Using functions which don't affect the derivative in differentiable Swift

I'm trying to use Array.reserveCapacity in a function that is @differentiable. Reserving capacity should have no effect on the derivative, but I can't seem to convince the compiler of this. withoutDerivative(at: array.reserveCapacity(withoutDerivative(at: minimumCapacity))) doesn't seem to appease the compiler, and I also can't figure out how to add a derivative to reserveCapacity or what to specify for the pullback.

Anyone have any advice?