20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.5 Conforming implementations [conforming]

20.5.5.10 Protection within classes [protection.within.classes]

It is unspecified whether any function signature or class described in Clauses [language.support] through [thread] and Annex [depr] is a friend of another class in the C++ standard library.