When you need to create a generic for Array you may either need Sequence or Collection or just Array. I might lack some understanding here because it seems trickier to understand which to use then I expect it to be.

With this I made a somewhat unnecessary extension because after I made it I found replaceSubrange. Either way though I figured out how to make a generic extension on Array that returns an Array containing the type that the Array holds with the predefined type Element.

extension Array {
   func replace(index i: Int, element: Element)  -> [Element] {
       Array(self[0 ..< i]) + [element] + Array(self[(i+1) ..< count])
   }
}

While digging around in the code I found this extension on Array from Collection.Array.swift jumped out at me:

extension ContiguousArray {
    @inlinable public mutating func replaceSubrange<C>(
        _ subrange: Range<Int>,
        with newElements: C
    ) where Element == C.Element, C : Collection
}

It receives a generic Collection of type C and specifies that the elements of C are of type C.Element. This allows the caller to tell the compiler the type at the call site and for Swift to continue to know the type.