[Concurrency] Task.immediate returning Never error must not have throwing operation

rdar://153855920

Author: ktoso

stdlib/public/Concurrency/Task+immediate.swift.gyb

@@ -19,14 +19,15 @@ import Swift
 %   'THROWING',
 %   'NON_THROWING',
 % ]
-% OPERATION_PARAM = '@_inheritActorContext @_implicitSelfCapture _ operation: __owned @Sendable @escaping () async throws -> Success'
 % for METHOD_VARIANT in METHOD_VARIANTS:
 
 % IS_THROWING = METHOD_VARIANT == 'THROWING'
 % if IS_THROWING:
 %   FAILURE_TYPE = 'Error'
+%   THROWS = 'throws '
 % else:
 %   FAILURE_TYPE = 'Never'
+%   THROWS = ''
 % end
 
 @available(SwiftStdlib 6.2, *)
@@ -46,7 +47,7 @@ extension Task where Failure == ${FAILURE_TYPE} {
   public static func startSynchronously(
     name: String? = nil,
     priority: TaskPriority? = nil,
-    @_implicitSelfCapture @_inheritActorContext(always) _ operation: sending @isolated(any) @escaping () async throws -> Success
+    @_implicitSelfCapture @_inheritActorContext(always) _ operation: sending @isolated(any) @escaping () async ${THROWS} -> Success
   ) -> Task<Success, ${FAILURE_TYPE}> {
     immediate(name: name, priority: priority, operation: operation)
   }
@@ -79,7 +80,7 @@ extension Task where Failure == ${FAILURE_TYPE} {
   public static func immediate(
     name: String? = nil,
     priority: TaskPriority? = nil,
-    @_implicitSelfCapture @_inheritActorContext(always) operation: sending @isolated(any) @escaping () async throws -> Success
+    @_implicitSelfCapture @_inheritActorContext(always) operation: sending @isolated(any) @escaping () async ${THROWS} -> Success
   ) -> Task<Success, ${FAILURE_TYPE}> {
 
     let builtinSerialExecutor =
@@ -235,15 +236,15 @@ extension ${GROUP_TYPE} {
 %   'THROWING',
 %   'NON_THROWING',
 % ]
-% OPERATION_PARAM = '@_inheritActorContext @_implicitSelfCapture _ operation: __owned @Sendable @escaping @MainActor () async throws -> Success'
 % for METHOD_VARIANT in METHOD_VARIANTS:
 
 % IS_THROWING = METHOD_VARIANT == 'THROWING'
 % if IS_THROWING:
 %   FAILURE_TYPE = 'Error'
+%   THROWS = 'throws '
 % else:
 %   FAILURE_TYPE = 'Never'
-%   OPERATION_PARAM = OPERATION_PARAM.replace('throws', '')
+%   THROWS = ''
 % end
 
 #if !SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY && !SWIFT_CONCURRENCY_EMBEDDED
@@ -256,7 +257,7 @@ extension Task where Failure == ${FAILURE_TYPE} {
   @discardableResult
   public static func startOnMainActor(
     priority: TaskPriority? = nil,
-    ${OPERATION_PARAM}
+    @_inheritActorContext @_implicitSelfCapture _ operation: __owned @Sendable @escaping @MainActor () async ${THROWS} -> Success
   ) -> Task<Success, ${FAILURE_TYPE}> {
     let flags = taskCreateFlags(
       priority: priority,

test/Concurrency/startImmediatelyIsolation.swift

@@ -1,5 +1,4 @@
-// RUN: %target-build-swift -swift-version 6 %s -strict-concurrency=complete -Xfrontend -verify
-
+// RUN: %target-swift-frontend -parse-as-library -swift-version 6 -strict-concurrency=complete -emit-sil -verify %s
 // REQUIRES: concurrency
 
 @available(SwiftStdlib 6.2, *)
@@ -41,11 +40,30 @@ func async() async throws {
 
 @available(SwiftStdlib 6.2, *)
 actor TestSelfCapture {
-    func method() {}
+  func method() {}
+
+  func test() {
+    Task.immediate {
+      method() // Ok due to `@_implicitSelfCapture`
+    }
+  }
+}
 
-    func test() {
-        Task.immediate {
-            method() // Ok due to `@_implicitSelfCapture`
-        }
+@available(SwiftStdlib 6.2, *)
+struct TestThrowing {
+  func test() {
+    // expected-error@+1{{invalid conversion from throwing function of type '() throws -> Void' to non-throwing function type '@isolated(any) () async -> Void'}}
+    let t: Task<Void, Never> = Task.immediate {
+      throw Boom()
+    }
+    _ = t
+
+    // ok
+    let t2: Task<Void, Error> = Task.immediate {
+      throw Boom()
     }
+    _ = t2
+  }
 }
+
+struct Boom: Error {}