Implement return type deduction for normal functions with -std=c++1y.

	Implement return type deduction for normal functions with -std=c++1y.
	* cp-tree.h (FNDECL_USED_AUTO): New macro.
	(LAMBDA_EXPR_DEDUCE_RETURN_TYPE_P): Remove.
	(dependent_lambda_return_type_node): Remove.
	(CPTI_DEPENDENT_LAMBDA_RETURN_TYPE): Remove.
	(struct language_function): Add x_auto_return_pattern field.
	(current_function_auto_return_pattern): New.
	(enum tsubst_flags): Add tf_partial.
	* decl.c (decls_match): Handle auto return comparison.
	(duplicate_decls): Adjust error message for auto return.
	(cxx_init_decl_processing): Remove dependent_lambda_return_type_node.
	(cp_finish_decl): Don't do auto deduction for functions.
	(grokdeclarator): Allow auto return without trailing return type in
	C++1y mode.
	(check_function_type): Defer checking of deduced return type.
	(start_preparsed_function): Set current_function_auto_return_pattern.
	(finish_function): Set deduced return type to void if not previously
	deduced.
	* decl2.c (change_return_type): Handle error_mark_node.
	(mark_used): Always instantiate functions with deduced return type.
	Complain about use if deduction isn't done.
	* parser.c (cp_parser_lambda_declarator_opt): Use 'auto' for
	initial return type.
	(cp_parser_lambda_body): Don't deduce return type in a template.
	(cp_parser_conversion_type_id): Allow auto in C++1y.
	* pt.c (instantiate_class_template_1): Don't mess with
	LAMBDA_EXPR_DEDUCE_RETURN_TYPE_P.
	(tsubst_copy_and_build): Likewise.
	(fn_type_unification, tsubst): Don't reduce the template parm level
	of 'auto' during deduction.
	(unify): Compare 'auto' specially.
	(get_bindings): Change test.
	(always_instantiate_p): Always instantiate functions with deduced
	return type.
	(do_auto_deduction): Handle error_mark_node and lambda context.
	Don't check for use in initializer.
	(contains_auto_r): Remove.
	* search.c (lookup_conversions_r): Handle auto conversion function.
	* semantics.c (lambda_return_type): Handle null return.  Don't mess
	with dependent_lambda_return_type_node.
	(apply_deduced_return_type): Rename from apply_lambda_return_type.
	* typeck.c (merge_types): Handle auto.
	(check_return_expr): Do auto deduction.
	* typeck2.c (add_exception_specifier): Fix complain check.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@185768 138bc75d-0d04-0410-961f-82ee72b054a4