MSVC With C++20 (Or Later) Calls Path Constructor With `nullptr`

[ajorians]

Please consider the following sample code:

#include <functional>
#include <boost/filesystem.hpp>

int main()
{
   std::function<void(boost::filesystem::path)> myfunc;

   if ( !myfunc )//OK compiles just fine
      return 0;

   if ( myfunc == nullptr )//Latest MSVC + /std:c++20 this fails to build
      return 0;
}

Here is an online link where you can see with Boost 1.80.0 it builds just fine: https://godbolt.org/z/e6zdc8WfM but that is because Compiler Explorer only offers up to version 1.80.0 at the time being. But if it had version 1.81.0 I claim you would see the error I will describe below.

Using either Visuals Studio 2022 as well as Visual Studio 2019 and compiling as C++20 I get the following build error:

Rebuild started... 1>------ Rebuild All started: Project: TestBoostFilesystem, Configuration: Debug x64 ------ 1>main.cpp 1>C:\Users\A.J. Orians\.CommonCppCache\repocache\boost-1.81.0-win\0d44f38a132c865549ff3aee332d2b8e46482759\include\boost\filesystem\detail\path_traits.hpp(495,52): error C2668: 'boost::filesystem::detail::path_traits::is_convertible_to_path_source::_check_convertible_to_path_source': ambiguous call to overloaded function 1>C:\Users\A.J. Orians\.CommonCppCache\repocache\boost-1.81.0-win\0d44f38a132c865549ff3aee332d2b8e46482759\include\boost\filesystem\detail\path_traits.hpp(481,21): message : could be 'boost::filesystem::detail::path_traits::yes_type boost::filesystem::detail::path_traits::is_convertible_to_path_source::_check_convertible_to_path_source(const wchar_t *)' 1>C:\Users\A.J. Orians\.CommonCppCache\repocache\boost-1.81.0-win\0d44f38a132c865549ff3aee332d2b8e46482759\include\boost\filesystem\detail\path_traits.hpp(480,21): message : or 'boost::filesystem::detail::path_traits::yes_type boost::filesystem::detail::path_traits::is_convertible_to_path_source::_check_convertible_to_path_source(const char *)' 1>C:\Users\A.J. Orians\.CommonCppCache\repocache\boost-1.81.0-win\0d44f38a132c865549ff3aee332d2b8e46482759\include\boost\filesystem\detail\path_traits.hpp(495,52): message : while trying to match the argument list '(const T)' 1> with 1> [ 1> T=nullptr 1> ] 1>C:\Users\A.J. Orians\.CommonCppCache\repocache\boost-1.81.0-win\0d44f38a132c865549ff3aee332d2b8e46482759\include\boost\type_traits\disjunction.hpp(27,1): message : see reference to class template instantiation 'boost::filesystem::detail::path_traits::is_convertible_to_path_source' being compiled 1>C:\Users\A.J. Orians\.CommonCppCache\repocache\boost-1.81.0-win\0d44f38a132c865549ff3aee332d2b8e46482759\include\boost\type_traits\disjunction.hpp(31,1): message : see reference to class template instantiation 'boost::disjunction>' being compiled 1>C:\Users\A.J. Orians\.CommonCppCache\repocache\boost-1.81.0-win\0d44f38a132c865549ff3aee332d2b8e46482759\include\boost\type_traits\conjunction.hpp(31,1): message : see reference to class template instantiation 'boost::disjunction,boost::filesystem::detail::path_traits::is_convertible_to_path_source>' being compiled 1>C:\Users\A.J. Orians\.CommonCppCache\repocache\boost-1.81.0-win\0d44f38a132c865549ff3aee332d2b8e46482759\include\boost\filesystem\path.hpp(264,27): message : see reference to class template instantiation 'boost::conjunction,boost::filesystem::detail::path_traits::is_convertible_to_path_source>,boost::negation>>' being compiled 1>Done building project "TestBoostFilesystem.vcxproj" -- FAILED. ========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ========== ========== Elapsed 00:03.205 ==========

That error is pretty much the exact same output as when I write:

boost::filesystem::path p( nullptr );

I am inclined to believe this is a MSVC bug because I can use Boost 1.81.0 with both GCC & Clang on Compiler Explorer to which they both build in C++20 with that sample code. Here is an online link: https://godbolt.org/z/f1jzWM6or That link also has the extra line to try to which you can see a similar build error with Boost 1.81.0 that you don't see with Boost 1.80.0.

So I do have a workaround as well as perhaps an updated MSVC could fix this. So just so you are aware. I didn't see any compile option/preprocessor definition I could set to workaround my problem.

Thanks for considering!

[Lastique]

1. Constructing path from nullptr is always UB and never correct. The fact that it fails to compile is a good thing, although the error could be better.
2. As to why the compiler tries to construct path from nullptr when you compare it with std::function, I really have no idea. This looks like a compiler issue to me, you should report it to MS.

[ajorians]

2. As to why the compiler tries to construct path from nullptr when you compare it with std::function, I really have no idea. This looks like a compiler issue to me, you should report it to MS.

Thank you. This is more my issue and I'll report it to MS. I figured I would first mention here because if my std::function had a std::filesystem::path instead of the boost version it does build on MSVC with C++20. I'll link to the MS issue after I create it (edit: Here is the link: https://developercommunity.visualstudio.com/t/C20-And-Later-Calls-Boost-Path-Con/10270439?port=1025&fsid=e5b4246a-ea5e-466d-95fb-745d83dd8a0a&sort=newest&ref=native&refTime=1675430809006&refUserId=0e05bc7b-454a-461d-bc0b-3ea1e4b3e10c ).

Thanks again!