Check for replacing existing routes should not remove default arguments

Okay, makes sense to me.

See also #2559493: Allow slugs or placeholders for page paths

Rebase was needed.

As for as I understand I have to create a page without '%' and assert that the page works. I am doing that but I don't know why the only test is not failing. Probably this was already fixed or I am wrong, so I need some feedback.

The test-only is passing, something not right there.

I was a bit confused with the test module. So here is the test only and complete patch.

+++ b/tests/modules/page_manager_test/page_manager_test.routing.yml
@@ -2,5 +2,6 @@ page_manager_test.page_view:
+    page: 'something'

Missing this change in the test-only patch, hopefully that's why it didn't fail?

Yes, the default parameter was missing.

Great. Adding unit test coverage as well:

diff --git a/tests/src/Unit/PageManagerRoutesTest.php b/tests/src/Unit/PageManagerRoutesTest.php
index 0846303..2194e1d 100644
--- a/tests/src/Unit/PageManagerRoutesTest.php
+++ b/tests/src/Unit/PageManagerRoutesTest.php
@@ -192,7 +192,7 @@ public function testAlterRoutesOverrideExisting($page_path, $existing_route_path
     $this->cacheTagsInvalidator->invalidateTags(["page_manager_route_name:$route_name"])->shouldBeCalledTimes(1);
 
     $collection = new RouteCollection();
-    $collection->add($route_name, new Route($existing_route_path, [], [], ['parameters' => ['foo' => 'bar']]));
+    $collection->add($route_name, new Route($existing_route_path, ['default_exists' => 'default_value'], [], ['parameters' => ['foo' => 'bar']]));
     $route_event = new RouteBuildEvent($collection);
     $this->routeSubscriber->onAlterRoutes($route_event);
 
@@ -227,6 +227,8 @@ public function providerTestAlterRoutesOverrideExisting() {
     $data['no_slug'] = ['/test_route', '/test_route'];
     $data['slug'] = ['/test_route/{test_route}', '/test_route/{test_route}'];
     $data['placeholder'] = ['/test_route/%', '/test_route/{test_route}'];
+    $data['slug_with_default'] = ['/test_route/{default_exists}', '/test_route/{default_exists}'];
+    $data['placeholder_with_default'] = ['/test_route/%', '/test_route/{default_exists}'];
     return $data;
   }