P1965R0 Blanket Wording for Specifying "Hidden Friends"

Also fixes NB DE 165 and LWG3239

Author: jensmaurer

source/lib-intro.tex

@@ -3121,6 +3121,29 @@
 or add additional signatures for a member function name.
 \end{note}
 
+\rSec3[hidden.friends]{Friend functions}
+
+\pnum
+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.
+\begin{note}
+In particular,
+an implementation is not allowed to provide
+an additional declaration of that function or function template
+at namespace scope.
+\end{note}
+\begin{note}
+Such a friend function or function template declaration
+is known as a hidden friend,
+as it is visible neither
+to ordinary unqualified lookup\iref{basic.lookup.unqual} nor
+to qualified lookup\iref{basic.lookup.qual}.
+\end{note}
+
 \rSec3[constexpr.functions]{Constexpr functions and constructors}
 
 \pnum