[Concurrency] Diagnose loss of global actor isolation in async function conversions

Perform `Sendable` checking on parameter/result of the function
type when conversion between asynchroneous functions results in
a loss of global actor isolation attribute because access would
result in data crossing an isolation boundary.

This is a warning until Swift language mode 6.

Resolves: rdar://130168104

lib/Sema/TypeCheckConcurrency.cpp

@@ -2274,12 +2274,6 @@ static bool safeToDropGlobalActor(
   if (otherIsolation.isErased())
     return true;
 
-  // We currently allow unconditional dropping of global actors from
-  // async function types, despite this confusing Sendable checking
-  // in light of SE-338.
-  if (funcTy->isAsync())
-    return true;
-
   // If the argument is passed over an isolation boundary, it's not
   // safe to erase actor isolation, because the callee can call the
   // function synchronously from outside the isolation domain.
@@ -2629,7 +2623,8 @@ namespace {
     /// be correct AND the solver doesn't know, so we must emit a diagnostic.
     void checkFunctionConversion(Expr *funcConv, Type fromType, Type toType) {
       auto diagnoseNonSendableParametersAndResult =
-          [&](FunctionType *fnType, bool downgradeToWarning = false) {
+          [&](FunctionType *fnType,
+              std::optional<unsigned> warnUntilSwiftMode = std::nullopt) {
             auto *dc = getDeclContext();
             llvm::SmallPtrSet<Type, 2> nonSendableTypes;
 
@@ -2659,8 +2654,8 @@ namespace {
                     diag::invalid_function_conversion_with_non_sendable,
                     fromType, toType);
 
-                if (downgradeToWarning)
-                  diag.warnUntilFutureSwiftVersion();
+                if (warnUntilSwiftMode)
+                  diag.warnUntilSwiftVersion(*warnUntilSwiftMode);
               }
 
               for (auto type : nonSendableTypes) {
@@ -2677,20 +2672,27 @@ namespace {
           auto toIsolation = toFnType->getIsolation();
 
           if (auto fromActor = fromFnType->getGlobalActor()) {
-            if (toFnType->hasGlobalActor() ||
-                (toFnType->isAsync() && !toIsolation.isNonIsolatedCaller()))
-              return;
+            if (!toFnType->hasGlobalActor()) {
+              auto dc = const_cast<DeclContext *>(getDeclContext());
+              // If it's unsafe to drop global actor attribute:
+              //  - for Sendable types we are going to perform Sendability
+              //    checking of parameters/result.
+              //  - for non-Sendable types we either leave it to region-based
+              //    isolation to determine whether it's okay or not or
+              //    diagnose if types are not-async.
+              if (safeToDropGlobalActor(dc, fromActor, toType,
+                                        getImmediateApply())) {
+                return;
+              }
 
-            auto dc = const_cast<DeclContext*>(getDeclContext());
-            if (!safeToDropGlobalActor(dc, fromActor, toType,
-                                       getImmediateApply())) {
-              // otherwise, it's not a safe cast.
-              ctx.Diags
-                  .diagnose(funcConv->getLoc(),
-                            diag::converting_func_loses_global_actor, fromType,
-                            toType, fromActor)
-                  .warnUntilSwiftVersion(6);
-              return;
+              if (!toFnType->isAsync()) {
+                ctx.Diags
+                    .diagnose(funcConv->getLoc(),
+                              diag::converting_func_loses_global_actor,
+                              fromType, toType, fromActor)
+                    .warnUntilSwiftVersion(6);
+                return;
+              }
             }
           }
 
@@ -2756,11 +2758,12 @@ namespace {
             case FunctionTypeIsolation::Kind::NonIsolatedCaller:
             case FunctionTypeIsolation::Kind::Erased:
               diagnoseNonSendableParametersAndResult(
-                  toFnType, /*downgradeToWarning=*/true);
+                  toFnType, version::Version::getFutureMajorLanguageVersion());
               break;
 
             case FunctionTypeIsolation::Kind::GlobalActor: {
-              // Handled above by `safeToDropGlobalActor` check.
+              diagnoseNonSendableParametersAndResult(toFnType,
+                                                     /*warnUntilSwiftMode*/ 6);
               break;
             }
 
@@ -2768,7 +2771,8 @@ namespace {
               // nonisolated synchronous <-> @concurrent
               if (fromFnType->isAsync() != toFnType->isAsync()) {
                 diagnoseNonSendableParametersAndResult(
-                    toFnType, /*downgradeToWarning=*/true);
+                    toFnType,
+                    version::Version::getFutureMajorLanguageVersion());
               }
               break;
             }
@@ -2783,7 +2787,7 @@ namespace {
             case FunctionTypeIsolation::Kind::Parameter:
             case FunctionTypeIsolation::Kind::Erased:
               diagnoseNonSendableParametersAndResult(
-                  toFnType, /*downgradeToWarning=*/true);
+                  toFnType, version::Version::getFutureMajorLanguageVersion());
               break;
 
             case FunctionTypeIsolation::Kind::NonIsolated: {
@@ -2793,7 +2797,8 @@ namespace {
               // actor isolation.
               if (fromFnType->isAsync()) {
                 diagnoseNonSendableParametersAndResult(
-                  toFnType, /*downgradeToWarning=*/true);
+                    toFnType,
+                    version::Version::getFutureMajorLanguageVersion());
                 break;
               }
               // Runs on the actor.
@@ -2805,7 +2810,9 @@ namespace {
               break;
 
             case FunctionTypeIsolation::Kind::GlobalActor:
-              llvm_unreachable("invalid conversion");
+              diagnoseNonSendableParametersAndResult(
+                  toFnType, version::Version::getFutureMajorLanguageVersion());
+              break;
             }
             break;
           }

test/Concurrency/attr_execution/conversions.swift

@@ -117,6 +117,11 @@ func testNonSendableDiagnostics(
   let _: nonisolated(nonsending) () async -> NonSendable = globalActor2 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
   // expected-error@-1 {{cannot convert '@MainActor @Sendable () async -> NonSendable' to 'nonisolated(nonsending) () async -> NonSendable' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
 
+  let _: @concurrent (NonSendable) async -> Void = globalActor1 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
+  // expected-warning@-1 {{cannot convert '@MainActor @Sendable (NonSendable) async -> Void' to '(NonSendable) async -> Void' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
+  let _: @concurrent () async -> NonSendable = globalActor2 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
+  // expected-warning@-1 {{cannot convert '@MainActor @Sendable () async -> NonSendable' to '() async -> NonSendable' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
+  
   let _: nonisolated(nonsending) (NonSendable) async -> Void = erased1 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
   // expected-error@-1 {{cannot convert '@isolated(any) @Sendable (NonSendable) async -> Void' to 'nonisolated(nonsending) (NonSendable) async -> Void' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
   let _: nonisolated(nonsending) () async -> NonSendable = erased2 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}

test/Concurrency/attr_execution/conversions_silgen.swift

@@ -396,8 +396,8 @@ func globalActorConversions3(_ x: @escaping @concurrent (SendableKlass) async ->
   _ = await v5(SendableKlass())
 }
 
-// CHECK-LABEL: sil hidden [ossa] @$s21attr_execution_silgen26conversionsFromSyncToAsyncyyyAA16NonSendableKlassCYbc_yAA0jK0CYbScMYccyADYbScMYcctYaF : $@convention(thin) @async (@guaranteed @Sendable @callee_guaranteed (@guaranteed NonSendableKlass) -> (), @guaranteed @Sendable @callee_guaranteed (@guaranteed SendableKlass) -> (), @guaranteed @Sendable @callee_guaranteed (@guaranteed NonSendableKlass) -> ()) -> () {
-// CHECK: bb0([[X:%.*]] : @guaranteed $@Sendable @callee_guaranteed (@guaranteed NonSendableKlass) -> (), [[Y:%.*]] : @guaranteed $@Sendable @callee_guaranteed (@guaranteed SendableKlass) -> (), [[Z:%.*]] : @guaranteed $@Sendable @callee_guaranteed (@guaranteed NonSendableKlass) -> ()):
+// CHECK-LABEL: sil hidden [ossa] @$s21attr_execution_silgen26conversionsFromSyncToAsyncyyyAA16NonSendableKlassCYbc_yAA0jK0CYbScMYcctYaF : $@convention(thin) @async (@guaranteed @Sendable @callee_guaranteed (@guaranteed NonSendableKlass) -> (), @guaranteed @Sendable @callee_guaranteed (@guaranteed SendableKlass) -> ()) -> ()
+// CHECK: bb0([[X:%.*]] : @guaranteed $@Sendable @callee_guaranteed (@guaranteed NonSendableKlass) -> (), [[Y:%.*]] : @guaranteed $@Sendable @callee_guaranteed (@guaranteed SendableKlass) -> ()):
 
 // CHECK:   [[X_C:%.*]] = copy_value [[X]]
 // CHECK:   [[THUNK:%.*]] = function_ref @$s21attr_execution_silgen16NonSendableKlassCIeghg_ScA_pSgACIegHgg_TR : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>, @guaranteed NonSendableKlass, @guaranteed @Sendable @callee_guaranteed (@guaranteed NonSendableKlass) -> ()) -> ()
@@ -411,19 +411,13 @@ func globalActorConversions3(_ x: @escaping @concurrent (SendableKlass) async ->
 // CHECK:   [[THUNK:%.*]] = function_ref @$s21attr_execution_silgen13SendableKlassCIeghg_ACIegHg_TRScMTU : $@convention(thin) @async (@guaranteed SendableKlass, @guaranteed @Sendable @callee_guaranteed (@guaranteed SendableKlass) -> ()) -> ()
 // CHECK:   [[PAI:%.*]] = partial_apply [callee_guaranteed] [[THUNK]]([[Y_C]])
 
-// CHECK:   [[Z_C:%.*]] = copy_value [[Z]]
-// CHECK:   [[THUNK:%.*]] = function_ref @$s21attr_execution_silgen16NonSendableKlassCIeghg_ACIegHg_TRScMTU : $@convention(thin) @async (@guaranteed NonSendableKlass, @guaranteed @Sendable @callee_guaranteed (@guaranteed NonSendableKlass) -> ()) -> ()
-// CHECK:   [[PAI:%.*]] = partial_apply [callee_guaranteed] [[THUNK]]([[Z_C]])
-
-// CHECK: } // end sil function '$s21attr_execution_silgen26conversionsFromSyncToAsyncyyyAA16NonSendableKlassCYbc_yAA0jK0CYbScMYccyADYbScMYcctYaF'
+// CHECK: } // end sil function '$s21attr_execution_silgen26conversionsFromSyncToAsyncyyyAA16NonSendableKlassCYbc_yAA0jK0CYbScMYcctYaF'
 
 func conversionsFromSyncToAsync(_ x: @escaping @Sendable (NonSendableKlass) -> Void,
-                                _ y: @escaping @MainActor @Sendable (SendableKlass) -> Void,
-                                _ z: @escaping @MainActor @Sendable (NonSendableKlass) -> Void) async {
+                                _ y: @escaping @MainActor @Sendable (SendableKlass) -> Void) async {
   let _: nonisolated(nonsending) (NonSendableKlass) async -> Void = x
   let _: nonisolated(nonsending) (SendableKlass) async -> Void = y
   let _: @concurrent (SendableKlass) async -> Void = y
-  let _: @concurrent (NonSendableKlass) async -> Void = z
 }
 
 func testThatClosuresAssumeIsolation(fn: inout nonisolated(nonsending) (Int) async -> Void) {

test/Concurrency/global_actor_function_types_swift6.swift

@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift  -target %target-swift-5.1-abi-triple -language-mode 6
+
+final class NonSendable {
+}
+
+@available(*, unavailable)
+extension NonSendable: Sendable {}
+
+actor Test {
+  func testNonSendableCrossingIsolationinAsync(v: NonSendable) {
+    let _: () async -> NonSendable = { @MainActor in v }
+    // expected-error@-1 {{cannot convert '@MainActor @Sendable () async -> NonSendable' to '() async -> NonSendable' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
+    // expected-note@-2 {{type 'NonSendable' does not conform to 'Sendable' protocol}}
+  }
+}