8 files declare their use of AuthenticationProviderInterface

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

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