[concept.common]

# 18 Concepts library [[concepts]](./#concepts)

## 18.4 Language-related concepts [[concepts.lang]](concepts.lang#concept.common)

### 18.4.6 Concept common_with [concept.common]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L437)

If T and U can both be explicitly converted to some third type,C, then T and U share a [*common type*](#def:common_type),C[.](#1.sentence-1)

[*Note [1](#note-1)*:

C can be the same as T or U, or can be a
different type[.](#1.sentence-2)

C is not necessarily unique[.](#1.sentence-3)

â *end note*]

[ð](#concept:common_with)

`template<class T, class U>
  concept [common_with](#concept:common_with "18.4.6 Concept common_­with [concept.common]") =
    [same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<common_type_t<T, U>, common_type_t<U, T>> &&
    requires {
      static_cast<common_type_t<T, U>>(declval<T>());
      static_cast<common_type_t<T, U>>(declval<U>());
    } &&
    [common_reference_with](concept.commonref#concept:common_reference_with "18.4.5 Concept common_­reference_­with [concept.commonref]")<
      add_lvalue_reference_t<const T>,
      add_lvalue_reference_t<const U>> &&
    [common_reference_with](concept.commonref#concept:common_reference_with "18.4.5 Concept common_­reference_­with [concept.commonref]")<
      add_lvalue_reference_t<common_type_t<T, U>>,
      common_reference_t<
        add_lvalue_reference_t<const T>,
        add_lvalue_reference_t<const U>>>;
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L465)

Let C be common_type_t<T, U>[.](#2.sentence-1)

Let t1 and t2 be
equality-preserving expressions ([[concepts.equality]](concepts.equality "18.2 Equality preservation")) such thatdecltype((t1)) and decltype((t2)) are each T, and
let u1 and u2 be equality-preserving expressions such thatdecltype((u1)) and decltype((u2)) are each U[.](#2.sentence-2)

T and U model [common_with](#concept:common_with "18.4.6 Concept common_­with [concept.common]")<T, U> only if

- [(2.1)](#2.1)

C(t1) equals C(t2) if and only if t1 equals t2, and

- [(2.2)](#2.2)

C(u1) equals C(u2) if and only if u1 equals u2[.](#2.sentence-3)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L481)

[*Note [2](#note-2)*:

Users can customize the behavior of [common_with](#concept:common_with "18.4.6 Concept common_­with [concept.common]") by specializing thecommon_type class template ([[meta.trans.other]](meta.trans.other "21.3.9.7 Other transformations"))[.](#3.sentence-1)

â *end note*]
