c++: fold function template args sooner [PR101460]

c++: fold function template args sooner [PR101460]

As discussed in the PR, we were giving a lot of unnecessary errors for this
testcase because we didn't try to do constant evaluation until
convert_nontype_argument, which happens for each of the candidates.  But
when looking at a template-id as the function operand of a call, we can try
to fold arguments before we get into overload resolution.

	PR c++/101460

gcc/cp/ChangeLog:

	* cp-tree.h (cxx_constant_value_sfinae): Declare.
	* constexpr.c (cxx_constant_value_sfinae): New.
	* pt.c (fold_targs_r, maybe_fold_fn_template_args): New.
	(tsubst_copy_and_build) [CALL_EXPR]: Call
	maybe_fold_fn_template_args.

gcc/testsuite/ChangeLog:

	* g++.dg/template/explicit-args6.C: New test.