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### 3587. `std::three_way_comparable_with<T, U, void>` can be satisfied but can't be modeled

Section: 17.12.4 [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.

However, in this situation `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: