The member typedef type denotes T.
The member typedef type denotes remove_cv_t<remove_reference_t<T>>.
Let U be remove_reference_t<T>.
If is_array_v<U> is true, the member typedef type denotes remove_extent_t<U>*.
If is_function_v<U> is true, the member typedef type denotes add_pointer_t<U>.
Otherwise the member typedef type denotes remove_cv_t<U>.
This behavior is similar to the lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) conversions applied when an lvalue is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing.— end note]
If B is true, the member typedef type denotes T; otherwise, there shall be no member type.
template<bool B, class T, class F>
template<class... T> struct common_type;
Unless this trait is specialized (as specified in Note D, below), there shall be no member type.
The member typedef-name type is defined or omitted as specified in Note C, below.
Each type in the parameter pack T shall be complete or cv void.
If T is an enumeration type, the member typedef type denotes the underlying type of T ([dcl.enum]); otherwise, there is no member type.
Mandates: T is not an incomplete enumeration type.
If the expression INVOKE(declval<Fn>(), declval<ArgTypes>()...) ([func.require]) is well-formed when treated as an unevaluated operand ([expr.context]), the member typedef type denotes the type decltype(INVOKE(declval<Fn>(), declval<ArgTypes>()...)); otherwise, there shall be no member type.
Access checking is performed as if in a context unrelated to Fn and ArgTypes.
Only the validity of the immediate context of the expression is considered.
The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.
Such side effects are not in the “immediate context” and can result in the program being ill-formed.— end note]
Preconditions: Fn and all types in the template parameter pack ArgTypes are complete types, cv void, or arrays of unknown bound.
If T is a specialization reference_wrapper<X> for some type X, the member typedef type of unwrap_reference<T> denotes X&, otherwise type denotes T.
The member typedef type of unwrap_ref_decay<T> denotes the type unwrap_reference_t<decay_t<T>>.