[concept.same]
18 Concepts library [concepts]
18.4 Language-related concepts [concepts.lang]
18.4.2 Concept same_as [concept.same]
template<class T, class U>
concept same-as-impl = is_same_v<T, U>; // exposition only
template<class T, class U>
concept same_as = same-as-impl<T, U> && same-as-impl<U, T>;