Add a proper TaskGroup.Iterator.next(isolation:)

Implement this function to pass the isolation through to the task group's
`next(isolation:)`. Fixes rdar://129690995.

Author: DougGregor

stdlib/public/Concurrency/TaskGroup.swift

@@ -1211,6 +1211,29 @@ extension TaskGroup: AsyncSequence {
       return element
     }
 
+    /// Advances to and returns the result of the next child task.
+    ///
+    /// The elements returned from this method
+    /// appear in the order that the tasks *completed*,
+    /// not in the order that those tasks were added to the task group.
+    /// After this method returns `nil`,
+    /// this iterator is guaranteed to never produce more values.
+    ///
+    /// For more information about the iteration order and semantics,
+    /// see `TaskGroup.next()`.
+    ///
+    /// - Returns: The value returned by the next child task that completes,
+    ///   or `nil` if there are no remaining child tasks,
+    @available(SwiftStdlib 6.0, *)
+    public mutating func next(isolation actor: isolated (any Actor)?) async -> Element? {
+      guard !finished else { return nil }
+      guard let element = await group.next(isolation: actor) else {
+        finished = true
+        return nil
+      }
+      return element
+    }
+
     public mutating func cancel() {
       finished = true
       group.cancelAll()
@@ -1324,7 +1347,7 @@ extension ThrowingTaskGroup: AsyncSequence {
     public mutating func next(isolation actor: isolated (any Actor)?) async throws(Failure) -> Element? {
       guard !finished else { return nil }
       do {
-        guard let element = try await group.next() else {
+        guard let element = try await group.next(isolation: actor) else {
           finished = true
           return nil
         }

test/Concurrency/async_sequence_macosx.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -target %target-cpu-apple-macos14.0 %s -emit-sil -o /dev/null -verify
+// RUN: %target-swift-frontend -target %target-cpu-apple-macos14.0 %s -emit-sil -o /dev/null -verify -swift-version 6
 
 // REQUIRES: concurrency, OS=macosx
 
@@ -15,4 +15,14 @@ func f<S: AsyncSequence>(s: S) async throws {
   }
 }
 
-
+// Make sure we don't complain about crossing a concurrency boundary here.
+@MainActor
+class Store<Action: Sendable> {
+    private func intercept(_ action: Action) async {
+        await withTaskGroup(of: Optional<Action>.self) { group in
+            for await case let nextAction? in group {
+                _ = nextAction
+            }
+        }
+    }
+}