fix(router-core): matching competing optional routes uses proper fullPath for params extraction

packages/router-core/src/new-process-route-tree.ts

@@ -332,7 +332,13 @@ function parseSegments<TRouteLike extends RouteLike>(
       // but if there is *also* a layout route at this path, save it as notFound
       // we can use it when fuzzy matching to display the NotFound component in the layout route
       if (!isIndex) node.notFound = route
-      if (!node.route || (!node.isIndex && isIndex)) node.route = route
+      // does the new route take precedence over an existing one?
+      // yes if previous is not an index route and new one is an index route
+      if (!node.route || (!node.isIndex && isIndex)) {
+        node.route = route
+        // when replacing, replace all attributes that are route-specific (`fullPath` only at the moment)
+        node.fullPath = route.fullPath ?? route.from
+      }
       node.isIndex ||= isIndex
     }
   }

packages/router-core/tests/new-process-route-tree.test.ts

@@ -831,6 +831,37 @@ describe('findRouteMatch', () => {
         expect(result?.params).toEqual({ '**': decoded })
       })
     })
+    describe('edge-cases', () => {
+      it('#6012: optional index at root with param extraction', () => {
+        const tree = {
+          id: '__root__',
+          fullPath: '/',
+          path: '/',
+          isRoot: true,
+          options: {},
+          children: [
+            {
+              id: '/{-$year}/{-$month}/{-$day}',
+              fullPath: '/{-$year}/{-$month}/{-$day}',
+              path: '{-$year}/{-$month}/{-$day}',
+              isRoot: false,
+              options: {},
+            },
+            {
+              id: '/_pathless/{-$language}/',
+              fullPath: '/{-$language}/',
+              path: '{-$language}/',
+              isRoot: false,
+              options: {},
+            },
+          ],
+        }
+        const { processedTree } = processRouteTree(tree)
+        const result = findRouteMatch(`/sv`, processedTree)
+        expect(result?.route.id).toBe('/_pathless/{-$language}/')
+        expect(result?.params).toEqual({ language: 'sv' })
+      })
+    })
   })
 })