Clang modules cannot compile most recent Windows SDK header

[kamrann]

Recently Microsoft appears to have made 10.0.26100.0 the default version of the SDK. A change to wchar.h in this version has broken projects that were previously compiling successfully with clang on Windows with C++20 modules enabled.

Simple repro:

// repro.mxx

module;
#include <cwchar>

export module repro;

export inline wchar_t const* func()
{
    return wmemchr(L"x", L'x', 1);
}

// source.cxx

import repro;

#include <cwchar>

Output:

In file included from C:\Code\build2-test-projects\hello\hello\clang\source.cxx:4:
In file included from C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.44.34823\include\cwchar:12:
C:\Program Files (x86)\Windows Kits\10\Include\10.0.26100.0\ucrt\wchar.h:209:31: error: redefinition of '_Avx2WmemEnabledWeakValue'
  209 |     __declspec(selectany) int _Avx2WmemEnabledWeakValue = 0;
      |                               ^
C:\Program Files (x86)\Windows Kits\10\Include\10.0.26100.0\ucrt\wchar.h:209:31: note: previous definition is here
  209 |     __declspec(selectany) int _Avx2WmemEnabledWeakValue = 0;
      |                               ^
1 error generated.

The issue seems to relate to handling of the Microsoft-specific __declspec(selectany).

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Cameron Angus (kamrann)

Recently Microsoft appears to have made `10.0.26100.0` the default version of the SDK. A change to `wchar.h` in this version has broken projects that were previously compiling successfully with clang on Windows with C++20 modules enabled.

Simple repro:

// repro.mxx

module;
#include <cwchar>

export module repro;

export inline wchar_t const* func()
{
    return wmemchr(L"x", L'x', 1);
}

// source.cxx

import repro;

#include <cwchar>

Output:

In file included from C:\Code\build2-test-projects\hello\hello\clang\source.cxx:4:
In file included from C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.44.34823\include\cwchar:12:
C:\Program Files (x86)\Windows Kits\10\Include\10.0.26100.0\ucrt\wchar.h:209:31: error: redefinition of '_Avx2WmemEnabledWeakValue'
  209 |     __declspec(selectany) int _Avx2WmemEnabledWeakValue = 0;
      |                               ^
C:\Program Files (x86)\Windows Kits\10\Include\10.0.26100.0\ucrt\wchar.h:209:31: note: previous definition is here
  209 |     __declspec(selectany) int _Avx2WmemEnabledWeakValue = 0;
      |                               ^
1 error generated.

The issue seems to relate to handling of the Microsoft-specific __declspec(selectany).

[ChuanqiXu9]

Could you try to reduce it without windows environment?

[kamrann]

It's a bit of a weird one. Eventually got it down to this:

// repro_decl.hxx
#pragma once

extern "C"
{
    __declspec(selectany) int foo = 0;
}

// repro.ixx
module;
#include "repro_decl.hxx"

export module repro;

export inline int func()
{
    return foo;
}

// source.cxx
import repro;

#include "repro_decl.hxx" // redefinition error here

[ChuanqiXu9]

Take a quick look. I can't understand why MSVC just don't use inline or weak linkage in this case.

[ChuanqiXu9]

/cherry-pick 24c06a1