[Bug c++/66968] New: Incorrect template argument shown in diagnostic

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66968

            Bug ID: 66968
           Summary: Incorrect template argument shown in diagnostic
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Compiling this invalid code produces an error as expected:

template<typename T> struct thing { };

template<typename T, typename XXX>
  bool operator==(const thing<T>&, const thing<XXX>&);

struct nested { typedef int name; };

template<typename A, typename B>
void func(typename A::name const&, const B) { }

template<typename A, typename B>
void func(typename A::name const&, const thing<B>) { }

int main()
{
  thing<int> ti;
  func<nested>(1, ti);
}


However the error has an error:

y.cc: In function ‘int main()’:
y.cc:17:21: error: call of overloaded ‘func(int, thing<int>&)’ is ambiguous
   func<nested>(1, ti);
                     ^
y.cc:9:6: note: candidate: void func(const typename A::name&, B) [with A =
nested; B = thing<int>; typename A::name = int]
 void func(typename A::name const&, const B) { }
      ^
y.cc:12:6: note: candidate: void func(const typename A::name&, thing<XXX>)
[with A = nested; B = int; typename A::name = int]
 void func(typename A::name const&, const thing<B>) { }
      ^


Note that the second candidate shows thing<XXX>, where does that XXX come
from?!

The "[with ...]" output correctly shows the template parameter B but B doesn't
appear in the candidate. For some reason XXX is shown instead, which is only
ever mentioned in the operator== declaration above, which is never used!