Fix types for loaderData and actionData that contain Records

.changeset/orange-meals-repair.md

@@ -0,0 +1,25 @@
+---
+"react-router": patch
+---
+
+Fix types for loaderData and actionData that contained `Record`s
+
+UNSTABLE(BREAKING):
+
+`unstable_SerializesTo` added a way to register custom serialization types in Single Fetch for other library and framework authors like Apollo.
+It was implemented with branded type whose branded property that was made optional so that casting arbitrary values was easy:
+
+```ts
+// without the brand being marked as optional
+let x1 = 42 as unknown as unstable_SerializesTo<number>;
+//          ^^^^^^^^^^
+
+// with the brand being marked as optional
+let x2 = 42 as unstable_SerializesTo<number>;
+```
+
+However, this broke type inference in `loaderData` and `actionData` for any `Record` types as those would now (incorrectly) match `unstable_SerializesTo`.
+This affected all users, not just those that depended on `unstable_SerializesTo`.
+To fix this, the branded property of `unstable_SerializesTo` is marked as required instead of optional.
+
+For library and framework authors using `unstable_SerializesTo`, you may need to add `as unknown` casts before casting to `unstable_SerializesTo`.

packages/react-router/lib/types/serializes-to.ts

@@ -1,9 +1,21 @@
+import type { Equal, Expect } from "./utils";
+
 /**
  * A brand that can be applied to a type to indicate that it will serialize
  * to a specific type when transported to the client from a loader.
  * Only use this if you have additional serialization/deserialization logic
  * in your application.
  */
 export type unstable_SerializesTo<T> = {
-  unstable__ReactRouter_SerializesTo?: [T];
+  unstable__ReactRouter_SerializesTo: [T];
 };
+
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+type __tests = [
+  Expect<
+    Equal<
+      Record<string, any> extends unstable_SerializesTo<any> ? true : false,
+      false
+    >
+  >
+];