8 files declare their use of AuthenticationProviderInterface in Drupal 8

Auth.php in core/modules/locale/tests/modules/early_translation_test/src/Auth.php

AuthenticationCollectorTest.php in core/tests/Drupal/Tests/Core/Authentication/AuthenticationCollectorTest.php

Contains \Drupal\Tests\Core\Authentication\AuthenticationCollectorTest.

AuthenticationManagerTest.php in core/tests/Drupal/Tests/Core/Authentication/AuthenticationManagerTest.php

Contains \Drupal\Tests\Core\Authentication\AuthenticationManagerTest.

AuthenticationSubscriber.php in core/lib/Drupal/Core/EventSubscriber/AuthenticationSubscriber.php

BasicAuth.php in core/modules/basic_auth/src/Authentication/Provider/BasicAuth.php

Cookie.php in core/modules/user/src/Authentication/Provider/Cookie.php

TestAuth.php in core/modules/rest/tests/modules/rest_test/src/Authentication/Provider/TestAuth.php

TestAuthGlobal.php in core/modules/rest/tests/modules/rest_test/src/Authentication/Provider/TestAuthGlobal.php