Why can't I duplicate a slice with `copy()`?

I need to make a copy of a slice in Go and reading the docs there is a copy function at my disposal.

The copy built-in function copies elements from a source slice into a destination slice. (As a special case, it also will copy bytes from a string to a slice of bytes.) The source and destination may overlap. Copy returns the number of elements copied, which will be the minimum of len(src) and len(dst).

But when I do:

arr := []int{1, 2, 3}
tmp := []int{}
copy(tmp, arr)

My tmp is empty as it was before (I even tried to use arr, tmp ):

[1 2 3]

You can check it on go playground. So why can not I copy a slice?