7 Expressions [expr]

7.5 Primary expressions [expr.prim]

7.5.7 Requires expressions [expr.prim.req] Type requirements [expr.prim.req.type]

A type-requirement asserts the validity of a type.
The component names of a type-requirement are those of its nested-name-specifier (if any) and type-name.
[Note 1: 
The enclosing requires-expression will evaluate to false if substitution of template arguments fails.
— end note]
[Example 1: template<typename T, typename T::type = 0> struct S; template<typename T> using Ref = T&; template<typename T> concept C = requires { typename T::inner; // required nested member name typename S<T>; // required valid ([temp.names]) template-id; // fails if T​::​type does not exist as a type to which 0 can be implicitly converted typename Ref<T>; // required alias template substitution, fails if T is void }; — end example]
A type-requirement that names a class template specialization does not require that type to be complete ([basic.types.general]).