[Concurrency] Converting from @execution(concurrent) to actor-isolated crosses an isolation boundary

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -2804,8 +2804,21 @@ namespace {
               }
               break;
 
+            case FunctionTypeIsolation::Kind::NonIsolated: {
+              // Since @execution(concurrent) as an asynchronous
+              // function it would mean that without Sendable
+              // check it would be possible for non-Sendable state
+              // to escape from actor isolation.
+              if (fromFnType->isAsync()) {
+                diagnoseNonSendableParametersAndResult(
+                  toFnType, /*downgradeToWarning=*/true);
+                break;
+              }
+              // Runs on the actor.
+              break;
+            }
+
             // Runs on the actor.
-            case FunctionTypeIsolation::Kind::NonIsolated:
             case FunctionTypeIsolation::Kind::NonIsolatedCaller:
               break;
             }

test/Concurrency/attr_execution_conversions.swift

@@ -123,10 +123,12 @@ func testNonSendableDiagnostics(
   // expected-warning@-1 {{cannot convert '@execution(caller) @Sendable () async -> NonSendable' to '() async -> NonSendable' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
 
   let _: @MainActor (NonSendable) async -> Void = nonIsolated1 // Ok
-  let _: @MainActor (NonSendable) async -> Void = nonIsolated2 // Ok
+  let _: @MainActor (NonSendable) async -> Void = nonIsolated2 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
+  // expected-warning@-1 {{cannot convert '@Sendable (NonSendable) async -> Void' to '@MainActor (NonSendable) async -> Void' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
 
   let _: @MainActor () async -> NonSendable = nonIsolated3 // Ok
-  let _: @MainActor () async -> NonSendable = nonIsolated4 // Ok
+  let _: @MainActor () async -> NonSendable = nonIsolated4 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
+  // expected-warning@-1 {{cannot convert '@Sendable () async -> NonSendable' to '@MainActor () async -> NonSendable' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
 
   let _: @MainActor (NonSendable) async -> Void = caller1 // Ok
   let _: @MainActor () async -> NonSendable = caller2 // Ok