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### 3380. `common_type` and comparison categories

Section: 21.3.8.7 [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`:

- 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`.

- Otherwise, if `decay_t<decltype(false ? declval<D1>() : declval<D2>())>` denotes a valid type, let `C` denote that 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 21.3.8.7 [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. […]