`KeyPath` for immutable property works differently between Swift4.2 / 5 #CodePiece #tryswiftconf

CodePiece.swift

func replace<T>(_ root: inout T, keyPath: KeyPath<T, String>) {
    if let k = keyPath as? WritableKeyPath<T, String> {
        print("writable:", keyPath)
        root[keyPath: k] += " 🖍"
    } else {
        print("not writable:", keyPath)
    }
}
struct S {
    let a = "let"
    private(set) var b = "private(set) var"
    var c = "var"
}

var s = S()

replace(&s, keyPath: \S.a) //
replace(&s, keyPath: \S.b)
replace(&s, keyPath: \S.c)

dump(s)

/* @ Swift 4.2 (Xcode 10.1)
 writable: Swift.WritableKeyPath<__lldb_expr_2.S, Swift.String>
 writable: Swift.WritableKeyPath<__lldb_expr_2.S, Swift.String>
 writable: Swift.WritableKeyPath<__lldb_expr_2.S, Swift.String>
 ▿ __lldb_expr_2.S
 - a: "let 🖍"
 - b: "private(set) var 🖍"
 - c: "var 🖍"
 */

/* @ Swift 5 (Xcode 10.2 beta)
 not writable: Swift.KeyPath<__lldb_expr_14.S, Swift.String>
 writable: Swift.WritableKeyPath<__lldb_expr_14.S, Swift.String>
 writable: Swift.WritableKeyPath<__lldb_expr_14.S, Swift.String>
 ▿ __lldb_expr_14.S
 - a: "let"
 - b: "private(set) var 🖍"
 - c: "var 🖍"
 */