feat(task): improve type checking for tuples (lit#4836)

---------

Co-authored-by: Augustine Kim <[email protected]>

.changeset/wicked-rabbits-trade.md

@@ -0,0 +1,5 @@
+---
+'@lit/task': patch
+---
+
+Improve type inference of tuples returned by the args function being used as task function parameter.

packages/labs/task/src/test/task_test.ts

@@ -759,7 +759,7 @@ suite('Task', () => {
   });
 
   test('Elements only render once for pending tasks', async () => {
-    let resolveTask: (v: unknown) => void;
+    let resolveTask: (v: any) => void;
     let renderCount = 0;
     class TestElement extends ReactiveElement {
       task = new Task(this, {

packages/task/src/task.ts

@@ -166,8 +166,8 @@ export interface TaskConfig<T extends ReadonlyArray<unknown>, R> {
  * ```
  */
 export class Task<
-  T extends ReadonlyArray<unknown> = ReadonlyArray<unknown>,
-  R = unknown,
+  const T extends ReadonlyArray<unknown> = ReadonlyArray<unknown>,
+  const R = unknown,
 > {
   private _previousArgs?: T;
   private _task: TaskFunction<T, R>;
@@ -227,12 +227,12 @@ export class Task<
   private _rejectTaskComplete?: (e: unknown) => void;
   private _taskComplete?: Promise<R>;
 
+  constructor(host: ReactiveControllerHost, task: TaskConfig<T, R>);
   constructor(
     host: ReactiveControllerHost,
     task: TaskFunction<T, R>,
     args?: ArgsFunction<T>
   );
-  constructor(host: ReactiveControllerHost, task: TaskConfig<T, R>);
   constructor(
     host: ReactiveControllerHost,
     task: TaskFunction<T, R> | TaskConfig<T, R>,

packages/task/src/test/task_test.ts

@@ -759,7 +759,7 @@ suite('Task', () => {
   });
 
   test('Elements only render once for pending tasks', async () => {
-    let resolveTask: (v: unknown) => void;
+    let resolveTask: (v: any) => void;
     let renderCount = 0;
     class TestElement extends ReactiveElement {
       task = new Task(this, {
@@ -1072,4 +1072,31 @@ suite('Task', () => {
       })
     );
   });
+
+  test('tuple type arguments are inferred without need for "as const"', () => {
+    const expectType = <T>(x: T) => x;
+
+    class TestElement extends ReactiveElement {
+      string = '';
+      number = 1;
+
+      task = new Task(
+        this,
+        ([string, number]) => {
+          expectType<string>(string);
+          expectType<number>(number);
+        },
+        () => [this.string, this.number]
+      );
+
+      task2 = new Task(this, {
+        task: ([string, number]) => {
+          expectType<string>(string);
+          expectType<number>(number);
+        },
+        args: () => [this.string, this.number],
+      });
+    }
+    TestElement;
+  });
 });