fix(router-core): buildLocation must consider basePath when resolving fromPath (#4930)

when building the location we are already considering the `basepath` for
the `to` path resolution but we don't do the same for the `from` path.

This PR resolves the `from` path also with the `basepath` so as to
ensure a like for like comparison with `to`.

This PR resolves #4924 and also adds a test to check for this regression
going forward

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: nlynzaad

packages/react-router/tests/link.test.tsx

@@ -751,6 +751,118 @@ describe('Link', () => {
     expect(updatedFilter).toHaveTextContent('Filter: inactive')
   })
 
+  test('when navigation to . from /posts while updating search from / and using base path', async () => {
+    const RootComponent = () => {
+      return (
+        <>
+          <div data-testid="root-nav">
+            <Link
+              to="."
+              search={{ page: 2, filter: 'inactive' }}
+              data-testid="update-search"
+            >
+              Update Search
+            </Link>
+          </div>
+          <Outlet />
+        </>
+      )
+    }
+
+    const rootRoute = createRootRoute({
+      component: RootComponent,
+    })
+
+    const indexRoute = createRoute({
+      getParentRoute: () => rootRoute,
+      path: '/',
+      component: () => {
+        return (
+          <>
+            <h1>Index</h1>
+            <Link
+              to="/posts"
+              search={{ page: 1, filter: 'active' }}
+              data-testid="to-posts"
+            >
+              Go to Posts
+            </Link>
+          </>
+        )
+      },
+    })
+
+    const PostsComponent = () => {
+      const search = useSearch({ strict: false })
+      return (
+        <>
+          <h1>Posts</h1>
+          <span data-testid="current-page">Page: {search.page}</span>
+          <span data-testid="current-filter">Filter: {search.filter}</span>
+        </>
+      )
+    }
+
+    const postsRoute = createRoute({
+      getParentRoute: () => rootRoute,
+      path: 'posts',
+      validateSearch: (input: Record<string, unknown>) => {
+        return {
+          page: input.page ? Number(input.page) : 1,
+          filter: (input.filter as string) || 'all',
+        }
+      },
+      component: PostsComponent,
+    })
+
+    const router = createRouter({
+      routeTree: rootRoute.addChildren([indexRoute, postsRoute]),
+      history,
+    })
+
+    render(<RouterProvider router={router} basepath={'/Dashboard'} />)
+
+    // Start at index page
+    const toPostsLink = await screen.findByTestId('to-posts')
+    expect(toPostsLink).toHaveAttribute(
+      'href',
+      '/Dashboard/posts?page=1&filter=active',
+    )
+
+    // Navigate to posts with initial search params
+    await act(() => fireEvent.click(toPostsLink))
+
+    // Verify we're on posts with initial search
+    const postsHeading = await screen.findByRole('heading', { name: 'Posts' })
+    expect(postsHeading).toBeInTheDocument()
+    expect(window.location.pathname).toBe('/Dashboard/posts')
+    expect(window.location.search).toBe('?page=1&filter=active')
+
+    const currentPage = await screen.findByTestId('current-page')
+    const currentFilter = await screen.findByTestId('current-filter')
+    expect(currentPage).toHaveTextContent('Page: 1')
+    expect(currentFilter).toHaveTextContent('Filter: active')
+
+    // Navigate to current route (.) with updated search
+    const updateSearchLink = await screen.findByTestId('update-search')
+
+    expect(updateSearchLink).toHaveAttribute(
+      'href',
+      '/Dashboard/posts?page=2&filter=inactive',
+    )
+
+    await act(() => fireEvent.click(updateSearchLink))
+
+    // Verify search was updated
+    expect(window.location.pathname).toBe('/Dashboard/posts')
+    expect(window.location.search).toBe('?page=2&filter=inactive')
+
+    const updatedPage = await screen.findByTestId('current-page')
+    const updatedFilter = await screen.findByTestId('current-filter')
+    expect(updatedPage).toHaveTextContent('Page: 2')
+    expect(updatedFilter).toHaveTextContent('Filter: inactive')
+  })
+
   test('when navigating to /posts with invalid search', async () => {
     const rootRoute = createRootRoute()
     const onError = vi.fn()

packages/router-core/src/router.ts

@@ -1420,7 +1420,7 @@ export class RouterCore<
 
       // First let's find the starting pathname
       // By default, start with the current location
-      let fromPath = lastMatch.fullPath
+      let fromPath = this.resolvePathWithBase(lastMatch.fullPath, '.')
       const toPath = dest.to
         ? this.resolvePathWithBase(fromPath, `${dest.to}`)
         : this.resolvePathWithBase(fromPath, '.')
@@ -1461,6 +1461,8 @@ export class RouterCore<
         }
       }
 
+      fromPath = this.resolvePathWithBase(fromPath, '.')
+
       // From search should always use the current location
       const fromSearch = lastMatch.search
       // Same with params. It can't hurt to provide as many as possible