[Bug c++/104094] New: Alias template shown in diagnostic with wrong template parameter name

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104094

            Bug ID: 104094
           Summary: Alias template shown in diagnostic with wrong template
                    parameter name
           Product: gcc
           Version: 12.0
            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: ---

template<typename> struct trait;
template<typename A> struct trait { using type = A; };
template<typename B> struct cond { static constexpr bool value = true; };

template<bool> struct enable_if { using type = bool; };
template<> struct enable_if<false> { };

template<typename C>
class pair
{
  template<typename D>
    using constraint = cond<trait<D>>;

  template<typename E, typename enable_if<constraint<E>::value>::type = true>
    pair(E&&) { }
};

pair<int> p(1);


GCC prints:

name.C:18:14: error: 'pair<C>::pair(E&&) [with E = int; typename
enable_if<cond<trait<D> >::value>::type <anonymous> = true; C = int]' is
private within this context
   18 | pair<int> p(1);
      |              ^
name.C:15:5: note: declared private here
   15 |     pair(E&&) { }
      |     ^~~~


Why does it show cond<trait<D>> in the error? There is no D in that
constructor's parameter list, and D is not explained in the [with ...] template
arg list.

I think it should be shown as constraint<E> or as cond<trait<E>>, but showing D
is just confusing.