Enabling the Database Search Defaults module before the Search API is installed causes a fatal error

I can confirm the issue on a fresh 8.6.9 install.
Here is a patch using drupal_classloader_register() which works but I don't know for sure if this is the right way.

I too am getting this error. Sorry, not able to test patch now.

Ah, damn, that makes sense … Really tricky to use the same code during installation and without having the module installed.
At least drupal_get_path() already works there, that’s something. Otherwise, this would be even harder to fix. But as it looks, your proposed patch seems to be a very sensible solution.

However, since manually registering class loading seems pretty hack-ish to me, perhaps we should go with a more minimal approach, of only loading that one class. Seems to work fine, too.
I also added test coverage for this in the attached patch – please test/review!

I have tested the patch above and can confirm that the error does not occur anymore.

Steps I performed:

- Clean Drupal 8.6.10 install.
- Composer require drupal/search_api
- Enable "Database Search", "Database search defaults" and "Search API" at the same time.
- Got following error:

The website encountered an unexpected error. Please try again later.
Error: Class 'Drupal\search_api\IndexListBuilder' not found in search_api_db_defaults_requirements() (line 19 of modules/contrib/search_api/modules/search_api_db/search_api_db_defaults/search_api_db_defaults.install).
search_api_db_defaults_requirements('install')
call_user_func_array('search_api_db_defaults_requirements', Array) (Line: 392)
Drupal\Core\Extension\ModuleHandler->invoke('search_api_db_defaults', 'requirements', Array) (Line: 1035)
drupal_check_module('search_api_db_defaults') (Line: 419)
Drupal\system\Form\ModulesListForm->buildModuleList(Object) (Line: 437)
Drupal\system\Form\ModulesListForm->submitForm(Array, Object)
call_user_func_array(Array, Array) (Line: 111)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 51)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 589)
Drupal\Core\Form\FormBuilder->processForm('system_modules', Array, Object) (Line: 318)
Drupal\Core\Form\FormBuilder->buildForm('system_modules', Object) (Line: 93)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 582)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 693)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

- Made sure "Database Search", "Database search defaults" and "Search API" are disabled.
- Applied the patch in #5 with "patch -p1 < 3033040-5--db_defaults_module_install_error.patch"
- Enabled modules "Database Search", "Database search defaults" and "Search API" are disabled.
- Error is not occurring anymore.

Awesome, thanks a lot for testing!
Committed.
Thanks again, everyone!