Compile failures with clang-cl version 9 (MSVC 2019).

[KineticTheory]

FYI, I can't compile the current version of mdspan when using clang-cl.exe that ships with Visual Studio 2019 (LLVM clang version 9.0). The first couple of build errors are shown below.

C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\Llvm\bin\clang-cl.exe /c /Z7 /nologo /W0 /WX- /diagnostics:column /FS /Od /D WIN32 /D _WINDOWS /D _CRT_SECURE_NO_DEPRECATE /D _SCL_SECURE_NO_DEPRECATE /D _SECURE_SCL=0 /D DEBUG /D _DEBUG/MDd /D _DEBUG /D _HAS_ITERATOR_DEBUGGING=0 /D __STDC_CONSTANT_MACROS /D _POSIX_C_SOURCE=200112 /D _XOPEN_SOURCE=600 /D "CMAKE_INTDIR=\"Debug\"" /D _MBCS /EHa /MDd /GS /Gy /fp:precise /std:c++14 /Fo"Ut_experimental_tst_mdspan_exe.dir\Debug\\" /Gd /TP -m64   /Zc:__cplusplus C:\work\KineticTheory\Draco\src\experimental\test\tst_mdspan.cc
           In file included from C:\work\KineticTheory\Draco\src\experimental\test\tst_mdspan.cc:12:
           In file included from C:\work\KineticTheory\Draco\src\experimental/mdspan:61:
C:\work\KineticTheory\Draco\src\experimental\__p0009_bits\array_workaround.hpp(112,3): error : too many arguments provided to function-like macro invocation 
         C:\work\KineticTheory\Draco\src\experimental/__p0009_bits/macros.hpp(150,3): message : expanded from macro 'MDSPAN_FUNCTION_REQUIRES' [C:\work\vs2019-x64-Clang-Debug\draco\src\experimental\test\Ut_experimental_tst_mdspan_exe.vcxproj]
         C:\work\KineticTheory\Draco\src\experimental/__p0009_bits/macros.hpp(158,17): message : expanded from macro 'MDSPAN_TEMPLATE_REQUIRES' [C:\work\vs2019-x64-Clang-Debug\draco\src\experimental\test\Ut_experimental_tst_mdspan_exe.vcxproj]
         C:\work\KineticTheory\Draco\src\experimental/__p0009_bits/macros.hpp(124,29): message : expanded from macro 'MDSPAN_PP_CAT' [C:\work\vs2019-x64-Clang-Debug\draco\src\experimental\test\Ut_experimental_tst_mdspan_exe.vcxproj]
           note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
         (107,1): message : expanded from here [C:\work\vs2019-x64-Clang-Debug\draco\src\experimental\test\Ut_experimental_tst_mdspan_exe.vcxproj]
         C:\work\KineticTheory\Draco\src\experimental/__p0009_bits/macros.hpp(170,17): message : expanded from macro 'MDSPAN_TEMPLATE_REQUIRES_2' [C:\work\vs2019-x64-Clang-Debug\draco\src\experimental\test\Ut_experimental_tst_mdspan_exe.vcxproj]
         C:\work\KineticTheory\Draco\src\experimental/__p0009_bits/macros.hpp(147,35): message : expanded from macro 'MDSPAN_CLOSE_ANGLE_REQUIRES' [C:\work\vs2019-x64-Clang-Debug\draco\src\experimental\test\Ut_experimental_tst_mdspan_exe.vcxproj]
         C:\work\KineticTheory\Draco\src\experimental/__p0009_bits/macros.hpp(123,9): message : macro 'MDSPAN_PP_CAT_IMPL' defined here [C:\work\vs2019-x64-Clang-Debug\draco\src\experimental\test\Ut_experimental_tst_mdspan_exe.vcxproj]
           In file included from C:\work\KineticTheory\Draco\src\experimental\test\tst_mdspan.cc:12:
           In file included from C:\work\KineticTheory\Draco\src\experimental/mdspan:61:
C:\work\KineticTheory\Draco\src\experimental\__p0009_bits\array_workaround.hpp(112,3): error : unknown type name 'MDSPAN_PP_CAT_IMPL' 
         C:\work\KineticTheory\Draco\src\experimental/__p0009_bits/macros.hpp(150,3): message : expanded from macro 'MDSPAN_FUNCTION_REQUIRES' [C:\work\vs2019-x64-Clang-Debug\draco\src\experimental\test\Ut_experimental_tst_mdspan_exe.vcxproj]
         C:\work\KineticTheory\Draco\src\experimental/__p0009_bits/macros.hpp(158,3): message : expanded from macro 'MDSPAN_TEMPLATE_REQUIRES' [C:\work\vs2019-x64-Clang-Debug\draco\src\experimental\test\Ut_experimental_tst_mdspan_exe.vcxproj]
         C:\work\KineticTheory\Draco\src\experimental/__p0009_bits/macros.hpp(124,29): message : expanded from macro 'MDSPAN_PP_CAT' [C:\work\vs2019-x64-Clang-Debug\draco\src\experimental\test\Ut_experimental_tst_mdspan_exe.vcxproj]
           In file included from C:\work\KineticTheory\Draco\src\experimental\test\tst_mdspan.cc:12:
           In file included from C:\work\KineticTheory\Draco\src\experimental/mdspan:61:

[dhollman]

Hmmm...this is interesting. It looks to me like clang-cl.exe may think that it's MSVC when it comes to preprocessing (since MSVC has some weird preprocessor behavior I had to account for). I don't have easy access to a Windows machine to test this, but I'll poke around and see what I can find.

[KineticTheory]

@dhollman This is new territory for me too. Here is what I have found so far...

• CMake's MSVC and WIN32 variables are both true for cl.exe and clang-cl.exe.
• CMake's CMAKE_<LANG>_COMPILER_ID is MSVC or Clang for cl.exe and clang-cl.exe, respectively.

[dhollman]

This was fixed in pull request #25. The problem was that clang sets the _MSC_VER variable even though it actually has a working C preprocessor, unlike MSVC.