boost::hash needs c++20 to compile with clang 15 and gcc 10

[martinsjogren]

I have a blob type with begin/end member functions that return const void* and when I try to use that with boost::unordered_map I get a compilation error with gcc 10 and clang 15 unless I compile with -std=c++20 (funny enough it works with gcc 11+ and msvc)

Am I doing something strange or are the ADL changes in c++20 relevant for this case? I attached a small repro as well as the output from compiling with gcc-10 -std=c++17 and clang-15 -std=c++17

Either way it feels like boost shouldn't require c++20 to compile this example where I provide my own hash_value function, right? I'm really confused whether this is a bug in boost or a bug somewhere in gcc/clang or if my code is doing something wrong.

hash_repro.tar.gz

[pdimov]

That's a pretty interesting issue.

The reason for the error is that std::iterator_traits causes a hard error when instantiated with const void*, and boost::hash uses std::iterator_traits with the return type of begin() and end() to detect whether the type is a container and enable its built-in support for containers.

Looks like std::iterator_traits has been fixed in C++20, which is why it works there.

I'll look into getting this fixed for the upcoming 1.85. In the meantime, you can make this work by overriding boost::container_hash::is_range<foo::bar> to report false:

template<> struct boost::container_hash::is_range<foo::bar>: std::false_type {};

as in https://godbolt.org/z/fc8h7KjsM.

[pdimov]

Should be fixed for 1.85. The beta will be going out in a few days.

[martinsjogren]

Super, and thank you for the workaround!