[region-isolation] Add a test case for a reported failure that I already fixed.

This is just to make sure that we do not break it again.

rdar://128195659

Author: gottesmm

test/Concurrency/transfernonsendable.swift

@@ -26,6 +26,8 @@ class NonSendableKlass { // expected-complete-note 53{{}}
   func asyncCall() async {}
   func asyncCallWithIsolatedParameter(isolation: isolated (any Actor)? = #isolation) async {
   }
+
+  func getSendableGenericStructAsync() async -> SendableGenericStruct { fatalError() }
 }
 
 class SendableKlass : @unchecked Sendable {}
@@ -82,6 +84,10 @@ class TwoFieldKlassClassBox {
   var recursive: TwoFieldKlassClassBox? = nil
 }
 
+struct SendableGenericStruct : Sendable {
+  var x = SendableKlass()
+}
+
 ////////////////////////////
 // MARK: Actor Self Tests //
 ////////////////////////////
@@ -1811,6 +1817,16 @@ actor FunctionWithSendableResultAndIsolationActor {
     }
 }
 
+// This was a test case that we used to emit an "pattern the compiler doesn't
+// understand" error. We now accept it, so lets make sure we keep doing so!
+@MainActor
+func previouslyBrokenTestCase(ns: NonSendableKlass) async -> SendableGenericStruct? {
+  return await { () -> SendableGenericStruct? in
+    return await ns.getSendableGenericStructAsync() // expected-warning {{sending 'ns' risks causing data races}}
+    // expected-note @-1 {{sending main actor-isolated 'ns' to nonisolated instance method 'getSendableGenericStructAsync()' risks causing data races between nonisolated and main actor-isolated uses}}
+  }()
+}
+
 @MainActor
 func testThatGlobalActorTakesPrecedenceOverActorIsolationOnMethods() async {
   let a = MyActor()