cxxstd not working when building using b2 under Windows

[alervd]

Please provide the following information

libtorrent version (or branch): 1.2.19.

platform/architecture: Windows 10.

compiler and compiler version: Microsoft Visual C++ 2022.

please describe what symptom you see, what you would expect to see instead and
how to reproduce it.

1. Download prebuild boost 1.85.0 binaries from the official site.
2. Build b2 as stated in the docs (https://www.libtorrent.org/building.html).
3. Try to build boost with b2 cxxstd=20.

Result:

compile-c-c++ bin\msvc-14.3\debug\crypto-openssl\cxxstd-20-iso\deprecated-functions-off\link-static\logging-off\threading-multi\src\block_cache.obj
block_cache.cpp
D:\Work\Source\libtorrent-build\openssl3\libtorrent-x86\include\libtorrent/units.hpp(168): error C2039: 'result_type': is not a member of 'std::hash<int>'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\system_error(410): note: see declaration of 'std::hash<int>'

I've found that when it builds libtorrent source files, __cplusplus is set to 199711L (however as you can see above it's using c++20 somewhere else so we get a conflict).

It works with cxxstd=11 (and I think 14 too), because the "culprit" was not deprecated in this standards yet.

[alervd]

It seems this is caused by this: https://learn.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=msvc-170.
Is there a way to pass this compiler switch to b2?

[alervd]

So, it's required to pass cxxflags="\"/Zc:__cplusplus\"" for the build to work properly under MSVC compiler. Please update docs / fix build system so one do not have to specify it manually.

So, the working command would be something like b2.exe cxxstd=20 cxxflags="\"/Zc:__cplusplus\"".

[arvidn]

I believe this is related #7545

[arvidn]

I believe the RC_2_0 branch builds as C++20 (but with some warnings)