Make ThrowingTaskGroup.nextResult() non-throwing.

We erroneously marked this API as `throws`, even though it returns its
error via a `Result` instead. Remove the `throws` in a strange-looking
manner so that we maintain the existing ABI.

Fixes rdar://81585954.

Author: DougGregor

stdlib/public/Concurrency/TaskGroup.swift

@@ -577,8 +577,9 @@ public struct ThrowingTaskGroup<ChildTaskResult, Failure: Error> {
     return try await _taskGroupWaitNext(group: _group)
   }
 
-  /// - SeeAlso: `next()`
-  public mutating func nextResult() async throws -> Result<ChildTaskResult, Failure>? {
+  @_silgen_name("$sScg10nextResults0B0Oyxq_GSgyYaKF")
+  @usableFromInline
+  mutating func nextResultForABI() async throws -> Result<ChildTaskResult, Failure>? {
     do {
       guard let success: ChildTaskResult = try await _taskGroupWaitNext(group: _group) else {
         return nil
@@ -590,6 +591,12 @@ public struct ThrowingTaskGroup<ChildTaskResult, Failure: Error> {
     }
   }
 
+  /// - SeeAlso: `next()`
+  @_alwaysEmitIntoClient
+  public mutating func nextResult() async -> Result<ChildTaskResult, Failure>? {
+    return try! await nextResultForABI()
+  }
+
   /// Query whether the group has any remaining tasks.
   ///
   /// Task groups are always empty upon entry to the `withTaskGroup` body, and

test/Concurrency/Runtime/async_taskgroup_throw_recover.swift

@@ -21,7 +21,7 @@ func boom() async throws -> Int {
 @available(SwiftStdlib 5.5, *)
 func test_taskGroup_throws() async {
   let got: Int = try await withThrowingTaskGroup(of: Int.self) { group in
-    group.spawn { try await boom()  }
+    group.addTask { try await boom()  }
 
     do {
       while let r = try await group.next() {
@@ -33,19 +33,24 @@ func test_taskGroup_throws() async {
       let gc = group.isCancelled
       print("group cancelled: \(gc)")
 
-      group.spawn { () async -> Int in
+      group.addTask { () async -> Int in
         let c = Task.isCancelled
         print("task 3 (cancelled: \(c))")
         return 3
       }
 
-      guard let third = try! await group.next() else {
+      switch await group.nextResult() {
+      case .success(let third):
+        print("task group returning normally: \(third)")
+        return third
+
+      case .failure(let error):
+        fatalError("got an erroneous third result")
+
+      case .none:
         print("task group failed to get 3")
         return 0
       }
-
-      print("task group returning normally: \(third)")
-      return third
     }
 
     fatalError("Should have thrown and handled inside the catch block")