From a8f3bb5e5efd433c79a1708385b8cc395bf760e8 Mon Sep 17 00:00:00 2001
From: Doug Gregor <dgregor@apple.com>
Date: Thu, 28 Mar 2024 11:20:14 -0700
Subject: [PATCH] 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.
---
 .../Concurrency/AsyncIteratorProtocol.swift     | 17 +++++++++++++++--
 test/Concurrency/async_sequence_rethrows.swift  | 10 ++++++++++
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/stdlib/public/Concurrency/AsyncIteratorProtocol.swift b/stdlib/public/Concurrency/AsyncIteratorProtocol.swift
index 56ff75bc56f..bc390d431bb 100644
--- a/stdlib/public/Concurrency/AsyncIteratorProtocol.swift
+++ b/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)
+  }
+}
diff --git a/test/Concurrency/async_sequence_rethrows.swift b/test/Concurrency/async_sequence_rethrows.swift
index 187af6e903c..9a215f07eda 100644
--- a/test/Concurrency/async_sequence_rethrows.swift
+++ b/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 }
+}