20 Library introduction [library]

20.4 Method of description (Informative) [description]

20.4.2 Other conventions [conventions]

20.4.2.1 Type descriptions [type.descriptions]

20.4.2.1.2 Exposition-only types [expos.only.types]

Several types defined in [language.support] through [thread] and [depr] that are used as function parameter or return types are defined for the purpose of exposition only in order to capture their language linkage.
The declarations of such types are followed by a comment ending in exposition only.
[Example
:
namespace std {
  extern "C" using some-handler = int(int, void*, double);  // exposition only
}
The type placeholder some-handler can now be used to specify a function that takes a callback parameter with C language linkage.
end example
]