Implement AsyncIteratorProtocol.next() in terms of next(isolation:).

New async iterators should be able to implement only `next(isolation:)` and
get the older `next()` implementation via a default. Implement the
appropriate default witness.

Fixes rdar://125447861.

Author: DougGregor

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

@@ -75,3 +75,13 @@ enum MyError: Error {
 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 }
+}