[router] fix relative navigation on hoisted routes (#27778)

# Why

Fix #27753

# How

This `isIndexPath` function was not taking in account hoisted index
routes that may not have the `param` structure

Take the following structure.
```
_layout // Stack
parent/
└── child/
    ├── index // name: parent/child/index
    └── page  // name: parent/child/page 
```

Because there are no route params or nested `_layout`, all routes are
hoisted to the root `_layout`. The existing logic for only checking for
`route.params` is insufficient as it will not exist. Additionally we
cannot check for `name: 'index'` as hoisted routes have complex names.


# Test Plan

I think our existing tests are fine, just added one for this scenario

# Checklist

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- [ ] Documentation is up to date to reflect these changes (eg:
https://docs.expo.dev and README.md).
- [ ] Conforms with the [Documentation Writing Style
Guide](https://github.com/expo/expo/blob/main/guides/Expo%20Documentation%20Writing%20Style%20Guide.md)
- [ ] This diff will work correctly for `npx expo prebuild` & EAS Build
(eg: updated a module plugin).

---------

Co-authored-by: Expo Bot <34669131+expo-bot@users.noreply.github.com>

packages/expo-router/CHANGELOG.md

@@ -31,6 +31,7 @@
 - Export `toHaveRouterState` and other matcher types from `expo-router/testing-library` ([#27646](https://github.com/expo/expo/pull/27646) by [@marklawlor](https://github.com/marklawlor))
 - Fix missing types from typed routes ([#27412](https://github.com/expo/expo/pull/27412) by [@marklawlor](https://github.com/marklawlor))
 - Fork NavigationContainer on web to use custom linking context ([#27712](https://github.com/expo/expo/pull/27712) by [@marklawlor](https://github.com/marklawlor))
+- Fix relative navigation on hoisted routes ([#27778](https://github.com/expo/expo/pull/27778) by [@marklawlor](https://github.com/marklawlor))
 
 ### 💡 Others
 

packages/expo-router/src/LocationProvider.tsx

@@ -33,10 +33,21 @@ function isIndexPath(state: State) {
   if (route.state) {
     return isIndexPath(route.state);
   }
-  // router.params is typed as 'object', so this usual syntax is to please TypeScript
+
+  // Index routes on the same level as a layout do not have `index` in their name
   if (route.params && 'screen' in route.params) {
     return route.params.screen === 'index';
   }
+
+  // The `params` key will not exist if there are no params
+  // So we need to do a positive lookahead to check if the route ends with /index
+  // Nested routes that are hoisted will have a name ending with /index
+  // e.g name could be /user/[id]/index
+  if (route.name.match(/.+\/index$/)) return true;
+
+  // The state will either have params (because there are multiple _layout) or it will be hoisted with a name
+  // If we don't match the above cases, then it's not an index route
+
   return false;
 }
 

packages/expo-router/src/__tests__/navigation.test.ios.tsx

@@ -1,5 +1,5 @@
 /* eslint-disable react-hooks/rules-of-hooks */
-import React, { Text } from 'react-native';
+import React, { Text, View } from 'react-native';
 
 import {
   useRouter,
@@ -9,6 +9,7 @@ import {
   Redirect,
   Slot,
   usePathname,
+  Link,
 } from '../exports';
 import { Stack } from '../layouts/Stack';
 import { Tabs } from '../layouts/Tabs';
@@ -821,6 +822,23 @@ it('can push relative links from index routes', async () => {
   expect(screen).toHavePathname('/test/bar');
 });
 
+it('can push relative links from hoisted routes', () => {
+  renderRouter(
+    {
+      _layout: () => <Stack />,
+      'parent/index': () => <Link testID="link" href="./child" />,
+      'parent/child': () => <View testID="child" />,
+    },
+    {
+      initialUrl: '/parent',
+    }
+  );
+
+  expect(screen.getByTestId('link')).toBeOnTheScreen();
+  fireEvent(screen.getByTestId('link'), 'press');
+  expect(screen.getByTestId('child')).toBeOnTheScreen();
+});
+
 it('can navigation to a relative route without losing path params', async () => {
   renderRouter(
     {