PR c++/68727
	* cp-tree.def (OFFSETOF_EXPR): Bump number of operands to 2.
	* cp-tree.h (finish_offsetof): Add OBJECT_PTR argument.
	* parser.c (cp_parser_builtin_offsetof): Pass result of
	build_static_cast of null_pointer_node to finish_offsetof.
	* semantics.c (finish_offsetof): Add OBJECT_PTR argument, use
	it for -Winvalid-offsetof pedwarn instead of trying to guess
	original offsetof type from EXPR.  Save OBJECT_PTR as a new
	second operand to OFFSETOF_EXPR.
	* pt.c (tsubst_copy_and_build) <case OFFSETOF_EXPR>: Adjust
	finish_offsetof caller, pass the second operand of OFFSETOF_EXPR
	as OBJECT_PTR.

	* g++.dg/other/offsetof8.C: Add expected error.
	* g++.dg/other/offsetof9.C: New test.

From-SVN: r244949