Dear all,
this is my first post to this group, and it might not even be the correct group to post to. Please correct me if I'm wrong.
The following code creates concept equivalents of every type trait that evaluates to a bool:
This is useful to constrain templates using a simple syntax, without relying on the more verbose requires keyword. To me, it is the cleanest way to use concepts. A toy example:
template <integral_c type>
auto function_only_accepting_integral_types() { ... }
std::integral is already a concept.
Many of the other cases are either already concepts with semantics identical to the respective type trait, or with a different implementation better suited to use as a concept: same_as is bidirectional for subsumption, derived_from is reversed from is_base_of (since bases are a closed set), constructible_from also checks destructibility. The concepts library is carefully designed to not just be a transcription of the traits library.
For those traits that don't have corresponding concepts (I would like to see an invocable_r alongside invocable) there's usually work to be done to ensure the concept is written in the most appropriate way.
Perhaps there's space for a facility that wraps an existing type trait as a concept, but I'm not sure whether that needs to be standardized:
template<class C, template<class...> class T, class... U>
concept satisfying = T<C, U...>::value;
Do you think this is useful? Useful enough to be considered into the standard?
I will build a more complete motivation/proposal with non-toy examples if you think its worth it.
Good day.
Best regards,
Ali Can Demiralp
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