is_hashable is broken in C++20 (std::hash<T>::result_type and ::argument_type were removed) #602
Description
In C++ 20, e.g. rxu::is_hashable<std::string> unexpectedly returns false because std::hash<T>::result_type and ::argument_type were removed (cppreference, P0619R2). See definition of is_hashable:
RxCpp/Rx/v2/src/rxcpp/rx-util.hpp
Lines 1022 to 1028 in 761b932
This means functions like observable<T>::distinct() do not work anymore.