Using std::hash through <typeindex> does not compile for builtin arithmetic types

[johnfranklinrickard]

As far as I am aware (hope I am not mistaken), this should be valid C++ code.

#include <typeindex>
std::size_t test(int i) { return std::hash<int>()(i); }

From the C++ draft http://wg21.link/N4950:
In [unord.hash] 2:

Each header that declares the template hash provides enabled specializations of hash for nullptr_t and all
cv-unqualified arithmetic, enumeration, and pointer types

And in [type.index.synopsis] it is mentioned that the header <typeindex> also declares the template std::hash.

In my opinion the msvc STL has it correctly in this case and the code compiles correctly.
Both libc++ and libstdc++ currently do not work for this code snippet and reject it with implicit instantiation of undefined template 'std::hash<int>'.

Godbolt link

I only checked it for <typeindex>, but this could affect more headers where std::hash is supposed to be defined as mentioned in cppreference.

Under the assumption that this is correct C++, would it be possible to fix these includes for the LLVM standard library?

[frederick-vs-ja]

See also LWG3653. I think some implementors want to only guarantee the definition of hash<type_index> to be provided when only <typeindex> is included.

[johnfranklinrickard]

For context: I want to specialize std::hash and/or use std::hash for arithmetic types. I know that this works with the <functional> include, but that pulls in a lot of things and is one of the heavier includes from the standard library. I would prefer to include something smaller if I just want to do a hash specialization. I found the leaner header <typeindex> under msvc, thats why I was asking for that one specifically.

I was hoping to either get the library more standards compliant (as long as my assumption still holds that this should be valid C++) or try to get a recommendation for a different "leaner" standard header that works for that purpose.

[frederick-vs-ja]

Thanks, I've roughly checked occurrences of hash in the current standard working draft and the availability of hash<int> in libc++'s standard headers.

• <bitset>
• <chrono> (since C++26; change not yet implemented)
• <coroutine>
• <filesystem>
• <functional>
• <memory>
• <optional>
• <stacktrace> (not yet implemented)
• <string>
• <string_view>
• <system_error>
• <text_encoding> (not yet implemented)
• <typeindex> (!!!)
• <thread>
• <variant>
• <vector>
• <utility> (since C++26)