Many of the concept definitions in the standard library have two definitions conditional on _HAS_EXACT_COMPOUND_REQUIREMENT. The intent is to be able to support both conforming implementations of C++20 concepts and clang-concepts, which until quite recently had not implemented WG21-P1084's change to return-type-requirements. I've verified with the author of clang-concepts that clang proper will certainly support P1084's semantics when it defines __cpp_concepts, so we can remove _HAS_EXACT_COMPOUND_REQUIREMENT from the STL as well as all conditional code that is used when _HAS_EXACT_COMPOUND_REQUIREMENT is 0.