std::filesystem::directory_iterator code compiles OK with <filesytem> fails with std module, 17.5 Preview 2

[davidhunter22]

The following code compiles OK when using the <filesystem> header but fails when using import std;. This is with 17.5 Preview 2. The only changes to a default C++ project in VS was /std:c++latest and adding the std.ixx file

import std;

// #include <filesystem>

int main( )
{
    for (auto a : std::filesystem::directory_iterator( "C:\\" )) { }

    return 0;
}

The error message when using the std module is

1>C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.35.32124\include\type_traits(2102,13): error C2678: binary '&': no operator found which takes a left-hand operand of type '_Bitmask' (or there is no acceptable conversion)
1>        with
1>        [
1>            _Bitmask=__std_fs_file_attr
1>        ]
1>C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.35.32124\include\cstddef(49,39): message : could be 'std::byte std::operator &(const std::byte,const std::byte) noexcept'
1>C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.35.32124\include\type_traits(2102,13): message : while trying to match the argument list '(_Bitmask, _Bitmask)'
1>        with
1>        [
1>            _Bitmask=__std_fs_file_attr
1>        ]
1>C:\Users\david\devel\bugs\mudule_filesystem\Project1\Test.cpp(10,1): message : see reference to function template instantiation 'bool std::_Bitmask_includes_any<__std_fs_file_attr>(_Bitmask,_Bitmask) noexcept' being compiled
1>        with
1>        [
1>            _Bitmask=__std_fs_file_attr
1>        ]
1>Done building project "Project1.vcxproj" -- FAILED.

Which seemed really odd to me.

[fsb4000]

If I change the code

STL/stl/inc/xfilesystem_abi.h

Line 82 in 8ddf4da

_BITMASK_OPS(_EMPTY_ARGUMENT, __std_fs_file_attr)

to

_BITMASK_OPS(_EXPORT_STD, __std_fs_file_attr)

then your example works for me.

But I am unsure should we add _EXPORT_STD or is this a compiler bug?

[davidhunter22]

Beyond my pay grade! What got me was why it is "operator &" in what I assume is an assignment of the dereferenced directory iterator. operator & to me would normally be a boolean and of the bits in the attribute flags.

Anyway I'll leave it to the experts. My only other comment is that it's a bit annoying that the error message doesn't tell me what line in my code is causing the issue. The line number in my code in the the error is simply the last one in the main function not the one causing the error.

[StephanTLavavej]

Igor Zhukov (@fsb4000)'s library workaround, and Cameron DaCamara (@cdacamar)'s compiler fix for the root cause, will both ship in VS 2022 17.6 Preview 2. Thanks again for the report.

[matbech]

In 17.5 Preview 3, I do have the same issue with __std_fs_stats_flags

1>C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.35.32213\include\type_traits(2102,13): error C2678: binary '&': no operator found which takes a left-hand operand of type '_Bitmask' (or there is no acceptable conversion)
1>        with
1>        [
1>            _Bitmask=__std_fs_stats_flags
1>        ]

With the same library workaround as with __std_fs_file_attr it compiles:

#if 1 // TRANSITION, DevCom-10247495
_BITMASK_OPS(_EXPORT_STD, __std_fs_stats_flags)
#else // ^^^ workaround / no workaround vvv
_BITMASK_OPS(_EMPTY_ARGUMENT, __std_fs_stats_flags)
#endif // ^^^ no workaround ^^^