std::three_way_comparable_with<T, U, void> can be satisfied but can't be modeledSection: 17.11.4 [cmp.concept] Status: New Submitter: Jiang An Opened: 2021-09-06 Last modified: 2021-10-23
Priority: 3
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Discussion:
Due to the current specification of std::common_comparison_category,
compare-as<void, void> is satisfied. And thus given two types T
and U with regular ==, !=, <, >, <=,
and >= but irregular <=> that has return type void,
std::three_way_comparable_with<T, U, void> is satisfied.
std::three_way_comparable_with<T, U, void>
can't be modeled, because Cat(t <=> u) == Cat(C(t) <=> C(u))
is ill-formed if Cat is void (see this godbolt link).
I don't think this issue has any impact on meaningful codes, but it seems to be a hole in the
current specification.
It seems that compares-as<void, void> should be changed to false.
compares-as<NonComparisonCategoryType, void> might need refinement.
(Note: The same problem does not exists for std::three_way_comparable, because
std::three_way_comparable<T, void> can be modeled if the <=> on T
has "regular" definition or its return type is not a comparison category type, but the return value
has "regular" semantics)
[2021-09-20; Reflector poll]
Set priority to 3 after reflector poll.
Proposed resolution: