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Clang |release| |ReleaseNotesTitle|

Written by the LLVM Team

.. only:: PreRelease

  .. warning::
     These are in-progress notes for the upcoming Clang |version| release.
     Release notes for previous releases can be found on
     `the Releases Page <https://llvm.org/releases/>`_.

Introduction

This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release |release|. 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 |release| because of the opportunity they pose for disruption to existing code bases.

C/C++ Language Potentially Breaking Changes

C++ Specific Potentially Breaking Changes

  • Clang now diagnoses function/variable templates that shadow their own template parameters, e.g. template<class T> void T();. This error can be disabled via -Wno-strict-primary-template-shadow for compatibility with previous versions of clang.

ABI Changes in This Version

AST Dumping Potentially Breaking Changes

Clang Frontend Potentially Breaking Changes

  • Removed support for constructing on-stack TemplateArgumentList``s; interfaces should instead use ``ArrayRef<TemplateArgument> to pass template arguments. Transitioning internal uses to ArrayRef<TemplateArgument> reduces AST memory usage by 0.4% when compiling clang, and is expected to show similar improvements on other workloads.
  • The -Wgnu-binary-literal diagnostic group no longer controls any diagnostics. Binary literals are no longer a GNU extension, they're now a C23 extension which is controlled via -pedantic or -Wc23-extensions. Use of -Wno-gnu-binary-literal will no longer silence this pedantic warning, which may break existing uses with -Werror.

What's New in Clang |release|?

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

C++20 Feature Support

  • Clang won't perform ODR checks for decls in the global module fragment any more to ease the implementation and improve the user's using experience. This follows the MSVC's behavior. Users interested in testing the more strict behavior can use the flag '-Xclang -fno-skip-odr-check-in-gmf'. (#GH79240).
  • Implemented the __is_layout_compatible intrinsic to support P0466R5: Layout-compatibility and Pointer-interconvertibility Traits.
  • Clang now implements [module.import]p7 fully. Clang now will import module units transitively for the module units coming from the same module of the current module units. Fixes #84002.
  • Initial support for class template argument deduction (CTAD) for type alias templates (P1814R0). (#GH54051).

C++23 Feature Support

C++2c Feature Support

Resolutions to C++ Defect Reports

C Language Changes

C23 Feature Support

  • No longer diagnose use of binary literals as an extension in C23 mode. Fixes #GH72017.

  • Corrected parsing behavior for the alignas specifier/qualifier in C23. We previously handled it as an attribute as in C++, but there are parsing differences. The behavioral differences are:

    struct alignas(8) /* was accepted, now rejected */ S {
  char alignas(8) /* was rejected, now accepted */ C;
};
int i alignas(8) /* was accepted, now rejected */ ;

    Fixes (#GH81472).

  • Clang now generates predefined macros of the form __TYPE_FMTB__ and __TYPE_FMTb__ (e.g., __UINT_FAST64_FMTB__) in C23 mode for use with macros typically exposed from <inttypes.h>, such as PRIb8. (#81896:).

  • Clang now supports N3018 The constexpr specifier for object definitions <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3018.htm>`_.

Non-comprehensive list of changes in this release

  • Added __builtin_readsteadycounter for reading fixed frequency hardware counters.
  • __builtin_addc, __builtin_subc, and the other sizes of those builtins are now constexpr and may be used in constant expressions.
  • Added __builtin_popcountg as a type-generic alternative to __builtin_popcount{,l,ll} with support for any unsigned integer type. Like the previous builtins, this new builtin is constexpr and may be used in constant expressions.

New Compiler Flags

  • -Wmissing-designated-field-initializers, grouped under -Wmissing-field-initializers. This diagnostic can be disabled to make -Wmissing-field-initializers behave like it did before Clang 18.x. Fixes (#56628)

Deprecated Compiler Flags

Modified Compiler Flags

  • Added a new diagnostic flag -Wreturn-mismatch which is grouped under -Wreturn-type, and moved some of the diagnostics previously controlled by -Wreturn-type under this new flag. Fixes #GH72116.

Removed Compiler Flags

  • The -freroll-loops flag has been removed. It had no effect since Clang 13.

Attribute Changes in Clang

  • Introduced a new function attribute __attribute__((amdgpu_max_num_work_groups(x, y, z))) or [[clang::amdgpu_max_num_work_groups(x, y, z)]] for the AMDGPU target. This attribute can be attached to HIP or OpenCL kernel function definitions to provide an optimization hint. The parameters x, y, and z specify the maximum number of workgroups for the respective dimensions, and each must be a positive integer when provided. The parameter x is required, while y and z are optional with default value of 1.

Improvements to Clang's diagnostics

  • Clang now applies syntax highlighting to the code snippets it prints.
  • Clang now diagnoses member template declarations with multiple declarators.
  • Clang now diagnoses use of the template keyword after declarative nested name specifiers.
  • The -Wshorten-64-to-32 diagnostic is now grouped under -Wimplicit-int-conversion instead
    of -Wconversion. Fixes #GH69444.
  • Clang now uses thousand separators when printing large numbers in integer overflow diagnostics. Fixes #GH80939.
  • Clang now diagnoses friend declarations with an enum elaborated-type-specifier in language modes after C++98.
  • Added diagnostics for C11 keywords being incompatible with language standards before C11, under a new warning group: -Wpre-c11-compat.
  • Now diagnoses an enumeration constant whose value is larger than can be represented by unsigned long long, which can happen with a large constant using the wb or uwb suffix. The maximal underlying type is currently unsigned long long, but this behavior may change in the future when Clang implements WG14 N3029. (#GH69352).
  • Clang now diagnoses extraneous template parameter lists as a language extension.
  • Clang now diagnoses declarative nested name specifiers that name alias templates.
  • Clang now diagnoses lambda function expressions being implicitly cast to boolean values, under -Wpointer-bool-conversion. Fixes #GH82512.

Improvements to Clang's time-trace

Bug Fixes in This Version

  • Fixed missing warnings when comparing mismatched enumeration constants in C (#29217 <llvm#29217>).
  • Clang now accepts elaborated-type-specifiers that explicitly specialize a member class template for an implicit instantiation of a class template.
  • Fixed missing warnings when doing bool-like conversions in C23 (#GH79435).
  • Clang's -Wshadow no longer warns when an init-capture is named the same as a class field unless the lambda can capture this. Fixes (#GH71976)
  • Clang now accepts qualified partial/explicit specializations of variable templates that are not nominable in the lookup context of the specialization.
  • Clang now doesn't produce false-positive warning -Wconstant-logical-operand for logical operators in C23. Fixes (#GH64356).
  • __is_trivially_relocatable no longer returns false for volatile-qualified types. Fixes (#GH77091).
  • Clang no longer produces a false-positive -Wunused-variable warning for variables created through copy initialization having side-effects in C++17 and later. Fixes (#GH64356) (#GH79518).
  • Clang now emits errors for explicit specializations/instatiations of lambda call operator. Fixes (#GH83267).
  • Clang now correctly generates overloads for bit-precise integer types for builtin operators in C++. Fixes #GH82998.

Bug Fixes to Compiler Builtins

Bug Fixes to Attribute Support

Bug Fixes to C++ Support

  • Fix crash when calling the constructor of an invalid class. (#GH10518) (#GH67914) (#GH78388)
  • Fix crash when using lifetimebound attribute in function with trailing return. (#GH73619)
  • Addressed an issue where constraints involving injected class types are perceived distinct from its specialization types. (#GH56482)
  • Fixed a bug where variables referenced by requires-clauses inside nested generic lambdas were not properly injected into the constraint scope. (#GH73418)
  • Fixed a crash where substituting into a requires-expression that refers to function parameters during the equivalence determination of two constraint expressions. (#GH74447)
  • Fixed deducing auto& from const int in template parameters of partial specializations. (#GH77189)
  • Fix for crash when using a erroneous type in a return statement. (#GH63244) (#GH79745)
  • Fixed an out-of-bounds error caused by building a recovery expression for ill-formed function calls while substituting into constraints. (#GH58548)
  • Fix incorrect code generation caused by the object argument of static operator() and static operator[] calls not being evaluated. (#GH67976)
  • Fix crash and diagnostic with const qualified member operator new. Fixes (#GH79748)
  • Fixed a crash where substituting into a requires-expression that involves parameter packs during the equivalence determination of two constraint expressions. (#GH72557)
  • Fix a crash when specializing an out-of-line member function with a default parameter where we did an incorrect specialization of the initialization of the default parameter. (#GH68490)
  • Fix a crash when trying to call a varargs function that also has an explicit object parameter. Fixes (#GH80971)
  • Reject explicit object parameters on new and delete operators. (#GH82249)
  • Fix a crash when trying to call a varargs function that also has an explicit object parameter. (#GH80971)
  • Fixed a bug where abbreviated function templates would append their invented template parameters to an empty template parameter lists.
  • Fix parsing of abominable function types inside type traits. Fixes (#77585)
  • Clang now classifies aggregate initialization in C++17 and newer as constant or non-constant more accurately. Previously, only a subset of the initializer elements were considered, misclassifying some initializers as constant. Partially fixes #GH80510.
  • Clang now ignores top-level cv-qualifiers on function parameters in template partial orderings. (#GH75404)
  • No longer reject valid use of the _Alignas specifier when declaring a local variable, which is supported as a C11 extension in C++. Previously, it was only accepted at namespace scope but not at local function scope.
  • Clang no longer tries to call consteval constructors at runtime when they appear in a member initializer. (#GH82154)
  • Fix crash when using an immediate-escalated function at global scope. (#GH82258)
  • Correctly immediate-escalate lambda conversion functions. (#GH82258)
  • Fixed an issue where template parameters of a nested abbreviated generic lambda within a requires-clause lie at the same depth as those of the surrounding lambda. This, in turn, results in the wrong template argument substitution during constraint checking. (#GH78524)
  • Clang no longer instantiates the exception specification of discarded candidate function templates when determining the primary template of an explicit specialization.
  • Fixed a crash in Microsoft compatibility mode where unqualified dependent base class lookup searches the bases of an incomplete class.
  • Fix a crash when an unresolved overload set is encountered on the RHS of a .* operator. (#GH53815)
  • In __restrict-qualified member functions, attach __restrict to the pointer type of this rather than the pointee type. Fixes (#GH82941), (#GH42411) and (#GH18121).
  • Clang now properly reports supported C++11 attributes when using __has_cpp_attribute and parses attributes with arguments in C++03 (#GH82995)
  • Clang now properly diagnoses missing 'default' template arguments on a variety of templates. Previously we were diagnosing on any non-function template instead of only on class, alias, and variable templates, as last updated by CWG2032. Fixes (#GH83461)
  • Fixed an issue where an attribute on a declarator would cause the attribute to be destructed prematurely. This fixes a pair of Chromium that were brought to our attention by an attempt to fix in (#GH77703). Fixes (#GH83385).
  • Fix evaluation of some immediate calls in default arguments. Fixes (#GH80630)
  • Fixed an issue where the RequiresExprBody was involved in the lambda dependency calculation. (#GH56556), (#GH82849).
  • Fix a bug where overload resolution falsely reported an ambiguity when it was comparing a member-function against a non member function or a member-function with an explicit object parameter against a member function with no explicit object parameter when one of the function had more specialized templates. Fixes (#82509) and (#74494)
  • Clang now supports direct lambda calls inside of a type alias template declarations. This addresses (#GH70601), (#GH76674), (#GH79555), (#GH81145) and (#GH82104).
  • Allow access to a public template alias declaration that refers to friend's private nested type. (#GH25708).

Bug Fixes to AST Handling

Miscellaneous Bug Fixes

Miscellaneous Clang Crashes Fixed

  • Do not attempt to dump the layout of dependent types or invalid declarations when -fdump-record-layouts-complete is passed. Fixes (#83684).

OpenACC Specific Changes

Target Specific Changes

AMDGPU Support

X86 Support

Arm and AArch64 Support

  • ARMv7+ targets now default to allowing unaligned access, except Armv6-M, and Armv8-M without the Main Extension. Baremetal targets should check that the new default will work with their system configurations, since it requires that SCTLR.A is 0, SCTLR.U is 1, and that the memory in question is configured as "normal" memory. This brings Clang in-line with the default settings for GCC and Arm Compiler. Aside from making Clang align with other compilers, changing the default brings major performance and code size improvements for most targets. We have not changed the default behavior for ARMv6, but may revisit that decision in the future. Users can restore the old behavior with -m[no-]unaligned-access.
  • An alias identifier (rdma) has been added for targeting the AArch64 Architecture Extension which uses Rounding Doubling Multiply Accumulate instructions (rdm). The identifier is available on the command line as a feature modifier for -march and -mcpu as well as via target attributes like target_version or target_clones.
  • Support has been added for the following processors (-mcpu identifiers in parenthesis):
    • Arm Cortex-A78AE (cortex-a78ae).

Android Support

Windows Support

LoongArch Support

RISC-V Support

  • __attribute__((rvv_vector_bits(N))) is now supported for RVV vbool*_t types.

CUDA/HIP Language Changes

  • PTX is no longer included by default when compiling for CUDA. Using --cuda-include-ptx=all will return the old behavior.

CUDA Support

AIX Support

WebAssembly Support

AVR Support

DWARF Support in Clang

Floating Point Support in Clang

Fixed Point Support in Clang

AST Matchers

  • isInStdNamespace now supports Decl declared with extern "C++".
  • Add isExplicitObjectMemberFunction.
  • Fixed forEachArgumentWithParam and forEachArgumentWithParamType to not skip the explicit object parameter for operator calls.

clang-format

  • AlwaysBreakTemplateDeclarations is deprecated and renamed to BreakTemplateDeclarations.
  • AlwaysBreakAfterReturnType is deprecated and renamed to BreakAfterReturnType.

libclang

Static Analyzer

New features

Crash and bug fixes

Improvements

  • Support importing C++20 modules in clang-repl.
  • Added support for TypeLoc::dump() for easier debugging, and improved textual and JSON dumping for various TypeLoc-related nodes.

Moved checkers

Sanitizers

  • -fsanitize=signed-integer-overflow now instruments signed arithmetic even when -fwrapv is enabled. Previously, only division checks were enabled.

    Users with -fwrapv as well as a sanitizer group like -fsanitize=undefined or -fsanitize=integer enabled may want to manually disable potentially noisy signed integer overflow checks with -fno-sanitize=signed-integer-overflow

Python Binding Changes

  • Exposed CXRewriter API as class Rewriter.

OpenMP Support

  • Added support for the [[omp::assume]] attribute.

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.

If you have any questions or comments about Clang, please feel free to contact us on the Discourse forums (Clang Frontend category).