[13.x] Keep the assertion failure message when session errors are JSON serialized

src/Illuminate/Testing/TestResponseAssert.php

@@ -4,6 +4,7 @@
 
 use Illuminate\Http\RedirectResponse;
 use Illuminate\Support\Arr;
+use Illuminate\Support\ViewErrorBag;
 use PHPUnit\Framework\ExpectationFailedException;
 use ReflectionProperty;
 
@@ -78,7 +79,17 @@ protected function injectResponseContext($exception)
             $session = $this->response->baseResponse->getSession();
 
             if (! is_null($session) && $session->has('errors')) {
-                return $this->appendErrorsToException($session->get('errors')->all(), $exception);
+                $errors = $session->get('errors');
+
+                if (! $errors instanceof ViewErrorBag && ! $session->isStarted()) {
+                    $session->start();
+
+                    $errors = $session->get('errors');
+                }
+
+                if ($errors instanceof ViewErrorBag) {
+                    return $this->appendErrorsToException($errors->all(), $exception);
+                }
             }
         }
 

tests/Testing/TestResponseTest.php

@@ -3177,6 +3177,44 @@ public function testValidationErrorsAreIncludedInAssertionFailure(): void
         $response->assertStatus(200);
     }
 
+    public function testValidationErrorsAreIncludedInAssertionFailureWhenSessionIsJsonSerialized(): void
+    {
+        $session = new Store('test-session', new NullSessionHandler(), null, 'json');
+
+        $response = TestResponse::fromBaseResponse(
+            tap(new RedirectResponse('/'))
+                ->setSession($session)
+                ->withErrors([
+                    'first_name' => 'The first name field is required.',
+                    'last_name' => 'The last name field is required.',
+                ])
+        );
+
+        $session->save(); // Required to serialize error bag to JSON
+
+        $this->expectException(ExpectationFailedException::class);
+        $this->expectExceptionMessageMatches('/Expected response status code \[200\] but received 302.*The first name field is required.*The last name field is required/s');
+
+        $response->assertStatus(200);
+    }
+
+    public function testValidationErrorsAreIncludedInAssertionFailureWhenJsonSerializedSessionIsNotSaved(): void
+    {
+        $response = TestResponse::fromBaseResponse(
+            tap(new RedirectResponse('/'))
+                ->setSession(new Store('test-session', new NullSessionHandler(), null, 'json'))
+                ->withErrors([
+                    'first_name' => 'The first name field is required.',
+                    'last_name' => 'The last name field is required.',
+                ])
+        );
+
+        $this->expectException(ExpectationFailedException::class);
+        $this->expectExceptionMessageMatches('/Expected response status code \[200\] but received 302.*The first name field is required.*The last name field is required/s');
+
+        $response->assertStatus(200);
+    }
+
     public function testJsonErrorsAreIncludedInAssertionFailure(): void
     {
         $response = TestResponse::fromBaseResponse(new JsonResponse([