[Tests] NFC: Add a test-case for invalid optional promotion

Author: xedin

test/Constraints/optional.swift

@@ -598,3 +598,19 @@ do {
   test(x!) // expected-error {{no exact matches in call to local function 'test'}}
   // expected-error@-1 {{cannot force unwrap value of non-optional type 'Double'}}
 }
+
+// Diagnose cases of invalid chaining when parameter is not optional based on context.
+do {
+  class Test {
+    var value: Int = 42
+  }
+
+  class Container {
+    let test: Test = Test()
+
+    func loop() {
+      [test].forEach { $0?.value = 42 }
+      // expected-error@-1 {{cannot use optional chaining on non-optional value of type 'Test'}}
+    }
+  }
+}