cxx26 warning has been added to warn about array comparison starting from C++26, this warning is enabled as an error by default.

  • clang-cl and clang-dxc now support -fdiagnostics-color=[auto|never|always] in addition to -f[no-]color-diagnostics.

Deprecated Compiler Flags

  • -fheinous-gnu-extensions is deprecated; it is now equivalent to specifying -Wno-error=invalid-gnu-asm-cast and may be removed in the future.

Modified Compiler Flags

  • The -ffp-model option has been updated to enable a more limited set of optimizations when the fast argument is used and to accept a new argument, aggressive. The behavior of -ffp-model=aggressive is equivalent to the previous behavior of -ffp-model=fast. The updated -ffp-model=fast behavior no longer assumes finite math only and uses the promoted algorithm for complex division when possible rather than the less basic (limited range) algorithm.

  • The -fveclib option has been updated to enable -fno-math-errno for -fveclib=ArmPL and -fveclib=SLEEF. This gives Clang more opportunities to utilize these vector libraries. The behavior for all other vector function libraries remains unchanged.

  • The -Wnontrivial-memcall warning has been added to warn about passing non-trivially-copyable destination parameter to memcpy, memset and similar functions for which it is a documented undefined behavior. It is implied by -Wnontrivial-memaccess

  • Added -fmodules-reduced-bmi flag corresponding to -fexperimental-modules-reduced-bmi flag. The -fmodules-reduced-bmi flag is intended to be enabled by default in the future.

Removed Compiler Flags

  • The compiler flag -Wenum-constexpr-conversion (and the Wno-, Wno-error- derivatives) is now removed, since it’s no longer possible to suppress the diagnostic (see above). Users can expect an unknown warning diagnostic if it’s still in use.

Attribute Changes in Clang

  • The swift_attr can now be applied to types. To make it possible to use imported APIs in Swift safely there has to be a way to annotate individual parameters and result types with relevant attributes that indicate that e.g. a block is called on a particular actor or it accepts a Sendable or global-actor (i.e. @MainActor) isolated parameter.

    For example:

    @interface MyService
  -(void) handle: (void (^ __attribute__((swift_attr("@Sendable"))))(id)) handler;
@end

  • Clang now disallows more than one __attribute__((ownership_returns(class, idx))) with different class names attached to one function.

  • Introduced a new format attribute __attribute__((format(syslog, 1, 2))) from OpenBSD.

  • The hybrid_patchable attribute is now supported on ARM64EC targets. It can be used to specify that a function requires an additional x86-64 thunk, which may be patched at runtime.

  • The attribute [[clang::no_specializations]] has been added to warn users that a specific template shouldn’t be specialized. This is useful for e.g. standard library type traits, where adding a specialization results in undefined behaviour.

  • [[clang::lifetimebound]] is now explicitly disallowed on explicit object member functions where they were previously silently ignored.

  • Clang now automatically adds [[clang::lifetimebound]] to the parameters of std::span, std::string_view constructors, this enables Clang to capture more cases where the returned reference outlives the object. (#100567)

  • Clang now correctly diagnoses the use of btf_type_tag in C++ and ignores it; this attribute is a C-only attribute, and caused crashes with template instantiation by accidentally allowing it in C++ in some circumstances. (#106864)

  • Introduced a new attribute [[clang::coro_await_elidable]] on coroutine return types to express elideability at call sites where the coroutine is invoked under a safe elide context.

  • Introduced a new attribute [[clang::coro_await_elidable_argument]] on function parameters to propagate safe elide context to arguments if such function is also under a safe elide context.

  • The documentation of the [[clang::musttail]] attribute was updated to note that the lifetimes of all local variables end before the call. This does not change the behaviour of the compiler, as this was true for previous versions.

  • Fix a bug where clang doesn’t automatically apply the [[gsl::Owner]] or [[gsl::Pointer]] to STL explicit template specialization decls. (#109442)

  • Clang now supports [[clang::lifetime_capture_by(X)]]. Similar to lifetimebound, this can be used to specify when a reference to a function parameter is captured by another capturing entity X.

  • The target_version attribute is now only supported for AArch64 and RISC-V architectures.

  • Clang now permits the usage of the placement new operator in [[msvc::constexpr]] context outside of the std namespace. (#74924)

Improvements to Clang’s diagnostics

  • Some template related diagnostics have been improved.

    void foo() { template <typename> int i; } // error: templates can only be declared in namespace or class scope

struct S {
 template <typename> int i; // error: non-static data member 'i' cannot be declared as a template
};

  • Clang now has improved diagnostics for functions with explicit ‘this’ parameters. Fixes #97878

  • Clang now diagnoses dangling references to fields of temporary objects. Fixes #81589.

  • Clang now diagnoses undefined behavior in constant expressions more consistently. This includes invalid shifts, and signed overflow in arithmetic.

  • -Wdangling-assignment-gsl is enabled by default.

  • Clang now always preserves the template arguments as written used to specialize template type aliases.

  • Clang now diagnoses the use of main in an extern context as invalid according to [basic.start.main] p3. Fixes #101512.

  • Clang now diagnoses when the result of a [[nodiscard]] function is discarded after being cast in C. Fixes #104391.

  • Don’t emit duplicated dangling diagnostics. (#93386).

  • Improved diagnostic when trying to befriend a concept. (#45182).

  • Added the -Winvalid-gnu-asm-cast diagnostic group to control warnings about use of “noop” casts for lvalues (a GNU extension). This diagnostic is a warning which defaults to being an error, is enabled by default, and is also controlled by the now-deprecated -fheinous-gnu-extensions flag.

  • Added the -Wdecls-in-multiple-modules option to assist users to identify multiple declarations in different modules, which is the major reason of the slow compilation speed with modules. This warning is disabled by default and it needs to be explicitly enabled or by -Weverything.

  • Improved diagnostic when trying to overload a function in an extern "C" context. (#80235)

  • Clang now respects lifetimebound attribute for the assignment operator parameter. (#106372).

  • The lifetimebound and GSL analysis in clang are coherent, allowing clang to detect more use-after-free bugs. (#100549).

  • Clang now diagnoses dangling cases where a gsl-pointer is constructed from a gsl-owner object inside a container (#100384).

  • Clang now warns for u8 character literals used in C23 with -Wpre-c23-compat instead of -Wpre-c++17-compat.

  • Clang now diagnose when importing module implementation partition units in module interface units.

  • Don’t emit bogus dangling diagnostics when [[gsl::Owner]] and [[clang::lifetimebound]] are used together (#108272).

  • The -Wreturn-stack-address warning now also warns about addresses of local variables passed to function calls using the [[clang::musttail]] attribute.

  • Clang now diagnoses cases where a dangling GSLOwner<GSLPointer> object is constructed, e.g. std::vector<string_view> v = {std::string()}; (#100526).

  • Clang now diagnoses when a requires expression has a local parameter of void type, aligning with the function parameter (#109831).

  • Clang now emits a diagnostic note at the class declaration when the method definition does not match any declaration (#110638).

  • Clang now omits warnings for extra parentheses in fold expressions with single expansion (#101863).

  • The warning for an unsupported type for a named register variable is now phrased unsupported type for named register variable, instead of bad type for named register variable. This makes it clear that the type is not supported at all, rather than being suboptimal in some way the error fails to mention (#111550).

  • Clang now emits a -Wdepredcated-literal-operator diagnostic, even if the name was a reserved name, which we improperly allowed to suppress the diagnostic.

  • Clang now diagnoses [[deprecated]] attribute usage on local variables (#90073).

  • Fix false positives when [[gsl::Owner/Pointer]] and [[clang::lifetimebound]] are used together.

  • Improved diagnostic message for __builtin_bit_cast size mismatch (#115870).

  • Clang now omits shadow warnings for enum constants in separate class scopes (#62588).

  • When diagnosing an unused return value of a type declared [[nodiscard]], the type itself is now included in the diagnostic.

  • Clang will now prefer the [[nodiscard]] declaration on function declarations over [[nodiscard]] declaration on the return type of a function. Previously, when both have a [[nodiscard]] declaration attached, the one on the return type would be preferred. This may affect the generated warning message:

    struct [[nodiscard("Reason 1")]] S {};
[[nodiscard("Reason 2")]] S getS();
void use()
{
  getS(); // Now diagnoses "Reason 2", previously diagnoses "Reason 1"
}

  • Clang now diagnoses = delete("reason") extension warnings only in pedantic mode rather than on by default. (#109311).

  • Clang now diagnoses missing return value in functions containing if consteval (#116485).

  • Clang now correctly recognises code after a call to a [[noreturn]] constructor as unreachable (#63009).

  • Clang now omits shadowing warnings for parameter names in explicit object member functions (#95707).

  • Improved error recovery for function call arguments with trailing commas (#100921).

  • For an rvalue reference bound to a temporary struct with an integer member, Clang will detect constant integer overflow in the initializer for the integer member (#46755).

  • Fixed a false negative -Wunused-private-field diagnostic when a defaulted comparison operator is defined out of class (#116961).

  • Clang now diagnoses dangling references for C++20’s parenthesized aggregate initialization (#101957).

  • Fixed a bug where Clang would not emit -Wunused-private-field warnings when an unrelated class defined a defaulted comparison operator (#116270).

    class A {
private:
  int a; // warning: private field 'a' is not used, no diagnostic previously
};

class C {
  bool operator==(const C&) = default;
};

  • Clang now emits -Wdangling-capture diangostic when a STL container captures a dangling reference.

    void test() {
  std::vector<std::string_view> views;
  views.push_back(std::string("123")); // warning
}

  • Clang now emits a -Wtautological-compare diagnostic when a check for pointer addition overflow is always true or false, because overflow would be undefined behavior.

    bool incorrect_overflow_check(const char *ptr, size_t index) {
  return ptr + index < ptr; // warning
}

  • Fix -Wdangling false positives on conditional operators (#120206).

  • Fixed a bug where Clang hung on an unsupported optional scope specifier :: when parsing Objective-C. Clang now emits a diagnostic message instead of hanging.

  • The Thread Safety Analysis now supports passing scoped capabilities into functions: an attribute on the scoped capability parameter indicates both the expected associated capabilities and, like in the case of attributes on the function declaration itself, their state before and after the call.

    #include "mutex.h"

Mutex mu1, mu2;
int a GUARDED_BY(mu1);

void require(MutexLocker& scope REQUIRES(mu1)) {
  scope.Unlock();
  a = 0; // Warning!  Requires mu1.
  scope.Lock();
}

void testParameter() {
  MutexLocker scope(&mu1), scope2(&mu2);
  require(scope2); // Warning! Mutex managed by 'scope2' is 'mu2' instead of 'mu1'
  require(scope); // OK.
  scope.Unlock();
  require(scope); // Warning!  Requires mu1.
}

Improvements to Clang’s time-trace

Improvements to Coverage Mapping

Bug Fixes in This Version

  • Fixed the definition of ATOMIC_FLAG_INIT in <stdatomic.h> so it can be used in C++.

  • Fixed a failed assertion when checking required literal types in C context. (#101304).

  • Fixed a crash when trying to transform a dependent address space type. Fixes #101685.

  • Fixed a crash when diagnosing format strings and encountering an empty delimited escape sequence (e.g., "\o{}"). #102218

  • Fixed a crash using __array_rank on 64-bit targets. (#113044).

  • The warning emitted for an unsupported register variable type now points to the unsupported type instead of the register keyword (#109776).

  • Fixed a crash when emit ctor for global variant with flexible array init (#113187).

  • Fixed a crash when GNU statement expression contains invalid statement (#113468).

  • Fixed a failed assertion when using __attribute__((noderef)) on an _Atomic-qualified type (#116124).

  • No longer return false for noexcept expressions involving a delete which resolves to a destroying delete but the type of the object being deleted has a potentially throwing destructor (#118660).

Bug Fixes to Compiler Builtins

  • Fix crash when atomic builtins are called with pointer to zero-size struct (#90330)

  • Clang now allows pointee types of atomic builtin arguments to be complete template types that was not instantiated elsewhere.

  • __noop can now be used in a constant expression. (#102064)

  • Fix __has_builtin incorrectly returning false for some C++ type traits. (#111477)

  • Fix __builtin_source_location incorrectly returning wrong column for method chains. (#119129)

Bug Fixes to Attribute Support

Bug Fixes to C++ Support

  • Fixed a crash when an expression with a dependent __typeof__ type is used as the operand of a unary operator. (#97646)

  • Fixed incorrect pack expansion of init-capture references in requires expresssions.

  • Fixed a failed assertion when checking invalid delete operator declaration. (#96191)

  • Fix a crash when checking destructor reference with an invalid initializer. (#97230)

  • Clang now correctly parses potentially declarative nested-name-specifiers in pointer-to-member declarators.

  • Fix a crash when checking the initializer of an object that was initialized with a string literal. (#82167)

  • Fix a crash when matching template template parameters with templates which have parameters of different class type. (#101394)

  • Clang now correctly recognizes the correct context for parameter substitutions in concepts, so it doesn’t incorrectly complain of missing module imports in those situations. (#60336)

  • Fix init-capture packs having a size of one before being instantiated. (#63677)

  • Clang now preserves the unexpanded flag in a lambda transform used for pack expansion. (#56852), (#85667), (#99877).

  • Fixed a bug when diagnosing ambiguous explicit specializations of constrained member functions.

  • Fixed an assertion failure when selecting a function from an overload set that includes a specialization of a conversion function template.

  • Correctly diagnose attempts to use a concept name in its own definition; A concept name is introduced to its scope sooner to match the C++ standard. (#55875)

  • Properly reject defaulted relational operators with invalid types for explicit object parameters, e.g., bool operator==(this int, const Foo&) (#100329), and rvalue reference parameters.

  • Properly reject defaulted copy/move assignment operators that have a non-reference explicit object parameter.

  • Clang now properly handles the order of attributes in extern blocks. (#101990).

  • Fixed an assertion failure by preventing null explicit object arguments from being deduced. (#102025).

  • Correctly check constraints of explicit instantiations of member functions. (#46029)

  • When performing partial ordering of function templates, clang now checks that the deduction was consistent. Fixes (#18291).

  • Fixed an assertion failure about a constraint of a friend function template references to a value with greater template depth than the friend function template. (#98258)

  • Clang now rebuilds the template parameters of out-of-line declarations and specializations in the context of the current instantiation in all cases.

  • Fix evaluation of the index of dependent pack indexing expressions/types specifiers (#105900)

  • Correctly handle subexpressions of an immediate invocation in the presence of implicit casts. (#105558)

  • Clang now correctly handles direct-list-initialization of a structured bindings from an array. (#31813)

  • Mangle placeholders for deduced types as a template-prefix, such that mangling of template template parameters uses the correct production. (#106182)

  • Fixed an assertion failure when converting vectors to int/float with invalid expressions. (#105486)

  • Template parameter names are considered in the name lookup of out-of-line class template specialization right before its declaration context. (#64082)

  • Fixed a constraint comparison bug for friend declarations. (#78101)

  • Fix handling of _ as the name of a lambda’s init capture variable. (#107024)

  • Fix an issue with dependent source location expressions (#106428), (#81155), (#80210), (#85373)

  • Fixed a bug in the substitution of empty pack indexing types. (#105903)

  • Clang no longer tries to capture non-odr used default arguments of template parameters of generic lambdas (#107048)

  • Fixed a bug where defaulted comparison operators would remove const from base classes. (#102588)

  • Fix a crash when using source_location in the trailing return type of a lambda expression. (#67134)

  • A follow-up fix was added for (#61460), as the previous fix was not entirely correct. (#86361), (#112352)

  • Fixed a crash in the typo correction of an invalid CTAD guide. (#107887)

  • Fixed a crash when clang tries to substitute parameter pack while retaining the parameter pack. (#63819), (#107560)

  • Fix a crash when a static assert declaration has an invalid close location. (#108687)

  • Avoided a redundant friend declaration instantiation under a certain consteval context. (#107175)

  • Fixed an assertion failure in debug mode, and potential crashes in release mode, when diagnosing a failed cast caused indirectly by a failed implicit conversion to the type of the constructor parameter.

  • Fixed an assertion failure by adjusting integral to boolean vector conversions (#108326)

  • Fixed a crash when mixture of designated and non-designated initializers in union. (#113855)

  • Fixed an issue deducing non-type template arguments of reference type. (#73460)

  • Fixed an issue in constraint evaluation, where type constraints on the lambda expression containing outer unexpanded parameters were not correctly expanded. (#101754)

  • Fixes crashes with function template member specializations, and increases conformance of explicit instantiation behaviour with MSVC. (#111266)

  • Fixed a bug in constraint expression comparison where the sizeof... expression was not handled properly in certain friend declarations. (#93099)

  • Clang now instantiates the correct lambda call operator when a lambda’s class type is merged across modules. (#110401)

  • Fix a crash when parsing a pseudo destructor involving an invalid type. (#111460)

  • Fixed an assertion failure when invoking recovery call expressions with explicit attributes and undeclared templates. (#107047), (#49093)

  • Clang no longer crashes when a lambda contains an invalid block declaration that contains an unexpanded parameter pack. (#109148)

  • Fixed overload handling for object parameters with top-level cv-qualifiers in explicit member functions (#100394)

  • Fixed a bug in lambda captures where constexpr class-type objects were not properly considered ODR-used in certain situations. (#47400), (#90896)

  • Fix erroneous templated array size calculation leading to crashes in generated code. (#41441)

  • During the lookup for a base class name, non-type names are ignored. (#16855)

  • Fix a crash when recovering an invalid expression involving an explicit object member conversion operator. (#112559)

  • Clang incorrectly considered a class with an anonymous union member to not be const-default-constructible even if a union member has a default member initializer. (#95854).

  • Fixed an assertion failure when evaluating an invalid expression in an array initializer. (#112140)

  • Fixed an assertion failure in range calculations for conditional throw expressions. (#111854)

  • Clang now correctly ignores previous partial specializations of member templates explicitly specialized for an implicitly instantiated class template specialization. (#51051)

  • Fixed an assertion failure caused by invalid enum forward declarations. (#112208)

  • Name independent data members were not correctly initialized from default member initializers. (#114069)

  • Fixed expression transformation for [[assume(...)]], allowing using pack indexing expressions within the assumption if they also occur inside of a dependent lambda. (#114787)

  • Lambdas now capture function types without considering top-level const qualifiers. (#84961)

  • Clang now uses valid deduced type locations when diagnosing functions with trailing return type missing placeholder return type. (#78694)

  • Fixed a bug where bounds of partially expanded pack indexing expressions were checked too early. (#116105)

  • Fixed an assertion failure caused by using consteval in condition in consumed analyses. (#117385)

  • Fixed an assertion failure caused by invalid default argument substitutions in non-defining friend declarations. (#113324)

  • Fix a crash caused by incorrect argument position in merging deduced template arguments. (#113659)

  • Fixed a parser crash when using pack indexing as a nested name specifier. (#119072)

  • Fixed a null pointer dereference issue when heuristically computing sizeof...(pack) expressions. (#81436)

  • Fixed an assertion failure caused by mangled names with invalid identifiers. (#112205)

  • Fixed an incorrect lambda scope of generic lambdas that caused Clang to crash when computing potential lambda captures at the end of a full expression. (#115931)

  • Clang no longer rejects deleting a pointer of incomplete enumeration type. (#99278)

  • Fixed recognition of std::initializer_list when it’s surrounded with extern "C++" and exported out of a module (which is the case e.g. in MSVC’s implementation of std module). (#118218)

  • Fixed a pack expansion issue in checking unexpanded parameter sizes. (#17042)

Bug Fixes to AST Handling

  • Fixed a crash that occurred when dividing by zero in complex integer division. (#55390).

  • Fixed a bug in ASTContext::getRawCommentForAnyRedecl() where the function could sometimes incorrectly return null even if a comment was present. (#108145)

  • Clang now correctly parses the argument of the relates, related, relatesalso, and relatedalso comment commands.

  • Clang now uses the location of the begin of the member expression for CallExpr involving deduced this. (#116928)

  • Fixed printout of AST that uses pack indexing expression. (#116486)

Miscellaneous Bug Fixes

Miscellaneous Clang Crashes Fixed

  • Fixed a crash in C due to incorrect lookup that members in nested anonymous struct/union can be found as ordinary identifiers in struct/union definition. (#31295)

  • Fixed a crash caused by long chains of sizeof and other similar operators that can be followed by a non-parenthesized expression. (#45061)

  • Fixed an crash when compiling #pragma STDC FP_CONTRACT DEFAULT with -ffp-contract=fast-honor-pragmas. (#104830)

  • Fixed a crash when function has more than 65536 parameters. Now a diagnostic is emitted. (#35741)

  • Fixed -ast-dump crashes on codes involving concept with -ast-dump-decl-types. (#94928)

  • Fixed internal assertion firing when a declaration in the implicit global module is found through ADL. (GH#109879)

OpenACC Specific Changes

Target Specific Changes

  • Clang now implements the Solaris-specific mangling of std::tm as tm, same for std::div_t, std::ldiv_t, and std::lconv, for Solaris ABI compatibility. (#33114)

AMDGPU Support

  • Initial support for gfx950

  • Added headers gpuintrin.h and amdgpuintrin.h that contains common definitions for GPU builtin functions. This header can be included for OpenMP, CUDA, HIP, OpenCL, and C/C++.

NVPTX Support

  • Added headers gpuintrin.h and nvptxintrin.h that contains common definitions for GPU builtin functions. This header can be included for OpenMP, CUDA, HIP, OpenCL, and C/C++.

X86 Support

  • The MMX vector intrinsic functions from *mmintrin.h which operate on __m64 vectors, such as _mm_add_pi8, have been reimplemented to use the SSE2 instruction-set and XMM registers unconditionally. These intrinsics are therefore no longer supported if MMX is enabled without SSE2 – either from targeting CPUs from the Pentium-MMX through the Pentium 3, or explicitly via passing arguments such as -mmmx -mno-sse2. MMX assembly code remains supported without requiring SSE2, including inside inline-assembly.

  • The compiler builtins such as __builtin_ia32_paddb which formerly implemented the above MMX intrinsic functions have been removed. Any uses of these removed functions should migrate to the functions defined by the *mmintrin.h headers. A mapping can be found in the file clang/www/builtins.py.

  • Support ISA of AVX10.2. * Supported MINMAX intrinsics of *_(mask(z)))_minmax(ne)_p[s|d|h|bh] and *_(mask(z)))_minmax_s[s|d|h].

  • Supported intrinsics for SM4 and AVX10.2. * Supported SM4 intrinsics of _mm512_sm4key4_epi32 and _mm512_sm4rnds4_epi32.

  • All intrinsics in adcintrin.h can now be used in constant expressions.

  • All intrinsics in adxintrin.h can now be used in constant expressions.

  • All intrinsics in lzcntintrin.h can now be used in constant expressions.

  • All intrinsics in bmiintrin.h can now be used in constant expressions.

  • All intrinsics in bmi2intrin.h can now be used in constant expressions.

  • All intrinsics in tbmintrin.h can now be used in constant expressions.

  • Supported intrinsics for MOVRS AND AVX10.2. * Supported intrinsics of _mm(256|512)_(mask(z))_loadrs_epi(8|16|32|64).

  • Support ISA of AMX-FP8.

  • Support ISA of AMX-TRANSPOSE.

  • Support ISA of AMX-MOVRS.

  • Support ISA of AMX-AVX512.

  • Support ISA of AMX-TF32.

  • Support ISA of MOVRS.

  • Supported -march/tune=diamondrapids

Arm and AArch64 Support

  • In the ARM Target, the frame pointer (FP) of a leaf function can be retained by using the -fno-omit-frame-pointer option. If you want to eliminate the FP in leaf functions after enabling -fno-omit-frame-pointer, you can do so by adding the -momit-leaf-frame-pointer option.

Android Support

Windows Support

  • clang-cl now supports /std:c++23preview which enables C++23 features.

  • Clang no longer allows references inside a union when emulating MSVC 1900+ even if fms-extensions is enabled. Starting with VS2015, MSVC 1900, this Microsoft extension is no longer allowed and always results in an error. Clang now follows the MSVC behavior in this scenario. When -fms-compatibility-version=18.00 or prior is set on the command line this Microsoft extension is still allowed as VS2013 and prior allow it.

LoongArch Support

RISC-V Support

CUDA/HIP Language Changes

CUDA Support

  • Clang now supports CUDA SDK up to 12.6

  • Added support for sm_100

  • Added support for __grid_constant__ attribute.

AIX Support

NetBSD Support

WebAssembly Support

The default target CPU, “generic”, now enables the -mnontrapping-fptoint and -mbulk-memory flags, which correspond to the [Bulk Memory Operations] and [Non-trapping float-to-int Conversions] language features, which are [widely implemented in engines].

A new Lime1 target CPU is added, -mcpu=lime1. This CPU follows the definition of the Lime1 CPU [here], and enables -mmultivalue, -mmutable-globals, -mcall-indirect-overlong, -msign-ext, -mbulk-memory-opt, -mnontrapping-fptoint, and -mextended-const.

[Bulk Memory Operations]: https://github.com/WebAssembly/bulk-memory-operations/blob/master/proposals/bulk-memory-operations/Overview.md [Non-trapping float-to-int Conversions]: https://github.com/WebAssembly/spec/blob/master/proposals/nontrapping-float-to-int-conversion/Overview.md [widely implemented in engines]: https://webassembly.org/features/ [here]: https://github.com/WebAssembly/tool-conventions/blob/main/Lime.md#lime1

AVR Support

  • Reject C/C++ compilation for avr1 devices which have no SRAM.

DWARF Support in Clang

Floating Point Support in Clang

  • Add __builtin_elementwise_atan2 builtin for floating point types only.

Fixed Point Support in Clang

AST Matchers

  • Fixed an issue with the hasName and hasAnyName matcher when matching inline namespaces with an enclosing namespace of the same name.

  • Fixed an ordering issue with the hasOperands matcher occurring when setting a binding in the first matcher and using it in the second matcher.

  • Fixed a crash when traverse lambda expr with invalid captures. (#106444)

  • Fixed isInstantiated and isInTemplateInstantiation to also match for variable templates. (#110666)

  • Ensure hasName matches template specializations across inline namespaces, making matchesNodeFullSlow and matchesNodeFullFast consistent.

  • Improved the performance of the getExpansionLocOfMacro by tracking already processed macros during recursion.

  • Add exportDecl matcher to match export declaration.

  • Ensure hasType and hasDeclaration match Objective-C interface declarations.

  • Ensure pointee matches Objective-C pointer types.

clang-format

  • Adds BreakBinaryOperations option.

  • Adds TemplateNames option.

  • Adds AlignFunctionDeclarations option to AlignConsecutiveDeclarations.

  • Adds IndentOnly suboption to ReflowComments to fix the indentation of multi-line comments without touching their contents, renames false to Never, and true to Always.

  • Adds RemoveEmptyLinesInUnwrappedLines option.

  • Adds KeepFormFeed option and set it to true for GNU style.

libclang

  • Add clang_isBeforeInTranslationUnit. Given two source locations, it determines whether the first one comes strictly before the second in the source code.

Static Analyzer

New features

  • Now CSA models __builtin_*_overflow functions. (#102602)

  • MallocChecker now checks for ownership_returns(class, idx) and ownership_takes(class, idx) attributes with class names different from “malloc”. Clang static analyzer now reports an error if class of allocation and deallocation function mismatches. Documentation.

  • Function effects, e.g. the nonblocking and nonallocating “performance constraint” attributes, are now verified. For example, for functions declared with the nonblocking attribute, the compiler can generate warnings about the use of any language features, or calls to other functions, which may block.

  • Introduced -warning-suppression-mappings flag to control diagnostic suppressions per file. See documentation <https://clang.llvm.org/docs/WarningSuppressionMappings.html>_ for details.

Crash and bug fixes

Improvements

  • Improved the handling of the ownership_returns attribute. Now, Clang reports an error if the attribute is attached to a function that returns a non-pointer value. Fixes (#99501)

Moved checkers

  • The checker alpha.core.IdenticalExpr was deleted because it was duplicated in the clang-tidy checkers misc-redundant-expression and bugprone-branch-clone.

  • The checker alpha.security.MallocOverflow was deleted because it was badly implemented and its aggressive logic produced too many false positives. To detect too large arguments passed to malloc, consider using the checker alpha.taint.TaintedAlloc.

  • The checkers alpha.nondeterministic.PointerSorting and alpha.nondeterministic.PointerIteration were moved to a new bugprone checker named bugprone-nondeterministic-pointer-iteration-order. The original checkers were implemented only using AST matching and make more sense as a single clang-tidy check.

  • The checker alpha.unix.Chroot was modernized, improved and moved to unix.Chroot. Testing was done on open source projects that use chroot(), and false issues addressed in the improvements based on real use cases. Open source projects used for testing include nsjail, lxroot, dive and ruri. This checker conforms to SEI Cert C recommendation POS05-C. Limit access to files by creating a jail. Fixes (#34697). (#117791) [Documentation](https://clang.llvm.org/docs/analyzer/checkers.html#unix-chroot-c).

Sanitizers

  • Introduced Realtime Sanitizer, activated by using the -fsanitize=realtime flag. This sanitizer detects unsafe system library calls, such as memory allocations and mutex locks. If any such function is called during invocation of a function marked with the [[clang::nonblocking]] attribute, an error is printed to the console and the process exits non-zero.

  • Added the -fsanitize-undefined-ignore-overflow-pattern flag which can be used to disable specific overflow-dependent code patterns. The supported patterns are: add-signed-overflow-test, add-unsigned-overflow-test, negated-unsigned-const, and unsigned-post-decr-while. The sanitizer instrumentation can be toggled off for all available patterns by specifying all. Conversely, you may disable all exclusions with none which is the default.

    /// specified with ``-fsanitize-undefined-ignore-overflow-pattern=add-unsigned-overflow-test``
int common_overflow_check_pattern(unsigned base, unsigned offset) {
  if (base + offset < base) { /* ... */ } // The pattern of `a + b < a`, and other re-orderings, won't be instrumented
}

/// specified with ``-fsanitize-undefined-ignore-overflow-pattern=add-signed-overflow-test``
int common_overflow_check_pattern_signed(signed int base, signed int offset) {
  if (base + offset < base) { /* ... */ } // The pattern of `a + b < a`, and other re-orderings, won't be instrumented
}

/// specified with ``-fsanitize-undefined-ignore-overflow-pattern=negated-unsigned-const``
void negation_overflow() {
  unsigned long foo = -1UL; // No longer causes a negation overflow warning
  unsigned long bar = -2UL; // and so on...
}

/// specified with ``-fsanitize-undefined-ignore-overflow-pattern=unsigned-post-decr-while``
void while_post_decrement() {
  unsigned char count = 16;
  while (count--) { /* ... */ } // No longer causes unsigned-integer-overflow sanitizer to trip
}

    Many existing projects have a large amount of these code patterns present. This new flag should allow those projects to enable integer sanitizers with less noise.

  • -fsanitize=signed-integer-overflow, -fsanitize=unsigned-integer-overflow, -fsanitize=implicit-signed-integer-truncation, -fsanitize=implicit-unsigned-integer-truncation, -fsanitize=enum now properly support the “type” prefix within Sanitizer Special Case Lists (SSCL). See that link for examples.

  • Introduced an experimental Type Sanitizer, activated by using the -fsanitize=type flag. This sanitizer detects violations of C/C++ type-based aliasing rules.

  • Implemented -f[no-]sanitize-trap=local-bounds, and -f[no-]sanitize-recover=local-bounds.

  • -fsanitize-merge (default) and -fno-sanitize-merge have been added for fine-grained, unified control of which UBSan checks can potentially be merged by the compiler (for example, -fno-sanitize-merge=bool,enum,array-bounds,local-bounds).

Python Binding Changes

  • Fixed an issue that led to crashes when calling Type.get_exception_specification_kind.

OpenMP Support

  • Added support for ‘omp assume’ directive.

  • Added support for ‘omp scope’ directive.

  • Added support for allocator-modifier in ‘allocate’ clause.

Improvements

  • Improve the handling of mapping array-section for struct containing nested structs with user defined mappers

  • num_teams and thead_limit now accept multiple expressions when it is used along in target teams ompx_bare construct. This allows the target region to be launched with multi-dim grid on GPUs.

Additional Information

A wide variety of additional information is available on the Clang web page. The web page contains versions of the API documentation which are up-to-date with the Git version of the source code. You can access versions of these documents specific to this release by going into the “clang/docs/” directory in the Clang tree.

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