Forgot to actually use provide(); make it do the type cast!

ember-polaris-service/src/index.ts

@@ -1,5 +1,5 @@
 export { factory } from './factory.ts';
-export { lookup, override } from './primitives.ts';
+export { lookup, provide, override } from './primitives.ts';
 export {
   type ServiceFactory,
   type ServiceManager,

ember-polaris-service/src/manager.ts

@@ -6,6 +6,9 @@ export interface ServiceFactory<T> {
   [INSTANTIATE]: (scope: Scope) => T;
 }
 
+export type ServiceInstanceType<T extends ServiceFactory<unknown>> =
+  T extends ServiceFactory<infer Instance> ? Instance : unknown;
+
 export interface ServiceManager<D extends object, T> {
   createService(definition: D): T;
 }

ember-polaris-service/src/primitives.ts

@@ -1,6 +1,7 @@
 import { assert, runInDebug } from '@ember/debug';
 import {
   type ServiceFactory,
+  type ServiceInstanceType,
   instantiate,
   isServiceFactory,
 } from './manager.ts';
@@ -24,11 +25,12 @@ export function lookup<T>(scope: Scope, factory: ServiceFactory<T>): T {
   return service;
 }
 
-export function provide<T, U>(
-  factory: ServiceFactory<T>,
-  provider: ServiceFactory<T & U>,
-): void {
+export function provide<T1 extends ServiceFactory<unknown>, T2 extends T1>(
+  factory: T1,
+  provider: T2,
+): ServiceFactory<ServiceInstanceType<T2>> {
   Providers.set(factory, provider);
+  return factory as ServiceFactory<ServiceInstanceType<T2>>;
 }
 
 export function override<T>(

test-app/app/services/json-storage.ts

@@ -1,4 +1,4 @@
-import { type ServiceFactory, service } from 'ember-polaris-service';
+import { type ServiceFactory, provide, service } from 'ember-polaris-service';
 import JSONStorage, { type Deserializer } from 'ember-storage-json';
 import Storage from './storage';
 
@@ -25,7 +25,24 @@ export class AppJSONStorage extends JSONStorage {
   }
 }
 
-// We must export the original well-known token!
+// We must export the original well-known token! `provide` returns that.
+//
 // The `as ServiceFactory<AppJSONStorage>` here makes other parts of the app
-// automatically
-export default JSONStorage as ServiceFactory<unknown> as ServiceFactory<AppJSONStorage>;
+// automatically.
+//
+// The type signature in `provide` is intended to make this just work without
+// the type cast, and it does work for factory functions and stuff. So e.g.
+// `provide(singleton(localStorage), factory(() => extendedStorage)) actually
+// does work the way you expect.
+//
+// However, because of how classes work in TypesScript, all subclasses of
+// Service are just considered `ServiceFactory<Service>`. The overload in
+// `service` (the function in `service.ts`) is what actually make things
+// work.
+//
+// We can add a similar overload for the primitives too, but I can't do that
+// easily right now for circular dependencies reason.
+export default provide(
+  JSONStorage,
+  AppJSONStorage,
+) as ServiceFactory<AppJSONStorage>;

test-app/app/services/storage.ts

@@ -1,4 +1,9 @@
-import { type ServiceFactory, service, singleton } from 'ember-polaris-service';
+import {
+  type ServiceFactory,
+  provide,
+  service,
+  singleton,
+} from 'ember-polaris-service';
 import Storage from 'ember-storage';
 import { enable } from 'ember-storage/extensions';
 import { type StorageWithBatching } from 'ember-storage-batch';
@@ -81,7 +86,21 @@ export class AppStorage extends Storage implements StorageWithBatching {
 // you will still get an error here.
 enable(AppStorage, 'batch');
 
-// We must export the original well-known token!
+// We must export the original well-known token! `provide` returns that.
+//
 // The `as ServiceFactory<AppStorage>` here makes other parts of the app
-// automatically
-export default Storage as ServiceFactory<unknown> as ServiceFactory<AppStorage>;
+// automatically.
+//
+// The type signature in `provide` is intended to make this just work without
+// the type cast, and it does work for factory functions and stuff. So e.g.
+// `provide(singleton(localStorage), factory(() => extendedStorage)) actually
+// does work the way you expect.
+//
+// However, because of how classes work in TypesScript, all subclasses of
+// Service are just considered `ServiceFactory<Service>`. The overload in
+// `service` (the function in `service.ts`) is what actually make things
+// work.
+//
+// We can add a similar overload for the primitives too, but I can't do that
+// easily right now for circular dependencies reason.
+export default provide(Storage, AppStorage) as ServiceFactory<AppStorage>;