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Section: 18.4.2 [concept.same] Status: C++20 Submitter: Casey Carter Opened: 2019-01-05 Last modified: 2021-02-25
Priority: 0
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Discussion:
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. Paragraph 1 is trying to tell us that there's some magic that provides the stated subsumption relationship, but to a casual reader it appears to be a mis-annotated note. We should either add a note to explain what's actually happening here, or define the concept in such a way that it naturally provides the specified subsumption relationship.
Given that there's a straightforward library implementation of the symmetric subsumption idiom, the latter option seems preferable.
[2019-01-20 Reflector prioritization]
Set Priority to and status to Tentatively Ready
Proposed resolution:
This wording is relative to N4791.
Change 18.4.2 [concept.same] as follows:
template<class T, class U> concept same-impl = // exposition only is_same_v<T, U>; template<class T, class U> concept Same =is_same_v<T, U>same-impl<T, U> && same-impl<U, T>;-1- [Note: Same<T, U> subsumes Same<U, T> and vice versa.—end note]