re PR c++/45329 (When printing a list of candidate functions, explain why each function failed to match.)

	PR c++/45329
	PR c++/48934
	* cp-tree.h (fn_type_unification): Add `bool' parameter.
	* pt.c (enum template_base_result): Define.
	(unify_success, unify_unknown): Define.
	(unify_parameter_deduction_failure): Define.
	(unify_invalid, unify_cv_qual_mismatch, unify_type_mismatch): Define.
	(unify_parameter_pack_mismatch): Define.
	(unify_parameter_pack_inconsistent): Define.
	(unify_ptrmem_cst_mismatch, unify_vla_arg): Define.
	(unify_expression_unequal, unify_inconsistency): Define.
	(unify_method_type_error, unify_arity): Likewise.
	(unify_too_many_parameters, unify_too_few_parameters): Define.
	(unify_arg_conversion, unify_no_common_base): Define.
	(unify_illformed_ptrmem_cst_expr): Define.
	(unify_substitution_failure): Define.
	(unify_inconsistent_template_template_parameters): Define.
	(unify_template_deduction_failure): Define.
	(unify_template_argument_mismatch): Define.
	(unify_overload_resolution_failure): Define.
	(comp_template_args_with_info): New function, split out from...
	(comp_template_args): ...here.	Call it.
	(deduction_tsubst_fntype): Add `complain' parameter'.  Pass it
	to tsubst.
	(unify): Add `explain_p' parameter.  Pass to all relevant calls.
	Call above status functions when appropriate.
	(resolve_overloaded_unification, try_one_overload): Likewise.
	(type_unification, type_unification_real): Likewise.
	(unify_pack_expansion): Likewise.
	(get_template_base, try_class_unification): Likewise.
	(get_bindings, more_specialized_fn): Pass false to unification
	calls.
	(get_class_bindings, do_auto_deduction): Likewise.
	(convert_nontype_argument): Likewise.
	(fn_type_unification): Likewise.  Pass tf_warning_or_error if
	explain_p.
	(get_template_base): Add `explain_p' parameter and pass it to
	try_class_unification.	Return an enum template_base_result.
	* class.c (resolve_address_of_overloaded_function): Pass false to
	fn_type_unification.
	* call.c (enum rejection_reason_code): Add new codes.
	(struct rejection_reason): Add template_unification field.
	Add template_instantiation field.
	(template_unification_rejection): Define.
	(template_unification_error_rejection): Define.
	(template_instantiation_rejection): Define.
	(invalid_copy_with_fn_template_rejection): Define.
	(add_template_candidate): Pass false to unify.
	Provide more rejection reasons when possible.
	(print_template_unification_rejection): Define.
	(print_arity_rejection): Define, split out from...
	(print_z_candidate): ...here.  Add cases for new rejection
	reasons.

Co-Authored-By: Jason Merrill <jason@redhat.com>

From-SVN: r176365