[11.x] Allow implicit binding to have optional backed enums (#51178)

* Allow implicit binding to have optional backed enums

* Add route-based test

src/Illuminate/Routing/ImplicitRouteBinding.php

@@ -84,6 +84,10 @@ protected static function resolveBackedEnumsForRoute($route, $parameters)
 
             $parameterValue = $parameters[$parameterName];
 
+            if ($parameterValue === null) {
+                continue;
+            }
+
             $backedEnumClass = $parameter->getType()?->getName();
 
             $backedEnum = $backedEnumClass::tryFrom((string) $parameterValue);

tests/Routing/ImplicitRouteBindingTest.php

@@ -49,6 +49,24 @@ public function test_it_can_resolve_the_implicit_backed_enum_route_bindings_for_
         $this->assertSame('fruits', $route->parameter('category')->value);
     }
 
+    public function test_it_handles_optional_implicit_backed_enum_route_bindings_for_the_given_route_with_optional_parameter()
+    {
+        $action = ['uses' => function (?CategoryBackedEnum $category = null) {
+            return $category->value;
+        }];
+
+        $route = new Route('GET', '/test', $action);
+        $route->parameters = ['category' => null];
+
+        $route->prepareForSerialization();
+
+        $container = Container::getInstance();
+
+        ImplicitRouteBinding::resolveForRoute($container, $route);
+
+        $this->assertNull($route->parameter('category'));
+    }
+
     public function test_it_does_not_resolve_implicit_non_backed_enum_route_bindings_for_the_given_route()
     {
         $action = ['uses' => function (CategoryEnum $category) {

tests/Routing/RoutingRouteTest.php

@@ -42,6 +42,8 @@
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 use UnexpectedValueException;
 
+include_once __DIR__.'/Enums.php';
+
 class RoutingRouteTest extends TestCase
 {
     public function testBasicDispatchingOfRoutes()
@@ -1883,6 +1885,21 @@ public function testImplicitBindingsWithOptionalParameterWithExistingKeyInUri()
         $this->assertSame('taylor', $router->dispatch(Request::create('foo/taylor', 'GET'))->getContent());
     }
 
+    public function testOptionalBackedEnumsReturnNullWhenMissing()
+    {
+        $router = $this->getRouter();
+        $router->get('foo/{bar?}', [
+            'middleware' => SubstituteBindings::class,
+            'uses' => function (?CategoryBackedEnum $bar = null) {
+                $this->assertNull($bar);
+
+                return 'bar';
+            },
+        ]);
+
+        $router->dispatch(Request::create('foo', 'GET'))->getContent();
+    }
+
     public function testImplicitBindingsWithMissingModelHandledByMissing()
     {
         $router = $this->getRouter();