fix(react-router): allow union when assigning (#2994)

packages/react-router/src/utils.ts

@@ -54,13 +54,17 @@ export type IsUnion<T, U extends T = T> = (
   ? false
   : true
 
-export type Assign<TLeft, TRight> = keyof TLeft extends never
-  ? TRight
-  : keyof TRight extends never
-    ? TLeft
-    : keyof TLeft & keyof TRight extends never
-      ? TLeft & TRight
-      : Omit<TLeft, keyof TRight> & TRight
+export type Assign<TLeft, TRight> = TLeft extends any
+  ? TRight extends any
+    ? keyof TLeft extends never
+      ? TRight
+      : keyof TRight extends never
+        ? TLeft
+        : keyof TLeft & keyof TRight extends never
+          ? TLeft & TRight
+          : Omit<TLeft, keyof TRight> & TRight
+    : never
+  : never
 
 export type Timeout = ReturnType<typeof setTimeout>
 

packages/react-router/tests/useSearch.test-d.tsx

@@ -579,3 +579,32 @@ test('when a route has search params using SearchSchemaInput', () => {
     page: number
   }>
 })
+
+test('when route has a union of search params', () => {
+  const rootRoute = createRootRoute()
+
+  const postRoute = createRoute({
+    getParentRoute: () => rootRoute,
+    path: '/',
+    validateSearch: (): { status: 'in' } | { status: 'out' } => {
+      return { status: 'in' }
+    },
+  })
+
+  const indexRoute = createRoute({
+    getParentRoute: () => postRoute,
+    path: '/',
+    validateSearch: (): { detail: string } => {
+      return { detail: 'detail' }
+    },
+  })
+
+  const routeTree = rootRoute.addChildren([
+    indexRoute.addChildren([indexRoute]),
+  ])
+  // eslint-disable-next-line unused-imports/no-unused-vars
+  const router = createRouter({ routeTree })
+  expectTypeOf(useSearch<typeof router, '/'>).returns.toEqualTypeOf<
+    { status: 'in'; detail: string } | { status: 'out'; detail: string }
+  >
+})