16 Library introduction [library]

16.4 Library-wide requirements [requirements]

16.4.6 Conforming implementations [conforming]

16.4.6.6 Friend functions [hidden.friends]

Whenever this document specifies a friend declaration of a function or function template within a class or class template definition, that declaration shall be the only declaration of that function or function template provided by an implementation.
[Note 1: 
In particular, an implementation is not allowed to provide an additional declaration of that function or function template at namespace scope.
— end note]
[Note 2: 
Such a friend function or function template declaration is known as a hidden friend, as it is visible neither to ordinary unqualified lookup ([basic.lookup.unqual]) nor to qualified lookup ([basic.lookup.qual]).
— end note]