LWG-3657 `std::hash<std::filesystem::path>` is not enabled

[CaseyCarter]

LWG-3657 std::hash<std::filesystem::path> is not enabled

[crackedmind]

If you didn't mind i started implementation.

Is it must work under C++17 ?

[frederick-vs-ja]

I believe this is a DR against C++17 and hence needs to be work under C++17, and member typedefs result_type/argument_type should be kept but deprecated in C++17 mode, like this.

template <>
struct hash<filesystem::path> {
    _CXX17_DEPRECATE_ADAPTOR_TYPEDEFS typedef filesystem::path _ARGUMENT_TYPE_NAME;
    _CXX17_DEPRECATE_ADAPTOR_TYPEDEFS typedef size_t _RESULT_TYPE_NAME;
    _NODISCARD size_t operator()(const filesystem::path& _Path) const noexcept {
        return _STD filesystem::hash_value(_Path);
    }
};

This issue was submitted by me, and I was going to implement the resolution myself. But I'm just a bit too busy to implement it as soon as possible. @crackedmind I'll be very glad if you start implement it now.

[frederick-vs-ja]

@crackedmind Oh... I finally decide to implement it myself. Thanks for looking into this issue!

[jdoubleu]

Unfortunately, this didn't make it into Visual Studio 16.11 2019, yet: https://godbolt.org/z/6rMTedqMj

[frederick-vs-ja]

Unfortunately, this didn't make it into Visual Studio 16.11 2019, yet: https://godbolt.org/z/6rMTedqMj

I once wished every DR or bug fix for C++20 and former modes would be backported. But the STL team seemly decided only to backport ranges- and chrono-related ones.

[jdoubleu]

Well, I just noticed, that it's working with /std:c++17 but not /std:c++20 (for VS 16.11).

So it's not an issue of backporting the fix, but also enabling it for C++20 mode in VS 16.11.