17 Language support library [support]

17.12 Coroutines [support.coroutine]

17.12.3 Coroutine traits [coroutine.traits]

17.12.3.2 Class template coroutine_traits [coroutine.traits.primary]

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The header <coroutine> defines the primary template coroutine_traits such that if ArgTypes is a parameter pack of types and if the qualified-id R​::​promise_type is valid and denotes a type ([temp.deduct]), 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.
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Program-defined specializations of this template shall define a publicly accessible nested type named promise_type.