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
