Add test case from rdar://problem/54322807.

Author: DougGregor

test/Constraints/keypath.swift

@@ -34,3 +34,24 @@ class Demo {
 let some = Some(keyPath: \Demo.here)
 // expected-error@-1 {{cannot convert value of type 'ReferenceWritableKeyPath<Demo, (() -> Void)?>' to expected argument type 'KeyPath<_, ((_) -> Void)?>'}}
 
+// rdar://problem/54322807
+struct X<T> {
+  init(foo: KeyPath<T, Bool>) { }
+  init(foo: KeyPath<T, Bool?>) { }
+}
+
+struct Wibble {
+  var boolProperty = false
+}
+
+struct Bar {
+  var optWibble: Wibble? = nil
+}
+
+class Foo {
+  var optBar: Bar? = nil
+}
+
+func testFoo<T: Foo>(_: T) {
+  let _: X<T> = .init(foo: \.optBar!.optWibble?.boolProperty)
+}