Demonstration of mdspan failure on MSVC 2019

• Using:
  • Visual Studio

Microsoft Visual Studio Community 2019 Preview
Version 16.8.0 Preview 2.1

• mdspan hash 371250b2341e6196b823337e4ea4d3474c879310
  • I added an #ifdef old_msvc to traits_backports.hpp to elminate some build errors related to duplicate symbols.

Reproducer

• Review CMakeSettigns.json. Some paths might need to be modified for your local system.
• Start Visual Studio and open the folder containing the sources.
• Project -> Generate Cache

1> CMake generation started for configuration: 'x64-Debug'.
1> Found and using vcpkg toolchain file (C:/Tools/vcpkg/scripts/buildsystems/vcpkg.cmake).
1> The toolchain file has changed (CMAKE_TOOLCHAIN_FILE).
1> Environment settings:
1> Command line: "cmd.exe" /c "%SYSTEMROOT%\System32\chcp.com 65001 >NUL && "C:\Program Files\CMake\bin\cmake.exe"  -G "Visual Studio 16 2019" -A x64  -DCMAKE_CONFIGURATION_TYPES:STRING="Debug" -DCMAKE_INSTALL_PREFIX:PATH="C:\Users\kt\vs2019/x64-Debug/install/d"  -DCMAKE_TOOLCHAIN_FILE="C:/Tools/vcpkg/scripts/buildsystems/vcpkg.cmake" "C:\work\KineticTheory\mdspan-msvc2019" 2>&1"
1> Working directory: C:\Users\kt\vs2019/x64-Debug/mdspan-msvc2019
1> [CMake] -- Selecting Windows SDK version 10.0.18362.0 to target Windows 10.0.19042.
1> [CMake] -- The CXX compiler identification is MSVC 19.28.29213.0
1> [CMake] -- Detecting CXX compiler ABI info
1> [CMake] -- Detecting CXX compiler ABI info - done
1> [CMake] -- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Preview/VC/Tools/MSVC/14.28.29213/bin/Hostx64/x64/cl.exe - skipped
1> [CMake] -- Detecting CXX compile features
1> [CMake] -- Detecting CXX compile features - done
1> [CMake] -- Configuring done
1> [CMake] -- Generating done
1> [CMake] -- Build files have been written to: C:/Users/kt/vs2019/x64-Debug/mdspan-msvc2019
1> Extracted CMake variables.
1> Extracted source files and headers.
1> Extracted code model.
1> Extracted includes paths.
1> CMake generation finished.

• Build -> Build All

>------ Build All started: Project: mdspan-msvc2019, Configuration: x64-Debug ------
  Microsoft (R) Build Engine version 16.8.0-preview-20411-05+9a32a8063 for .NET Framework
  Copyright (C) Microsoft Corporation. All rights reserved.
  
    Checking Build System
    Building Custom Rule C:/work/KineticTheory/mdspan-msvc2019/CMakeLists.txt
    tst_mdspan.cc
C:\work\KineticTheory\mdspan-msvc2019\__p0009_bits\subspan.hpp(469,1): error C3546: '...': there are no parameter packs available to expand 
C:\work\KineticTheory\mdspan-msvc2019\__p0009_bits\extents.hpp(182,1): error C3546: '...': there are no parameter packs available to expand 
  C:\work\KineticTheory\mdspan-msvc2019\__p0009_bits/basic_mdspan.hpp(143): message : see reference to class template instantiation 'std::experimental::extents<-1,-1>' being compiled [C:\Users\kt\vs2019\x64-Debug\mdspan-msvc2019\mdspan_test_exe.vcxproj]
  C:\work\KineticTheory\mdspan-msvc2019\tst_mdspan.cc(75): message : see reference to class template instantiation 'std::experimental::basic_mdspan<int,std::experimental::extents<-1,-1>,std::experimental::layout_right,std::experimental::accessor_basic<int>>' being compiled [C:\Users\kt\vs2019\x64-Debug\mdspan-msvc2019\mdspan_test_exe.vcxproj]
C:\work\KineticTheory\mdspan-msvc2019\__p0009_bits\basic_mdspan.hpp(135,1): error C3546: '...': there are no parameter packs available to expand 
C:\work\KineticTheory\mdspan-msvc2019\__p0009_bits\basic_mdspan.hpp(244,1): error C3546: '...': there are no parameter packs available to expand 
C:\work\KineticTheory\mdspan-msvc2019\tst_mdspan.cc(75,53): error C2440: '<function-style-cast>': cannot convert from 'initializer list' to 'span_2d_dynamic' 
  C:\work\KineticTheory\mdspan-msvc2019\tst_mdspan.cc(75,53): message : No constructor could take the source type, or constructor overload resolution was ambiguous [C:\Users\kt\vs2019\x64-Debug\mdspan-msvc2019\mdspan_test_exe.vcxproj]

• (output truncated)