[all-commits] [llvm/llvm-project] 705b99: Allow weak/selectany external definitions in heade...

[akrieger via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 705b99607c0c1aadc75fddef8738f22be206da25
      https://github.com/llvm/llvm-project/commit/705b99607c0c1aadc75fddef8738f22be206da25
  Author: akrieger <andrewskrieger at gmail.com>
  Date:   2025-10-16 (Thu, 16 Oct 2025)

  Changed paths:
    M clang/lib/Sema/SemaDecl.cpp
    M clang/test/CXX/module/module.import/p6.cpp

  Log Message:
  -----------
  Allow weak/selectany external definitions in header units. (#162713)

weak and selectany are mechanisms for allowing the linker to resolve ODR
violations. [module.import/6] states

> A header unit shall not contain a definition of a non-inline function
or variable whose name has external linkage.

But this prevents compiling any headers with such weak symbols defined.
These occur in eg. some Windows SDK headers like `DirectXMath.h`.

```
#ifndef XMGLOBALCONST
#if defined(__GNUC__) && !defined(__MINGW32__)
#define XMGLOBALCONST extern const __attribute__((weak))
#else
#define XMGLOBALCONST extern const __declspec(selectany)
#endif
#endif

    XMGLOBALCONST XMVECTORF32 g_XMSinCoefficients0 = { { { -0.16666667f, +0.0083333310f, -0.00019840874f, +2.7525562e-06f } } };
```

Proposed solution: Do not emit `diag::err_extern_def_in_header_unit` if
the `FD` or `VDecl` have either `SelectAnyAttr` or `WeakAttr`.



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