* c-common.c (binary_op_error): Do not allow NOP_EXPR.
	(c_common_truthvalue_conversion): Handle NOP_EXPR the same as
	CONVERT_EXPR.
	(check_function_arguments_recurse): Allow both NOP_EXPR and
	CONVERT_EXPR but check conversions preserve precision.
	* c-typeck.c (pointer_diff): Allow both NOP_EXPR and CONVERT_EXPR
	but check conversions preserve precision.
	(build_unary_op): Don't allow NOP_EXPR.  Use gcc_unreachable () in
	default case.

testsuite:
	* gcc.dg/format/cast-1.c: New test.

From-SVN: r108020