Clang (In-Progress) Release Notes¶

Written by the LLVM Team

Warning

These are in-progress notes for the upcoming Clang 24 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 24.0.0git. 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 ¶

C/C++ Language Potentially Breaking Changes ¶

C++ Specific Potentially Breaking Changes ¶

ABI Changes in This Version ¶

AST Dumping Potentially Breaking Changes ¶

Clang Frontend Potentially Breaking Changes ¶

Templight support has been removed.

Clang Python Bindings Potentially Breaking Changes ¶

OpenCL Potentially Breaking Changes ¶

What’s New in Clang 24.0.0git?¶

C++ Language Changes ¶

C++2d Feature Support¶

C++2c Feature Support¶

Clang now supports P3533R2 (constexpr virtual inheritance).

C++23 Feature Support¶

C++20 Feature Support¶

C++17 Feature Support¶

Resolutions to C++ Defect Reports¶

C Language Changes ¶

C2y Feature Support¶

Clang now supports C2y’s new syntax for if and switch statements with initializer and condition variables, as specified in N3356_. For example:

  if (bool x = true; x) {
    // ...
  }

  if (bool x = true) {
    // ...
  }

  // attribute list on declarations are also supported
  switch ([[maybe_unused]] int x = 1) {
  default:
    // ...
  }

  if (bool x [[maybe_unused]] = true; x) {
    // ...
  }

C23 Feature Support¶

Objective-C Language Changes ¶

Non-comprehensive list of changes in this release ¶

New Compiler Flags ¶

Deprecated Compiler Flags ¶

Modified Compiler Flags ¶

Removed Compiler Flags ¶

Attribute Changes in Clang ¶

Improvements to Clang’s diagnostics ¶

Fixed bug in -Wdocumentation so that it correctly handles explicit function template instantiations (#64087).
Fixed concept template parameters not being recognized in -Wdocumentation when mentioned in tparam comments. (#64087)
-Wunused-but-set-variable now diagnoses file-scope variables with internal linkage (static storage class) that are assigned but never used. This new coverage is added under the subgroup -Wunused-but-set-global, allowing it to be disabled independently with -Wno-unused-but-set-global. (#148361)
-Wunused-template is now part of -Wunused (which is enabled by -Wall). It diagnoses unused function and variable templates with internal linkage, which in a header is a latent ODR hazard. It can be disabled with -Wno-unused-template. (#202945)
Added -Wlifetime-safety to enable lifetime safety analysis, a CFG-based intra-procedural analysis that detects use-after-free and related temporal safety bugs. See the RFC for more details. By design, this warning is enabled in -Weverything. To disable the analysis, use -Wno-lifetime-safety or -fno-lifetime-safety.

Added -Wlifetime-safety-suggestions to enable lifetime annotation suggestions. This provides suggestions for function parameters that should be marked [[clang::lifetimebound]] based on lifetime analysis. For example, for the following function:

int* p(int *in) { return in; }

Clang will suggest:

warning: parameter in intra-TU function should be marked [[clang::lifetimebound]]
int* p(int *in) { return in; }
       ^~~~~~~
               [[clang::lifetimebound]]
note: param returned here
int* p(int *in) { return in; }
                         ^~

Added -Wlifetime-safety-noescape to detect misuse of [[clang::noescape]] annotation where the parameter escapes through return. For example:

int* p(int *in [[clang::noescape]]) { return in; }

Clang will warn:

warning: parameter is marked [[clang::noescape]] but escapes
int* p(int *in [[clang::noescape]]) { return in; }
       ^~~~~~~
note: returned here
int* p(int *in [[clang::noescape]]) { return in; }
                                             ^~

Added -Wlifetime-safety-dangling-field to detect dangling field references when stack memory escapes to class fields. This is part of -Wlifetime-safety and detects cases where local variables or parameters are stored in fields but outlive their scope. For example:
```
struct DanglingView {
  std::string_view view;
  DanglingView(std::string s) : view(s) {}  // warning: address of stack memory escapes to a field
};
```
Improved -Wassign-enum performance by caching enum enumerator values. (#176454)
Fixed a false negative in -Warray-bounds where the warning was suppressed when accessing a member function on a past-the-end array element. (#179128)
Added a missing space to the FixIt for the implicit-int group of diagnostics and made sure that only one such diagnostic and FixIt is emitted per declaration group. (#179354)
Fixed the Fix-It insertion point for expected ';' after alias declaration when parsing alias declarations involving a token-split >> sequence (for example, using A = X<int>>;). (#184425)
Fixed incorrect implicitly deleted diagnostic for explicitly deleted candidate function. (#185693)
The -Wloop-analysis warning has been extended to catch more cases of variable modification inside lambda expressions (#132038).
Clang now emits -Wsizeof-pointer-memaccess when snprintf/vsnprintf use the sizeof the destination buffer(dynamically allocated) in the len parameter(#162366)
Added -Wmodule-map-path-outside-directory (off by default) to warn on header and umbrella directory paths that use .. to refer outside the module directory in module maps found via implicit search (-fimplicit-module-maps). This does not affect module maps specified explicitly via -fmodule-map-file=.
Honour [[maybe_unused]] attribute on private fields. -Wunused-private-field no longer emits a warning for annotated private fields.
Improved -Wgnu-zero-variadic-macro-arguments to suggest using __VA_OPT__ if the current language version supports it(#188624)
Clang now emits an error when implicitly casting a complex type to a built-in vector type. (#186805)
Added -Wnonportable-include-path-separator (off by default) to catch #include directives that use backslashes as a path separator. The warning includes a FixIt to change all the backslashes to forward slashes, so that the code can automatically be made portable to other host platforms that don’t support backslashes.
Clang now explains why template deduction fails for explicit template arguments.
No longer emitting a -Wpre-c2y-compat or extension diagnostic about use of octal literals with a 0o prefix, and no longer emitting a -Wdeprecated-octal-literals diagnostic for use of octal literals without a 0o prefix, when the literal is expanded from a macro defined in a system header. (#192389)
Improved error recovery for missing semicolons after class members. Clang now avoids skipping subsequent valid declarations when their previous decl is missing semicolon.
Removed the body of lambdas from some diagnostic messages.
Fixed false positive host-device mismatch errors in discarded if constexpr branches for CUDA/HIP; such calls are now correctly skipped.
Clang now errors when a function declaration aliases a variable or vice versa. (#195550)
Added -Wattribute-alias to diagnose type mismatches between an alias and its aliased function. (#195550)
The diagnostics around __block now explain why a variable cannot be marked __block. (#197213)
Extended -Wnonportable-include-path to warn about trailing whitespace and dots in #include paths. (#190610)
Clang now emits error when attribute is missing closing ]] followed by ;;. (#187223)
Clang now rejects inline asm constraints and clobbers that contain an embedded null character, instead of silently truncating them. (#173900)
Added -Wstringop-overread to warn when memcpy, memmove, memcmp, and related builtins read more bytes than the source buffer size (#83728).
Diagnostics for the C++11 range-based for statement now report the correct iterator type in notes for invalid iterator types.
-Wfortify-source now warns when the constant-evaluated argument to umask has bits set outside 0777. Those bits are silently discarded by the kernel, so setting them is almost always a typo (matching the bionic libc diagnose_if check).
Improved how Unicode characters are displayed in diagnostic messages.
-Wtautological-pointer-compare and -Wpointer-bool-conversion now diagnose a reference to a function (e.g. of type void (&)()) compared against or converted to a null pointer, the same as a bare function name. (#46362)

Improvements to Clang’s time-trace ¶

Improvements to Coverage Mapping ¶

Bug Fixes in This Version ¶

Fixed a constraint comparison bug in partial ordering. (#182671)
Fixed a rejected-valid case that used an explicit object parameter in an out-of-line definition of a nested class member. (#136472)

Bug Fixes to Compiler Builtins¶

Bug Fixes to Attribute Support¶

The counted_by/counted_by_or_null diagnostic that rejects a pointer whose pointee is a struct with a flexible array member (e.g. struct with_fam * __sized_by(size) ptr;) was incorrectly also applied to the sized_by/sized_by_or_null attributes. Because sized_by and sized_by_or_null describe the size in bytes rather than a count of elements, they are now correctly accepted on such pointers.

Bug Fixes to C++ Support¶

Fixed an issue where we tried to compare invalid NTTPs for variable declarations, which ended up in hitting an assertion with a constrained non-plain-auto NTTP, which we don’t quite implement yet. (#208658)
Fixed a crash when a using-declaration naming an unresolvable member of a dependent base was shadowed by an invalid using-declaration. (#209427)

Bug Fixes to AST Handling¶

Fixed a non-deterministic ordering of unused local typedefs that made serialized PCH/AST files and -Wunused-local-typedef diagnostics non-reproducible across runs. (#209639)

Miscellaneous Bug Fixes¶

Miscellaneous Clang Crashes Fixed¶

OpenACC Specific Changes ¶

OpenCL Specific Changes ¶

Extensions cl_khr_extended_bit_ops, cl_khr_integer_dot_product, cl_khr_subgroup_extended_types, cl_khr_subgroup_rotate, cl_khr_subgroup_shuffle, and cl_khr_subgroup_shuffle_relative are promoted to core features in OpenCL C 3.1. A target claiming OpenCL C 3.1 conformance without supporting one of these features is now diagnosed.

Target Specific Changes ¶

AMDGPU Support¶

Deprecated the following builtins in favor of __builtin_amdgcn_ballot_w32 or __builtin_amdgcn_ballot_w64:
- __builtin_amdgcn_uicmp
- __builtin_amdgcn_uicmpl
- __builtin_amdgcn_sicmpl
- __builtin_amdgcn_fcmp
- __builtin_amdgcn_fcmpf

NVPTX Support¶

X86 Support¶

Arm and AArch64 Support¶

On AArch64 Windows targets, -mbranch-protection=standard and -mbranch-protection=pac-ret now uses the B-key by default.

Android Support¶

Windows Support¶

LoongArch Support¶

RISC-V Support¶

CUDA/HIP Language Changes¶

CUDA Support¶

AIX Support¶

NetBSD Support¶

WebAssembly Support¶

AVR Support¶

SystemZ Support¶

DWARF Support in Clang ¶

Floating Point Support in Clang ¶

Fixed Point Support in Clang ¶

AST Matchers ¶

clang-format ¶

libclang ¶

Code Completion ¶

Static Analyzer ¶

Crash and bug fixes¶

Improvements¶

The lock-order-reversal check in alpha.unix.PthreadLock is now disabled by default. It can be re-enabled with the WarnOnLockOrderReversal option.

Moved checkers¶

Diagnostic changes¶

For self-assignments during initialization (T v = v;), core.uninitialized.Assign will not report them as uninitialized accesses (except C++ reference types), and the checks will be delayed until the first accesses of these variables; deadcode.DeadStores will not report them as dead stores. (#187530)

Sanitizers ¶

Python Binding Changes ¶

OpenMP Support ¶

Added parsing and semantic support for dims modifier in num_teams and thread_limit clauses for OpenMP 6.1 or later.

SYCL Support ¶

Improvements¶

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).