Operator annotation can reject explicit operand types in error #330

A copy-pasto in a condition and gap in test coverage let this one through, with an @operator with synthetic= must not specify operand types when the operator is not synthetic. As it is most convenient to put the @Operator annotation right on a method and let the operand type(s) be inferred, this would be rarely seen in practice, nor hard to work around. But it shouldn't have gotten through.