[6.0] AsyncIteratorProtocol conformance fixes

lib/Sema/AssociatedTypeInference.cpp

@@ -2426,13 +2426,17 @@ AssociatedTypeInference::computeFailureTypeWitness(
   for (const auto &witness : valueWitnesses) {
     if (isAsyncIteratorProtocolNext(witness.first)) {
       if (auto witnessFunc = dyn_cast<AbstractFunctionDecl>(witness.second)) {
+        auto thrownError = witnessFunc->getEffectiveThrownErrorType();
+
         // If it doesn't throw, Failure == Never.
-        if (!witnessFunc->hasThrows())
+        if (!thrownError)
           return AbstractTypeWitness(assocType, ctx.getNeverType());
 
-        // If it isn't 'rethrows', Failure == any Error.
-        if (!witnessFunc->getAttrs().hasAttribute<RethrowsAttr>())
-          return AbstractTypeWitness(assocType, ctx.getErrorExistentialType());
+        // If it isn't 'rethrows', use the thrown error type;.
+        if (!witnessFunc->getAttrs().hasAttribute<RethrowsAttr>()) {
+          return AbstractTypeWitness(assocType,
+                                     dc->mapTypeIntoContext(*thrownError));
+        }
 
         for (auto req : witnessFunc->getGenericSignature().getRequirements()) {
           if (req.getKind() == RequirementKind::Conformance) {

stdlib/public/Concurrency/AsyncIteratorProtocol.swift

@@ -111,8 +111,9 @@ public protocol AsyncIteratorProtocol<Element, Failure> {
 
 @available(SwiftStdlib 5.1, *)
 extension AsyncIteratorProtocol {
-  /// Default implementation of `next()` in terms of `next()`, which is
-  /// required to maintain backward compatibility with existing async iterators.
+  /// Default implementation of `next(isolation:)` in terms of `next()`, which
+  /// is required to maintain backward compatibility with existing async
+  /// iterators.
   @available(SwiftStdlib 6.0, *)
   @inlinable
   public mutating func next(isolation actor: isolated (any Actor)?) async throws(Failure) -> Element? {
@@ -123,3 +124,15 @@ extension AsyncIteratorProtocol {
     }
   }
 }
+
+@available(SwiftStdlib 5.1, *)
+extension AsyncIteratorProtocol {
+  /// Default implementation of `next()` in terms of `next(isolation:)`, which
+  /// is required to maintain backward compatibility with existing async
+  /// iterators.
+  @available(SwiftStdlib 6.0, *)
+  @inlinable
+  public mutating func next() async throws(Failure) -> Element? {
+    return try await next(isolation: nil)
+  }
+}

test/Concurrency/async_sequence_rethrows.swift

@@ -51,3 +51,37 @@ extension InheritsAsyncSequence {
     try await self.reduce(into: [Element]()) { $0.append($1) }
   }
 }
+
+// Ensure that we can get the thrown error type from next().
+struct Data { }
+
+struct ErrorSequence<Element, Failure: Error>: AsyncSequence, AsyncIteratorProtocol {
+    let throwError : Failure
+
+    func makeAsyncIterator() -> ErrorSequence<Element, Failure> {
+        self
+    }
+
+    mutating func next() async throws(Failure) -> Element? {
+        throw throwError
+    }
+}
+
+enum MyError: Error {
+    case foo
+}
+
+
+func getASequence() -> any AsyncSequence<Data, MyError> {
+    return ErrorSequence<Data, _>(throwError: MyError.foo) // ERROR: Cannot convert return expression of type 'any Error' to return type 'MyError'
+}
+
+// Test the default implementation of next() in terms of next(isolation:).
+struct AsyncIteratorWithOnlyNextIsolation: AsyncIteratorProtocol {
+  public mutating func next(isolation: (any Actor)?) throws(MyError) -> Int? { 0 }
+}
+
+// Test the default implementation of next(isolation:) in terms of next().
+struct AsyncIteratorWithOnlyNext: AsyncIteratorProtocol {
+  public mutating func next() throws(MyError) -> Int? { 0 }
+}

test/abi/macOS/arm64/concurrency.swift

@@ -276,6 +276,11 @@ Added: _$ss26withTaskExecutorPreference_9isolation9operationxSch_pSg_ScA_pSgYixy
 // async function pointer to Swift.withTaskExecutorPreference<A, B where B: Swift.Error>(_: Swift.TaskExecutor?, isolation: isolated Swift.Actor?, operation: () async throws(B) -> A) async throws(B) -> A
 Added: _$ss26withTaskExecutorPreference_9isolation9operationxSch_pSg_ScA_pSgYixyYaq_YKXEtYaq_YKs5ErrorR_r0_lFTu
 
+
+// next() default implementation in terms of next(isolation:)
+Added: _$sScIsE4next7ElementQzSgyYa7FailureQzYKF
+Added: _$sScIsE4next7ElementQzSgyYa7FailureQzYKFTu
+
 // === SerialExecutor.checkIsolated()
 Added: _swift_task_checkIsolated
 Added: _swift_task_checkIsolated_hook
@@ -284,4 +289,5 @@ Added: _$sScfsE13checkIsolatedyyF
 // dispatch thunk of Swift.SerialExecutor.checkIsolated() -> ()
 Added: _$sScf13checkIsolatedyyFTj
 // method descriptor for Swift.SerialExecutor.checkIsolated() -> ()
-Added: _$sScf13checkIsolatedyyFTq
+Added: _$sScf13checkIsolatedyyFTq
+

test/abi/macOS/x86_64/concurrency.swift

@@ -276,6 +276,11 @@ Added: _$ss26withTaskExecutorPreference_9isolation9operationxSch_pSg_ScA_pSgYixy
 // async function pointer to Swift.withTaskExecutorPreference<A, B where B: Swift.Error>(_: Swift.TaskExecutor?, isolation: isolated Swift.Actor?, operation: () async throws(B) -> A) async throws(B) -> A
 Added: _$ss26withTaskExecutorPreference_9isolation9operationxSch_pSg_ScA_pSgYixyYaq_YKXEtYaq_YKs5ErrorR_r0_lFTu
 
+
+// next() default implementation in terms of next(isolation:)
+Added: _$sScIsE4next7ElementQzSgyYa7FailureQzYKF
+Added: _$sScIsE4next7ElementQzSgyYa7FailureQzYKFTu
+
 // === SerialExecutor.checkIsolated()
 Added: _swift_task_checkIsolated
 Added: _swift_task_checkIsolated_hook
@@ -284,4 +289,5 @@ Added: _$sScfsE13checkIsolatedyyF
 // dispatch thunk of Swift.SerialExecutor.checkIsolated() -> ()
 Added: _$sScf13checkIsolatedyyFTj
 // method descriptor for Swift.SerialExecutor.checkIsolated() -> ()
-Added: _$sScf13checkIsolatedyyFTq
+Added: _$sScf13checkIsolatedyyFTq
+