[Bug c++/71227] [6/7 Regression] template friend function cannot be resolved

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71227

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> ---
7.3.1.2: If a friend declaration in a non-local class first declares a class,
function, class template or function template the friend is a member of the
innermost enclosing namespace. The friend declaration does not by itself make
the name visible to unqualified lookup (3.4.1) or qualified lookup (3.4.3).

14.7.3: A declaration of a function template, class template, or variable
template being explicitly specialized shall precede the declaration of the
explicit specialization.

I interpret this as meaning that there needs to be a visible declaration of f
before it can be specialized.  But we can certainly be more helpful about it.