Make CheckedContinuation and UnsafeContinuation unconditionally Sendable.

The continuation types were conditionally `Sendable` based on whether
the result type of the continuation was `Sendable`. However,
conceptually, the return value is never leaving the current task, so
it is never actually crossing concurrency boundaries. Therefore, we
can make the continuation types unconditionally `Sendable`.

Fixes rdar://98462858.

Author: DougGregor

stdlib/public/BackDeployConcurrency/CheckedContinuation.swift

@@ -119,7 +119,7 @@ internal final class CheckedContinuationCanary: @unchecked Sendable {
 /// you can replace one with the other in most circumstances,
 /// without making other changes.
 @available(SwiftStdlib 5.1, *)
-public struct CheckedContinuation<T, E: Error> {
+public struct CheckedContinuation<T, E: Error>: Sendable {
   private let canary: CheckedContinuationCanary
 
   /// Creates a checked continuation from an unsafe continuation.
@@ -186,9 +186,6 @@ public struct CheckedContinuation<T, E: Error> {
   }
 }
 
-@available(SwiftStdlib 5.1, *)
-extension CheckedContinuation: Sendable where T: Sendable { }
-
 @available(SwiftStdlib 5.1, *)
 extension CheckedContinuation {
   /// Resume the task awaiting the continuation by having it either

stdlib/public/BackDeployConcurrency/PartialAsyncTask.swift

@@ -69,7 +69,7 @@ public struct UnownedJob: Sendable {
 /// without making other changes.
 @available(SwiftStdlib 5.1, *)
 @frozen
-public struct UnsafeContinuation<T, E: Error> {
+public struct UnsafeContinuation<T, E: Error>: Sendable {
   @usableFromInline internal var context: Builtin.RawUnsafeContinuation
 
   @_alwaysEmitIntoClient
@@ -144,9 +144,6 @@ public struct UnsafeContinuation<T, E: Error> {
   }
 }
 
-@available(SwiftStdlib 5.1, *)
-extension UnsafeContinuation: Sendable where T: Sendable { }
-
 @available(SwiftStdlib 5.1, *)
 extension UnsafeContinuation {
   /// Resume the task that's awaiting the continuation

stdlib/public/Concurrency/CheckedContinuation.swift

@@ -119,7 +119,7 @@ internal final class CheckedContinuationCanary: @unchecked Sendable {
 /// you can replace one with the other in most circumstances,
 /// without making other changes.
 @available(SwiftStdlib 5.1, *)
-public struct CheckedContinuation<T, E: Error> {
+public struct CheckedContinuation<T, E: Error>: Sendable {
   private let canary: CheckedContinuationCanary
 
   /// Creates a checked continuation from an unsafe continuation.
@@ -186,9 +186,6 @@ public struct CheckedContinuation<T, E: Error> {
   }
 }
 
-@available(SwiftStdlib 5.1, *)
-extension CheckedContinuation: Sendable where T: Sendable { }
-
 @available(SwiftStdlib 5.1, *)
 extension CheckedContinuation {
   /// Resume the task awaiting the continuation by having it either

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -69,7 +69,7 @@ public struct UnownedJob: Sendable {
 /// without making other changes.
 @available(SwiftStdlib 5.1, *)
 @frozen
-public struct UnsafeContinuation<T, E: Error> {
+public struct UnsafeContinuation<T, E: Error>: Sendable {
   @usableFromInline internal var context: Builtin.RawUnsafeContinuation
 
   @_alwaysEmitIntoClient
@@ -144,9 +144,6 @@ public struct UnsafeContinuation<T, E: Error> {
   }
 }
 
-@available(SwiftStdlib 5.1, *)
-extension UnsafeContinuation: Sendable where T: Sendable { }
-
 @available(SwiftStdlib 5.1, *)
 extension UnsafeContinuation {
   /// Resume the task that's awaiting the continuation

test/Concurrency/async_tasks.swift

@@ -89,7 +89,6 @@ func test_unsafeThrowingContinuations() async throws {
 
 // ==== Sendability ------------------------------------------------------------
 class NotSendable { }
-// expected-note@-1{{class 'NotSendable' does not conform to the 'Sendable' protocol}}
 
 @available(SwiftStdlib 5.1, *)
 func test_nonsendableContinuation() async throws {
@@ -99,7 +98,7 @@ func test_nonsendableContinuation() async throws {
 
   let _: NotSendable = try await withUnsafeThrowingContinuation { continuation in
     Task {
-      continuation.resume(returning: NotSendable()) // expected-warning{{capture of 'continuation' with non-sendable type 'UnsafeContinuation<NotSendable, any Error>' in a `@Sendable` closure}}
+      continuation.resume(returning: NotSendable()) // okay
     }
   }
 }