* c-typeck.c (null_pointer_constant_p): New function.
	(build_conditional_expr, build_c_cast, convert_for_assignment,
	build_binary_op): Use it.

testsuite:
	* gcc.dg/c90-const-expr-5.c, gcc.dg/c99-const-expr-5.c: New tests.

From-SVN: r108022