[Bug c++/105481] [9/10/11/12/13 Regression] ICE: unexpected expression of kind template_parm_index

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105481

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
During type_unification_real as part of partial ordering, when considering
default template arguments we assume all previously deduced arguments are
non-dependent and therefore default argument instantiation will be
non-dependent.  But I don't think that's true for partial ordering, where we
may end up deducing dependent arguments e.g. deducing N for N in the comment #3
testcase.