Test: Update swift 5 keypath tests to reflect and test new behavior

Author: sl

test/Constraints/keypath_swift_5.swift

@@ -7,15 +7,30 @@ struct S {
     let _: WritableKeyPath<S, Int> = \.i // expected-error {{cannot convert value of type 'KeyPath<S, Int>' to specified type 'WritableKeyPath<S, Int>'}}
 
     S()[keyPath: \.i] = 1
-    // expected-error@-1 {{cannot assign through subscript: immutable key path}}
+    // expected-error@-1 {{cannot assign through subscript: the key path is a read-only key path}}
   }
 }
 
 func test() {
   let _: WritableKeyPath<C, Int> = \.i // expected-error {{cannot convert value of type 'KeyPath<C, Int>' to specified type 'WritableKeyPath<C, Int>'}}
 
   C()[keyPath: \.i] = 1
-  // expected-error@-1 {{cannot assign through subscript: immutable key path}}
+  // expected-error@-1 {{cannot assign through subscript: the key path is a read-only key path}}
 
   let _ = C()[keyPath: \.i] // no warning for a read
 }
+
+
+struct T {
+    private(set) var a: Int
+    init(a: Int) {
+        self.a = a
+    }
+}
+
+func testReadOnlyKeyPathDiagnostics() {
+  let path = \T.a
+  var t = T(a: 3)
+  t[keyPath: path] = 4 // expected-error {{cannot assign through subscript: 'path' is a read-only key path}}
+  t[keyPath: \T.a] = 4 // expected-error {{cannot assign through subscript: the key path is a read-only key path}}
+}