[transferring] Transferring results shouldn't have the following error emitted: "non-sendable type 'NonSendableKlass' returned by implicitly asynchronous call to main actor-isolated function cannot cross actor boundary"

Transferring makes this safe.

rdar://123712033

Author: gottesmm

lib/Sema/TypeCheckConcurrency.cpp

@@ -3508,8 +3508,11 @@ namespace {
         diagnoseApplyArgSendability(apply, getDeclContext());
       }
 
-      // Check for sendability of the result type.
-      if (diagnoseNonSendableTypes(
+      // Check for sendability of the result type if we do not have a
+      // transferring result.
+      if ((!ctx.LangOpts.hasFeature(Feature::TransferringArgsAndResults) ||
+           !fnType->hasTransferringResult()) &&
+          diagnoseNonSendableTypes(
              fnType->getResult(), getDeclContext(),
              /*inDerivedConformance*/Type(),
              apply->getLoc(),

test/Concurrency/transfernonsendable_strong_transferring_results.swift

@@ -49,6 +49,8 @@ func transferAsyncResultWithTransferringArg(_ x: transferring NonSendableKlass)
 func transferAsyncResultWithTransferringArg2(_ x: transferring NonSendableKlass, _ y: NonSendableKlass) async -> transferring NonSendableKlass { NonSendableKlass() }
 func transferAsyncResultWithTransferringArg2Throwing(_ x: transferring NonSendableKlass, _ y: NonSendableKlass) async throws -> transferring NonSendableKlass { NonSendableKlass() }
 
+@MainActor func transferAsyncResultMainActor() async -> transferring NonSendableKlass { NonSendableKlass() }
+
 @MainActor var globalNonSendableKlass = NonSendableKlass()
 
 /////////////////
@@ -106,3 +108,11 @@ func transferReturnArg(_ x: NonSendableKlass) -> transferring NonSendableKlass {
 func transferReturnArgTuple(_ x: transferring NonSendableKlass) -> transferring (NonSendableKlass, NonSendableKlass) {
   return (x, x)
 }
+
+func useTransferredResultMainActor() async {
+  let _ = await transferAsyncResultMainActor()
+}
+
+func useTransferredResult() async {
+  let _ = await transferAsyncResult()
+}