[coroutine.traits]

# 17 Language support library [[support]](./#support)

## 17.13 Coroutines [[support.coroutine]](support.coroutine#coroutine.traits)

### 17.13.3 Coroutine traits [coroutine.traits]

#### [17.13.3.1](#general) General [[coroutine.traits.general]](coroutine.traits.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L5798)

Subclause [coroutine.traits] defines requirements on classes representing[*coroutine traits*](#def:coroutine_traits),
and defines the class templatecoroutine_traits that meets those requirements[.](#general-1.sentence-1)

#### [17.13.3.2](#primary) Class template coroutine_traits [[coroutine.traits.primary]](coroutine.traits.primary)

[1](#primary-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L5808)

The header [<coroutine>](coroutine.syn#header:%3ccoroutine%3e "17.13.2 Header <coroutine> synopsis [coroutine.syn]") defines the primary templatecoroutine_traits such that
if ArgTypes is a parameter pack of types and
if the [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") R​::​promise_type is valid and
denotes a type ([[temp.deduct]](temp.deduct "13.10.3 Template argument deduction")),
then coroutine_traits<R, ArgTypes...> has the following publicly
accessible member:using promise_type = typename R::promise_type;

Otherwise, coroutine_traits<R, ArgTypes...> has no members[.](#primary-1.sentence-2)

[2](#primary-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L5822)

Program-defined specializations of this template shall define a publicly
accessible nested type named promise_type[.](#primary-2.sentence-1)