Correct types for overriden services

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@snap/ts-inject",
-  "version": "0.1.0",
+  "version": "0.1.1",
   "description": "100% typesafe dependency injection framework for TypeScript projects",
   "license": "MIT",
   "author": "Snap Inc.",

src/Container.ts

@@ -1,7 +1,7 @@
 import { isMemoized, memoize } from "./memoize";
 import type { Memoized } from "./memoize";
 import { PartialContainer } from "./PartialContainer";
-import type { AddService, InjectableClass, InjectableFunction, TokenType, ValidTokens } from "./types";
+import type { AddService, AddServices, InjectableClass, InjectableFunction, TokenType, ValidTokens } from "./types";
 import { ConcatInjectable } from "./Injectable";
 import { entries } from "./entries";
 
@@ -344,7 +344,7 @@ export class Container<Services = {}> {
     // `this` type, we ensure this Container can provide all the Dependencies required by the PartialContainer.
     this: Container<FulfilledDependencies>,
     container: PartialContainer<AdditionalServices, Dependencies>
-  ): Container<Services & AdditionalServices>;
+  ): Container<AddServices<Services, AdditionalServices>>;
 
   /**
    * Merges services from another `Container` into this container, creating a new `Container` instance.
@@ -362,7 +362,9 @@ export class Container<Services = {}> {
    * @returns A new `Container` instance that combines services from this container with those from the
    *          provided container, with services from the provided container taking precedence in case of conflicts.
    */
-  provides<AdditionalServices>(container: Container<AdditionalServices>): Container<Services & AdditionalServices>;
+  provides<AdditionalServices>(
+    container: Container<AdditionalServices>
+  ): Container<AddServices<Services, AdditionalServices>>;
 
   /**
    * Registers a new service in this Container using an `InjectableFunction`. This function defines how the service
@@ -400,7 +402,7 @@ export class Container<Services = {}> {
       return new Container({
         ...this.factories,
         ...factories,
-      } as unknown as MaybeMemoizedFactories<Services & AdditionalServices>);
+      } as unknown as MaybeMemoizedFactories<AddServices<Services, AdditionalServices>>);
     }
     return this.providesService(fnOrContainer);
   }

src/__tests__/Container.spec.ts

@@ -305,6 +305,27 @@ describe("Container", () => {
       let childContainerWithOverride = parentContainer.providesValue("value", 2);
       expect(childContainerWithOverride.get("service")).toBe(1);
     });
+
+    test("overriding with a different type changes resulting container's type", () => {
+      const parentContainer = Container.providesValue("value", 1);
+      let childContainerWithOverride = parentContainer.providesValue("value", "two");
+
+      // @ts-expect-error should be failing to compile as the type of the container has changed
+      let numberValue: number = childContainerWithOverride.get("value");
+
+      let value: string = childContainerWithOverride.get("value");
+      expect(value).toBe("two");
+
+      const partialContainer = new PartialContainer({}).provides(Injectable("value", () => "three"));
+      childContainerWithOverride = parentContainer.provides(partialContainer);
+      value = childContainerWithOverride.get("value");
+      expect(value).toBe("three");
+
+      let extraContainer = Container.fromObject({ value: "four" });
+      childContainerWithOverride = parentContainer.provides(extraContainer);
+      value = childContainerWithOverride.get("value");
+      expect(value).toBe("four");
+    });
   });
 
   describe("when making a copy of the Container", () => {

src/types.ts

@@ -71,7 +71,30 @@ export type ServicesFromInjectables<Injectables extends readonly AnyInjectable[]
 // will see the mapped type instead of the AddService type alias. This produces better hints.
 export type AddService<ParentServices, Token extends TokenType, Service> = ParentServices extends any
   ? // A mapped type produces better, more concise type hints than an intersection type.
-    { [K in keyof ParentServices | Token]: K extends keyof ParentServices ? ParentServices[K] : Service }
+    {
+      [K in keyof ParentServices | Token]: K extends keyof ParentServices
+        ? K extends Token
+          ? Service
+          : ParentServices[K]
+        : Service;
+    }
+  : never;
+
+/**
+ * Same as AddService above, but is merging multiple services at once. Services types override those of the parent.
+ */
+// Using a conditional type forces TS language services to evaluate the type -- so when showing e.g. type hints, we
+// will see the mapped type instead of the AddService type alias. This produces better hints.
+export type AddServices<ParentServices, Services> = ParentServices extends any
+  ? Services extends any
+    ? {
+        [K in keyof Services | keyof ParentServices]: K extends keyof Services
+          ? Services[K]
+          : K extends keyof ParentServices
+            ? ParentServices[K]
+            : never;
+      }
+    : never
   : never;
 
 /**