Allow TaskGroup's ChildTaskResult Type To Be Inferred (swiftlang#74517)

Co-authored-by: Konrad `ktoso` Malawski <[email protected]>

Author: rlziii

stdlib/public/Concurrency/TaskGroup.swift

@@ -67,7 +67,7 @@ import Swift
 @backDeployed(before: SwiftStdlib 6.0)
 @inlinable
 public func withTaskGroup<ChildTaskResult, GroupResult>(
-  of childTaskResultType: ChildTaskResult.Type,
+  of childTaskResultType: ChildTaskResult.Type = ChildTaskResult.self,
   returning returnType: GroupResult.Type = GroupResult.self,
   isolation: isolated (any Actor)? = #isolation,
   body: (inout TaskGroup<ChildTaskResult>) async -> GroupResult
@@ -202,7 +202,7 @@ public func _unsafeInheritExecutor_withTaskGroup<ChildTaskResult, GroupResult>(
 @backDeployed(before: SwiftStdlib 6.0)
 @inlinable
 public func withThrowingTaskGroup<ChildTaskResult, GroupResult>(
-  of childTaskResultType: ChildTaskResult.Type,
+  of childTaskResultType: ChildTaskResult.Type = ChildTaskResult.self,
   returning returnType: GroupResult.Type = GroupResult.self,
   isolation: isolated (any Actor)? = #isolation,
   body: (inout ThrowingTaskGroup<ChildTaskResult, Error>) async throws -> GroupResult

test/Concurrency/async_task_groups_and_type_inference.swift

@@ -0,0 +1,139 @@
+// RUN: %target-swift-frontend  -disable-availability-checking %s -emit-sil -o /dev/null -verify -strict-concurrency=complete
+
+// REQUIRES: concurrency
+// REQUIRES: asserts
+
+// === OK cases
+
+let _ = await withTaskGroup { group in
+  group.addTask { }
+  group.addTask { }
+}
+let _ = await withThrowingTaskGroup { group in
+  group.addTask { }
+  group.addTask { }
+}
+
+let _ = await withTaskGroup { group in
+  group.addTask { 1 }
+  group.addTask { 2 }
+  return "result"
+}
+let _ = await withThrowingTaskGroup { group in
+  group.addTask { 1 }
+  group.addTask { 2 }
+  return "result"
+}
+
+let _ = await withTaskGroup { group in
+  group.addTask {
+    if Bool.random() {
+      return 1
+    } else {
+      return 0
+    }
+  }
+  group.addTask {
+    if Bool.random() {
+      return 1
+    } else {
+      return 0
+    }
+  }
+  return "result"
+}
+let _ = await withThrowingTaskGroup { group in
+  group.addTask {
+    if Bool.random() {
+      return 1
+    } else {
+      return 0
+    }
+  }
+  group.addTask {
+    if Bool.random() {
+      return 1
+    } else {
+      return 0
+    }
+  }
+  return "result"
+}
+
+// === Cases where inference fails
+
+let _: Int = await withTaskGroup { group in
+  // expected-error@-1{{generic parameter 'ChildTaskResult' could not be inferred}}
+  return 1
+}
+let _: Int = await withThrowingTaskGroup { group in
+  // expected-error@-1{{generic parameter 'ChildTaskResult' could not be inferred}}
+  return 1
+}
+
+// we infer the ChildTaskResult from a addTask, so if some other method
+// precedes it, inference will fail.
+
+let _: Int = await withTaskGroup { group in
+  // expected-error@-1{{generic parameter 'ChildTaskResult' could not be inferred}}
+  await group.next()
+  return 1
+}
+let _: Int = await withThrowingTaskGroup { group in
+  // expected-error@-1{{generic parameter 'ChildTaskResult' could not be inferred}}
+  try await group.next()
+  return 1
+}
+
+let _: Int = await withTaskGroup { group in
+  // expected-error@-1{{generic parameter 'ChildTaskResult' could not be inferred}}
+  await group.next()
+  group.addTask { 1 }
+
+  return 1
+}
+let _: Int = await withThrowingTaskGroup { group in
+  // expected-error@-1{{generic parameter 'ChildTaskResult' could not be inferred}}
+  try await group.next()
+  group.addTask { 1 }
+
+  return 1
+}
+
+let _ = await withTaskGroup { group in
+  // expected-error@-1{{generic parameter 'ChildTaskResult' could not be inferred}}
+}
+let _ = await withThrowingTaskGroup { group in
+  // expected-error@-1{{generic parameter 'ChildTaskResult' could not be inferred}}
+}
+
+// Notable exception, defer does not cause inference to break:
+
+let _: Int = await withTaskGroup { group in
+  defer { group.cancelAll() }
+  group.addTask { 1 }
+
+  return 1
+}
+let _: Int = await withThrowingTaskGroup { group in
+  defer { group.cancelAll() }
+  group.addTask { 1 }
+
+  return 1
+}
+
+// Check conflicting types inside addTask {}
+
+let _ = await withTaskGroup { group in
+  group.addTask { 1 }
+  group.addTask { "x" } // expected-error{{cannot convert value of type 'String' to closure result type 'Int'}}
+
+  return 1
+}
+let _ = await withThrowingTaskGroup { group in
+  group.addTask { 1 }
+  group.addTask { "x" } // expected-error{{cannot convert value of type 'String' to closure result type 'Int'}}
+
+  return 1
+}
+