Add availability for the Failure associated type of async sequences

Author: DougGregor

stdlib/public/Concurrency/AsyncFlatMapSequence.swift

@@ -74,6 +74,7 @@ extension AsyncSequence {
   @preconcurrency 
   @_alwaysEmitIntoClient
   @inlinable
+  @available(SwiftStdlib 5.11, *)
   public __consuming func flatMap<SegmentOfResult: AsyncSequence>(
     _ transform: @Sendable @escaping (Element) async -> SegmentOfResult
   ) -> AsyncFlatMapSequence<Self, SegmentOfResult>
@@ -110,6 +111,7 @@ extension AsyncSequence {
   @preconcurrency 
   @_alwaysEmitIntoClient
   @inlinable
+  @available(SwiftStdlib 5.11, *)
   public __consuming func flatMap<SegmentOfResult: AsyncSequence>(
     _ transform: @Sendable @escaping (Element) async -> SegmentOfResult
   ) -> AsyncFlatMapSequence<Self, SegmentOfResult>
@@ -146,6 +148,7 @@ extension AsyncSequence {
   @preconcurrency 
   @_alwaysEmitIntoClient
   @inlinable
+  @available(SwiftStdlib 5.11, *)
   public __consuming func flatMap<SegmentOfResult: AsyncSequence>(
     _ transform: @Sendable @escaping (Element) async -> SegmentOfResult
   ) -> AsyncFlatMapSequence<Self, SegmentOfResult>

stdlib/public/Concurrency/AsyncIteratorProtocol.swift

@@ -90,6 +90,7 @@ public protocol AsyncIteratorProtocol<Element, Failure> {
   associatedtype Element
 
   /// The type of failure produced by iteration.
+  @available(SwiftStdlib 5.11, *)
   associatedtype Failure: Error = any Error
 
   /// Asynchronously advances to the next element and returns it, or ends the

stdlib/public/Concurrency/AsyncSequence.swift

@@ -80,6 +80,7 @@ public protocol AsyncSequence<Element, Failure> {
   associatedtype Element
 
   /// The type of errors produced when iteration over the sequence fails.
+  @available(SwiftStdlib 5.11, *)
   associatedtype Failure: Error = AsyncIterator.Failure
       where AsyncIterator.Failure == Failure
 

test/Concurrency/typed_throws.swift

@@ -7,14 +7,14 @@ enum MyError: Error {
   case epicFailed
 }
 
-@available(SwiftStdlib 5.1, *)
+@available(SwiftStdlib 5.11, *)
 func testAsyncFor<S: AsyncSequence>(seq: S) async throws(MyError) {
   // expected-error@+1{{thrown expression type 'S.Failure' cannot be converted to error type 'MyError'}}
   for try await _ in seq {
   }
 }
 
-@available(SwiftStdlib 5.1, *)
+@available(SwiftStdlib 5.11, *)
 func testAsyncFor<S: AsyncSequence>(seq: S) async throws(MyError)
   where S.Failure == MyError
 {