name lookup in qualified friend declaration is extremely suspicious

[zygoloid]

Bugzilla Link	38882
Version	5.0
OS	All
CC	@DougGregor

Extended Description

Testcase:

using T = int;

namespace N {
  struct X {
    template<typename T> void f(T);
  };
  template<typename> struct Y {
    template<typename T> friend void X::f(T); // #1
    friend void X::f<>(Y); // #2
  };
}

Here, #​1 resolves T as ::T, not as the template parameter. Strangely, we fail to diagnose this, and instead silently befriend nothing in line #​1.

In #​2, Y resolves to N::Y, not to the injected-class-name of N::Y, and as a result declaration #​2 is rejected because Y is lacking its template arguments.

These results both seem extremely surprising. The standard's rules here are highly unclear, but this cannot be the right behavior...

[jwakely]

Is this another case of the same problem?

namespace ns
{
  struct S { };

  void func(S) { }
}

struct X
{
  friend void ns::func(S);
};

Other compilers accept this, but clang wants the friend decl to use ns::S:

<source>:10:24: error: unknown type name 'S'; did you mean 'ns::S'?
  friend void ns::func(S);
                       ^
                       ns::S
<source>:3:10: note: 'ns::S' declared here
  struct S { };
         ^

[stbergmann]

Other compilers accept this, but clang wants the friend decl to use ns::S:

...or is the Clang behavior in line with [basic.scope.namespace]/1, since C++23 P1787R6: Declarations and where to find them explicitly stating "non-friend": "For each non-friend redeclaration or specialization whose target scope is or is contained by the scope, the portion after the declarator-id, class-head-name, or enum-head-name is also included in the scope."

[stbergmann]

(which would be CWG2370 friend declarations of namespace-scope functions (which states "There is implementation divergence on these points."), reportedly "resolved by performing a search in (only) the immediate scope of any friend", P1787R6: Declarations and where to find them)

[zygoloid]

I think the relevant rule is:

In a friend declaration declarator whose declarator-id is a qualified-id whose lookup context is ([basic.lookup.qual]) a class or namespace S, lookup for an unqualified name that appears after the declarator-id performs a search in the scope associated with S. If that lookup finds nothing, it undergoes unqualified name lookup.

So in the new example, we should first look for S in namespace ns (but not in its enclosing scopes), then look in class X and its enclosing scopes as usual if that first lookup fails.

In the original example, we should look for T and Y as member names of N::X only (which finds nothing) and then fall back to normal unqualified lookup, so T should name the template parameter of the friend declaration and Y should name the injected-class-name.