为什么 Same<T, U> 不包含 Same<U, T>？
Why doesn't Same<T, U> subsume Same<U, T>?
在LWG 3182中指出
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>定义为is_same_v<T, U>，Same<T, U>怎么不包含Same<U, T>？
该问题涉及标准概念 Same，已重命名为 same_as，并由
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>;
直接定义不对称的原因（即Same<T, U>没有包含Same<U, T>）
是因为,
这就是为什么只有说明的概念 same-as-impl 在规范中是必要的。
在LWG 3182中指出
The specification of the
Sameconcept 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>subsumesSame<U, T>nor vice versa.
但是，从 cppreference、
... Commutativity is satisfied, i.e. for any two types
TandU,is_same<T, U>::value == trueif and only ifis_same<U, T>::value == true.
注意Same<T, U>定义为is_same_v<T, U>，Same<T, U>怎么不包含Same<U, T>？
该问题涉及标准概念 Same，已重命名为 same_as，并由
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>;
直接定义不对称的原因（即Same<T, U>没有包含Same<U, T>）
是因为same-as-impl 在规范中是必要的。