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3380. common_type and comparison categories

Section: 21.3.9.7 [meta.trans.other] Status: C++20 Submitter: Casey Carter Opened: 2020-01-23 Last modified: 2021-02-25

Priority: 0

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Discussion:

There are two paragraphs in the the definition of common_type:

P1614R2 added the first bullet so that common_type_t<strong_equality, T> would be the same type as common_comparison_category_t<strong_equality, T>; other cases are correctly handled by the second (pre-existing) bullet. After application of P1959R0 in Belfast, std::strong_equality is no more. We can now strike the first bullet without changing the behavior of common_type.

[2020-02-08 Status set to Tentatively Ready after seven positive votes on the reflector.]

Proposed resolution:

This wording is relative to N4849.

  1. Modify 21.3.9.7 [meta.trans.other] as indicated:

    -3- Note A: For the common_type trait applied to a template parameter pack T of types, the member type shall be either defined or not present as follows:

    1. (3.1) — […]

    2. […]

    3. (3.3) — If sizeof...(T) is two, let the first and second types constituting T be denoted by T1 and T2, respectively, and let D1 and D2 denote the same types as decay_t<T1> and decay_t<T2>, respectively.

      1. (3.3.1) — If is_same_v<T1, D1> is false or is_same_v<T2, D2> is false, let C denote the same type, if any, as common_type_t<D1, D2>.

      2. (3.3.2) — [Note: None of the following will apply if there is a specialization common_type<D1, D2>. — end note]

      3. (3.3.3) — Otherwise, if both D1 and D2 denote comparison category types (17.12.2.1 [cmp.categories.pre]), let C denote the common comparison type (11.10.3 [class.spaceship]) of D1 and D2.

      4. (3.3.4) — Otherwise, if

        decay_t<decltype(false ? declval<D1>() : declval<D2>())>

        denotes a valid type, let C denote that type.

      5. (3.3.5) — […]

    4. […]