[TypeChecker] NFC: Test key path with contextual optional typealias type

Author: xedin

test/expr/unary/keypath/keypath.swift

@@ -807,6 +807,25 @@ func test_keypath_in_any_context() {
   _ = foo(\String.count) // Ok
 }
 
+protocol PWithTypeAlias {
+  typealias Key = WritableKeyPath<Self, Int?>
+  static var fooKey: Key? { get }
+  static var barKey: Key! { get }
+  static var fazKey: Key?? { get }
+  static var bazKey: Key?! { get }
+}
+
+func test_keypath_inference_with_optionals() {
+  final class S : PWithTypeAlias {
+    static var fooKey: Key? { return \.foo }
+    static var barKey: Key! { return \.foo }
+    static var fazKey: Key?? { return \.foo }
+    static var bazKey: Key?! { return \.foo }
+
+    var foo: Int? = nil
+  }
+}
+
 func testSyntaxErrors() { // expected-note{{}}
   _ = \.  ; // expected-error{{expected member name following '.'}}
   _ = \.a ;