c++ - Same 为什么不包含Same ？

Question

在LWG 3182中，它指出

The specification of the Same concept in 18.4.2 [concept.same]:

template<class T, class U>
  concept Same = is_same_v<T, U>;

-1- Same<T, U> subsumes Same<U, T> and vice versa.

seems contradictory. From the concept definition alone, it is not the case that Same<T, U> subsumes Same<U, T> nor vice versa.

但是，从 cppreference，

... Commutativity is satisfied, i.e. for any two types T and U, is_same<T, U>::value == true if and only if is_same<U, T>::value == true.

注意 Same<T, U>被定义为 is_same_v<T, U>， Same<T, U>如何不包含 Same<U, T>？

Answer 1

该问题涉及标准概念Same，该概念已重命名为same_as，并已由
将其指定为[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>;

简单定义不对称的原因(即 Same<T, U>不包含 Same<U, T>)
是因为 only concepts can be subsumed，
这就是为什么在规范中仅需要博览会概念 same-as-impl的原因。