ICE with VisualStudio 16.5.4

[BobSteagall]

#include <experimental/mdspan> in an otherwise empty source file yields the following set of error messages and an ICE:

1>------ Build started: Project: lin_alg_test, Configuration: Debug x64 ------
1>pch.cpp
1>D:\Repos\wg21\linear_algebra\code\extsrc\mdspan\include\experimental\__p0009_bits\trait_backports.hpp(64,1): error C2371: 'std::is_assignable_v': redefinition; different basic types
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\type_traits(683): message : see declaration of 'std::is_assignable_v'
1>D:\Repos\wg21\linear_algebra\code\extsrc\mdspan\include\experimental\__p0009_bits\trait_backports.hpp(65,1): error C2371: 'std::is_constructible_v': redefinition; different basic types
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\type_traits(636): message : see declaration of 'std::is_constructible_v'
1>D:\Repos\wg21\linear_algebra\code\extsrc\mdspan\include\experimental\__p0009_bits\trait_backports.hpp(66,1): error C2371: 'std::is_convertible_v': redefinition; different basic types
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\type_traits(473): message : see declaration of 'std::is_convertible_v'
1>D:\Repos\wg21\linear_algebra\code\extsrc\mdspan\include\experimental\__p0009_bits\trait_backports.hpp(67,1): error C2371: 'std::is_default_constructible_v': redefinition; different basic types
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\type_traits(654): message : see declaration of 'std::is_default_constructible_v'
1>D:\Repos\wg21\linear_algebra\code\extsrc\mdspan\include\experimental\__p0009_bits\trait_backports.hpp(68,1): error C2371: 'std::is_trivially_destructible_v': redefinition; different basic types
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\type_traits(822): message : see declaration of 'std::is_trivially_destructible_v'
1>D:\Repos\wg21\linear_algebra\code\extsrc\mdspan\include\experimental\__p0009_bits\trait_backports.hpp(69,1): error C2371: 'std::is_same_v': redefinition; different basic types
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\xtr1common(79): message : see declaration of 'std::is_same_v'
1>D:\Repos\wg21\linear_algebra\code\extsrc\mdspan\include\experimental\__p0009_bits\trait_backports.hpp(70,1): error C2371: 'std::is_empty_v': redefinition; different basic types
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\type_traits(536): message : see declaration of 'std::is_empty_v'
1>D:\Repos\wg21\linear_algebra\code\extsrc\mdspan\include\experimental\__p0009_bits\trait_backports.hpp(71,1): error C2371: 'std::is_void_v': redefinition; different basic types
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\type_traits(156): message : see declaration of 'std::is_void_v'
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\xstring(4246,5): error C2976: 'std::_Compressed_pair': too few template arguments
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\xmemory(1319): message : see declaration of 'std::_Compressed_pair'
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\xstring(4555): message : see reference to class template instantiation 'std::basic_string<char,std::char_traits<char>,std::allocator<char>>' being compiled
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\xstring(4246,3): error C2955: 'std::_Compressed_pair': use of class template requires template argument list
1>C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\include\xmemory(1319): message : see declaration of 'std::_Compressed_pair'
1>D:\Repos\wg21\linear_algebra\code\extsrc\mdspan\include\experimental\__p0009_bits\basic_mdspan.hpp(264,1): fatal error C1903: unable to recover from previous error(s); stopping compilation
1>INTERNAL COMPILER ERROR in 'C:\DevTools\VisualStudio\2019\VC\Tools\MSVC\14.25.28610\bin\HostX86\x64\CL.exe'
1>    Please choose the Technical Support command on the Visual C++
1>    Help menu, or open the Technical Support help file for more information
1>Done building project "lin_alg_test.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

[crtrott]

Wait is this VisualStudio 16 or 19? or is the versioning just weird?

[BobSteagall]

This is Visual Studio Community 2019 (product name), version number 16.5.4 (i.e., the latest, at least as of last week). It's MS's numbering convention, sorry for any confusion.

[BobSteagall]

Yes, their numbering is weird, at least to me.....

[crtrott]

Ok we identified that __cplusplus is the problem: This thing asserts this MSVC:

static_assert(__cplusplus >= 201102L, "__cplusplus is too small: MDSpan requires C++11 or later.");

__cplusplus is defined to 199711L right now even though the compile line includes -std:c++latest.

[crtrott]

Here is the explanation: https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/

[dhollman]

Pull request #25 should have addressed this. Thanks for reporting this, and please let us know if you have any other issues!