<execution>: Including with /clr enabled results in compiler error.

[AlexGuteniev]

Describe the bug
Including <execution> and compiling with /clr emits a lot of compiler errors.

Command-line test case

d:\Temp2>type repro.cpp
#include <execution>

int main() {}

d:\Temp2>cl /clr /std:c++17 repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.27.29009.1
for Microsoft (R) .NET Framework version 4.08.4084.0
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.cpp
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(401): error C3646: '_Mtx': unknown override specifier
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(428): note: see reference to class template instantiation 'std::_Parallel_choose_min_chunk<_Ty>' being compiled
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(401): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(436): error C3646: '_Mtx': unknown override specifier
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(463): note: see reference to class template instantiation 'std::_Parallel_choose_max_chunk<_Ty>' being compiled
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(436): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(647): error C3646: '_Segment_lock': unknown override specifier
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(648): note: see reference to class template instantiation 'std::_Work_stealing_deque<_Ty>' being compiled
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(647): error C2059: syntax error: '{'
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(647): error C2334: unexpected token(s) preceding '{'; skipping apparent function body
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(749): error C3646: '_Available_mutex': unknown override specifier
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(751): note: see reference to class template instantiation 'std::_Work_stealing_team<_Ty>' being compiled
c:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\MSVC\14.27.29009\include\execution(749): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

Expected behavior
Either program compiles, or a short error message is emitted.
Depending on whether it is expected to compile,
(Generic issue for this question is #838 , but a specific answer should be enough)

STL version

Microsoft Visual Studio Professional 2019 Preview
Version 16.7.0 Preview 3.1

Additional context
This item is also tracked on Developer Community as DevCom-642475 and by Microsoft-internal VSO-947547 / AB#947547.

[StephanTLavavej]

What's happening is that <mutex> isn't completely blocked under /clr, it just declines to provide std::mutex etc. Then <execution> unconditionally needs std::mutex.

[CaseyCarter]

Then <execution> unconditionally needs std::mutex.

Which is peculiar, because there's some conditional compilation near the top of <execution>:

STL/stl/inc/execution

Lines 37 to 45 in 5be7d49

	#ifdef _M_CEE
	using __std_TP_WORK = void;
	using __std_TP_CALLBACK_INSTANCE = void;
	using __std_TP_CALLBACK_ENVIRON = void;
	#else // ^^^ _M_CEE ^^^ // vvv !_M_CEE vvv
	struct __std_TP_WORK; // not defined
	struct __std_TP_CALLBACK_INSTANCE; // not defined
	struct __std_TP_CALLBACK_ENVIRON; // not defined
	#endif // _M_CEE

that suggests this header is intended to tolerate /clr. We need to make up our mind ;)