[Bug c++/66850] New: Adding a forward declaration of a template containing a template template parm causes ICE on valid code

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66850

            Bug ID: 66850
           Summary: Adding a forward declaration of a template containing
                    a template template parm causes ICE on valid code
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

The following snippet emits an ICE in tsubst:

template <template <typename U, U> class> struct Sort;

template <template <typename U, U> class Comparator>
struct Sort
{
  template <int I>
  struct less_than
  {
    Comparator<int, I> a;
  };
};

If one removes the initial forward declaration of "struct Sort" then the code
compiles fine.

Branched off from PR65186.