[Bug c++/89636] New: Duplicate diagnostic when resolving ambiguity between variable and function template using implicit typename

[blitzrakete at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89636

            Bug ID: 89636
           Summary: Duplicate diagnostic when resolving ambiguity between
                    variable and function template using implicit typename
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: blitzrakete at gmail dot com
  Target Milestone: ---

namespace N {
  inline namespace A { template <typename T> int f(typename T::type); }
  inline namespace B { template <typename T> int f(T::type); }
}

template <typename T>
int N::f(T::type); // ill-formed

For the above code, compiled with -std=c++2a, gcc generates:

<source>:7:5: error: reference to 'f' is ambiguous

    7 | int N::f(T::type); // ill-formed

      |     ^

<source>:2:50: note: candidates are: 'template<class T> int N::A::f(typename
T::type)'

    2 |   inline namespace A { template <typename T> int f(typename T::type); }

      |                                                  ^

<source>:3:50: note:                 'template<class T> int N::B::f<T>'

    3 |   inline namespace B { template <typename T> int f(T::type); }

      |                                                  ^

<source>:7:17: error: reference to 'N::f' is ambiguous

    7 | int N::f(T::type); // ill-formed

      |                 ^

<source>:2:50: note: candidates are: 'template<class T> int N::A::f(typename
T::type)'

    2 |   inline namespace A { template <typename T> int f(typename T::type); }

      |                                                  ^

<source>:3:50: note:                 'template<class T> int N::B::f<T>'

    3 |   inline namespace B { template <typename T> int f(T::type); }

      |                                                  ^

Note the duplicate diagnostic. First it errors on just `f` being ambiguous,
then it does the same thing with `N::f`, but they're the same. I'd expect only
the second one to be emitted.