PR c++/48948
	* semantics.c (validate_constexpr_fundecl): Defer checking if
	an argument type is being defined.
	(is_valid_constexpr_fn): Add defer_ok parm.
	(cxx_eval_call_expression): Adjust.
	(check_deferred_constexpr_decls): New.
	(literal_type_p): Make sure type isn't being defined.
	(ensure_literal_type_for_constexpr_object): Handle type being defined.
	* cp-tree.h: Declare check_deferred_constexpr_decls.
	* decl.c (grokfndecl): Call validate_constexpr_fundecl here.
	(start_preparsed_function, cp_finish_decl): Not here.
	* class.c (finalize_literal_type_property): Don't call
	validate_constexpr_fundecl.
	(finish_struct): Call check_deferred_constexpr_decls.
	* pt.c (tsubst_decl): Call validate_constexpr_fundecl.
	(instantiate_class_template): Call check_deferred_constexpr_decls.

From-SVN: r173683