Use contrivances to present something close to act()'s real return type

Make it harder to accidentally use it in contexts where a Promise may be expected.

types/react-dom/react-dom-tests.tsx

@@ -191,5 +191,10 @@ describe('React dom test utils', () => {
             // $ExpectError
             ReactTestUtils.act(() => null);
         });
+        it('returns a Promise-like that errors out on use', () => {
+            const result = ReactTestUtils.act(() => {});
+            // $ExpectError
+            Promise.resolve(result);
+        });
     });
 });

types/react-dom/test-utils/index.d.ts

@@ -291,4 +291,13 @@ export function createRenderer(): ShallowRenderer;
  * @see https://reactjs.org/blog/2019/02/06/react-v16.8.0.html#testing-hooks
  */
 // the "void | undefined" is here to forbid any sneaky "Promise" returns.
-export function act(callback: () => void | undefined): void;
+// the actual return value is always a "DebugPromiseLike",
+// but having an "| {}" makes it harder to accidentally use.
+export function act(callback: () => void | undefined): DebugPromiseLike | {};
+
+// Intentionally doesn't extend PromiseLike<never>.
+// Ideally this should be as hard to accidentally use as possible.
+interface DebugPromiseLike {
+    // the actual then() in here is 0-ary, but that doesn't count as a PromiseLike.
+    then(onfulfilled: (value: never) => never, onrejected: (reason: never) => never): never;
+}

types/react-test-renderer/index.d.ts

@@ -64,4 +64,13 @@ export function create(nextElement: ReactElement<any>, options?: TestRendererOpt
  * @see https://reactjs.org/blog/2019/02/06/react-v16.8.0.html#testing-hooks
  */
 // the "void | undefined" is here to forbid any sneaky "Promise" returns.
-export function act(callback: () => void | undefined): void;
+// the actual return value is always a "DebugPromiseLike",
+// but having an "| {}" makes it harder to accidentally use.
+export function act(callback: () => void | undefined): DebugPromiseLike | {};
+
+// Intentionally doesn't extend PromiseLike<never>.
+// Ideally this should be as hard to accidentally use as possible.
+interface DebugPromiseLike {
+    // the actual then() in here is 0-ary, but that doesn't count as a PromiseLike.
+    then(onfulfilled: (value: never) => never, onrejected: (reason: never) => never): never;
+}

types/react-test-renderer/react-test-renderer-tests.ts

@@ -73,3 +73,5 @@ act(() => {});
 act(async () => {});
 // $ExpectError
 act(() => null);
+// $ExpectError
+Promise.resolve(act(() => {}));