MSVC: static struct std::atomic<bool> Test::g_test has different type

Question

I had the following warning with my Visual Studio 17 (2022) project, and I could reduce it to the following:

test1.cpp

#include <atomic>
#include "test.h"

int main() {
    Test::g_test = true;
}

test2.cpp

#include <atomic>

struct A {
    std::atomic<bool> m_test = false;
};

#include "test.h"

void a() {
    Test::g_test = true;
}

test.h

#pragma once

struct Test {
    static inline std::atomic<bool> g_test = false;
};

Result:

1>------ Build started: Project: ConsoleApplication1, Configuration: Release x64 ------
1>test1.cpp
1>test2.cpp
1>LINK : warning C4744: 'static struct std::atomic<bool> Test::g_test' has different type in 'c:\consoleapplication1\test2.cpp' and 'c:\consoleapplication1\test1.cpp': '__declspec(align(1)) struct (1 bytes)' and 'struct (1 bytes)'

Am I violating some C++ rules? Is it an MSVC bug? What would be the best fix/workaround?

Answer 1

It's a compiler bug. See https://github.com/microsoft/STL/issues/3241 and https://developercommunity.visualstudio.com/t/10207002.

Reduced from STL bug report https://github.com/microsoft/STL/issues/3241, this seems to actually be a compiler issue.

Create file a.cpp:

template <class T> struct atomic { alignas(sizeof(T)) T x; };

struct S0 { static inline atomic<bool> z; };

int main() { (void) S0::z; }

and file b.cpp:

template <class T> struct atomic { alignas(sizeof(T)) T x; };

struct S1 { atomic<bool> y; };

struct S0 { static inline atomic<bool> z; };

void f() { (void) S0::z; }

then compile with “cl /nologo /std:c++17 /GL a.cpp b.cpp” which emits:

a.cpp
b.cpp
warning C4744: 'static struct atomic<bool> S0::z' has different type in 'b.cpp' and 'a.cpp': '__declspec(align(1)) struct (1 bytes)' and 'struct (1 bytes)'
Generating code
Finished generating code

Notably, no warning is emitted if the declarations of S1 and S0 are reordered in b.cpp.