The code below shows warning, why? And how to implement it without warning?

An imported function has been extended without using 
module defined typed arguments.Julia(TypePiracy)

import Base: fill

fill(f::Function, length::Int) = [f(i) for i in 1:length] # <= warn

println(fill(1, 3))
println(fill((i) -> i^2, 3))

P.S.

Also, is there a way to differentiate fn with single int arg and no args, to make following code work:

println(fill(1, 3))
println(fill((i) -> i^2, 3))
println(fill(() -> 1, 3)) # <= Error