The non-allocating GradientCache constructor has a line that calls eltype(df), but I can't see df anywhere in the arguments list?

https://github.com/JuliaDiffEq/DiffEqDiffTools.jl/blob/master/src/gradients.jl#L57
https://github.com/JuliaDiffEq/DiffEqDiffTools.jl/blob/master/README.md#allocating-cache-constructor

@dextorious / @ChrisRackauckas