* tree.h (DECIMAL_FLOAT_TYPE_P): New.
	* c-typeck.c (c_common_type): Disallow operations on decimal float
	types and other float types.
	* convert.c (convert_to_real): Don't ignore conversions involving
	decimal float types.

testsuite:
	* gcc.dg/dfp/usual-arith-conv-bad.c: New test.

From-SVN: r114951