cpp/iterator/indirectly readable traits

Computes the associated value type of the type, if any. Users may specialize for a program-defined type.

@1@ Primary template has no member. @2@ Specialization for pointers. If is an object type, provides a member type  equal to. Otherwise, there is no member. @3@ Specialization for array types. Provides a member type equal to. @4@ Specialization for const-qualified types. @5@ Specialization for types that define a public and accessible member type (e.g., std). If is an object type, provides a member type  equal to. Otherwise, there is no member. @6@ Specialization for types that define a public and accessible member type (e.g., std). If is an object type, provides a member type  equal to. Otherwise, there is no member. @7-8@ Specialization for types that define public and accessible member types and  (e.g., ). If both and  are object types and they become the same type after removing cv-qualifiers on the top-level, provides a member type  equal to. Otherwise, there is no member.