Forgive me if this has already been fixed in latest commits. But trying to build 4.5.3 with Clang 13 I get a compile error:

type_traits.hpp:87:35: error: no type named 'result_of' in namespace 'std'
using result_of_t = typename std::result_of<T>::type;

And so on. This is because the deprecated std::result_of has finally been removed in Clang 13. I think the fix would be to use std::invoke_result instead. (I do not have a patch but it would be great if someone could provide one as we are blocked in our effort to turn on C++20 by this issue)