c++: wrong error with variadic concept [PR105268]

c++: wrong error with variadic concept [PR105268]

Here we issue a wrong error for the

  template<typename T = S<C_many<int>>> void g();

line in the testcase.  I surmise that's because we mistakenly parse
C_many<int> as a placeholder-type-specifier, and things go wrong from
there.  We are in a default argument so we should reject parsing C_many<int>
as a placeholder-type-specifier, which would mean creating a new parameter.
We want C_many<int> to be a concept-id instead.

It's interesting to see why the same problem didn't occur for C_one<int>.
In that case, cp_parser_placeholder_type_specifier -> finish_type_constraints
-> build_type_constraint -> build_concept_check -> build_standard_check ->
coerce_template_parms fails the parse here:

 8916   nargs = inner_args ? NUM_TMPL_ARGS (inner_args) : 0;
 8917   if ((nargs - variadic_args_p > nparms && !variadic_p)
 8918       || (nargs < nparms - variadic_p
 8919           && require_all_args
 8920           && !variadic_args_p
 8921           && (!use_default_args
 8922               || (TREE_VEC_ELT (parms, nargs) != error_mark_node
 8923                   && !TREE_PURPOSE (TREE_VEC_ELT (parms, nargs))))))
 8924     {
 8925     bad_nargs:
 ...
 8943       return error_mark_node;

because nargs is 2 (the targs are <WILDCARD_DECL, int>) while nparms is
1 (for the one 'typename' in the tparam list of C_one).  But for
C_many<int> variadic_p is true so we don't return error_mark_node but
<type_argument_pack>.

This patch does not issue any error for the !tentative case because
I didn't figure out how to trigger that.  So it adds an assert instead.

	PR c++/105268

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_placeholder_type_specifier): Return
	error_mark_node when trying to build up a constrained parameter in
	a default argument.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/variadic6.C: New test.