Grouping.swift

struct A { let a: String }
let array = [A(a: "b"), A(a: "C"), A(a: "b")]
let dictionary = Dictionary(grouping: array) { $0.a }
dictionary

extension Array {
    func group<K: Hashable, Element>(_ fn: (Element) -> K) -> Dictionary<K, [Element]> {
        return Dictionary<K, [Element]>(grouping: self, by: fn)
    }
}
let dictionary2 = array.group { $0.a }
dictionary2

Remove Element as a generic type local to the function (since that override's Array's associated Element type). So replace line 7 with this:

func group<K: Hashable>(_ fn: (Element) -> K) -> Dictionary<K, [Element]> {

And it'll work 🎉