c++: Error recovery in merge_default_template_args [PR108206]

c++: Error recovery in merge_default_template_args [PR108206]

We ICE on the following testcase during error recovery, both new_parm
and old_parm are error_mark_node, the ICE is on
          error ("redefinition of default argument for %q+#D", new_parm);
          inform (DECL_SOURCE_LOCATION (old_parm),
                  "original definition appeared here");
where we don't print anything useful for new_parm and ICE trying to
access DECL_SOURCE_LOCATION of old_parm.  I think we shouldn't diagnose
anything when either of the parms is erroneous, GCC 11 before
merge_default_template_args has been added was doing
      if (TREE_VEC_ELT (tmpl_parms, i) == error_mark_node
          || TREE_VEC_ELT (parms, i) == error_mark_node)
        continue;

      tmpl_parm = TREE_VALUE (TREE_VEC_ELT (tmpl_parms, i));
      if (error_operand_p (tmpl_parm))
        return false;
in redeclare_class_template.

2023-01-04  Jakub Jelinek  <jakub@redhat.com>

	PR c++/108206
	* decl.cc (merge_default_template_args): Return false if either
	new_parm or old_parm are erroneous.

	* g++.dg/template/pr108206.C: New test.