"Probably useless use of a constant in grep block" is a step in the wrong direction. There is definitely an error, and it's due to the use of a constant.

One could say that it's the grep that useless, but that's moving away from the cause of the problem, and that's only the case for true constants. In fact, it's impossible to talking in terms "useless use" for false constants in a grep block.

However, using "useless use" in the error message provides familiarity. Combined with identifying the constant as the source of the problem, this message allows us to identify the problem more easily than a more accurate message would.