Building error: 'result_type': is not a member of 'std::hash<int>'

[dwarfovich]

Hi. I'm trying to build libtorrent RC_2_0 on Win11 with Boost_1_83_0. The command to build is:
b2 cxxstd=20 variant=release link=static runtime-link=static debug-symbols=on boost-link=static openssl-lib="C:\Program Files\OpenSSL\lib" openssl-include="C:\Program Files\OpenSSL\include" logging=off lto=on alert-msg=off deprecated-functions=off --hash.

It gives me a few errors like:

> C:\Boo\Code\libtorrent\include\libtorrent/units.hpp(177): error C2039: 'result_type': is not a member of 'std::hash<int>'
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.37.32822\include\system_error(450): note: see declaration of 'std::hash<int>'
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.37.32822\include\xhash(126): note: see reference to class template instantiation 'std::hash<libtorrent::piece_index_t>' being compiled
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.37.32822\include\unordered_map(30): note: see reference to class template instantiation 'std::_Uhash_compare<_Kty,_Hasher,_Keyeq>' being compiled
        with
        [
            _Kty=libtorrent::piece_index_t,
            _Hasher=std::hash<libtorrent::piece_index_t>,
            _Keyeq=std::equal_to<libtorrent::piece_index_t>
        ]
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.37.32822\include\xhash(335): note: see reference to class template instantiation 'std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>' being compiled
        with
        [
            _Kty=libtorrent::piece_index_t,
            _Ty=int,
            _Hasher=std::hash<libtorrent::piece_index_t>,
            _Keyeq=std::equal_to<libtorrent::piece_index_t>,
            _Alloc=std::allocator<std::pair<const libtorrent::piece_index_t,int>>
        ]
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.37.32822\include\unordered_map(67): note: see reference to class template instantiation 'std::_Hash<std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>>' being compiled
        with
        [
            _Kty=libtorrent::piece_index_t,
            _Ty=int,
            _Hasher=std::hash<libtorrent::piece_index_t>,
            _Keyeq=std::equal_to<libtorrent::piece_index_t>,
            _Alloc=std::allocator<std::pair<const libtorrent::piece_index_t,int>>
        ]
C:\Boo\Code\libtorrent\include\libtorrent/piece_picker.hpp(808): note: see reference to class template instantiation 'std::unordered_map<libtorrent::piece_index_t,int,std::hash<libtorrent::piece_index_t>,std::equal_to<libtorrent::piece_index_t>,std::allocator<std::pair<const libtorrent::piece_index_t,int>>>' being compiled

What may cause this error?

[dwarfovich]

It seems, that error occurs with flag cxxstd=20 or cxxstd=17 - after switching to cxxstd=14, I was able to build the lib. despite I tried to build b2 with commands b2.exe --hash cxxstd=17 release and b2.exe --hash cxxstd=20 release

[arvidn]

The code attempts to detect C++17 and later by:

#if __cplusplus < 201402

which apparently is failing on msvc. I believe msvc isn't always updating this macro to reflect the version.

I think this might fix the issue: #7545