12 files declare their use of SelectInterface

Cache.php in core/lib/Drupal/Core/Cache/Cache.php
Condition.php in core/lib/Drupal/Core/Entity/Query/Sql/Condition.php
Condition.php in core/lib/Drupal/Core/Entity/Query/Sql/pgsql/Condition.php
ConditionAggregate.php in core/lib/Drupal/Core/Entity/Query/Sql/ConditionAggregate.php
database.api.php in core/lib/Drupal/Core/Database/database.api.php
Hooks related to the Database system and the Schema API.
MenuTreeStorage.php in core/lib/Drupal/Core/Menu/MenuTreeStorage.php
MigrateMissingDatabaseSource.php in core/modules/migrate/tests/modules/migrate_missing_database_test/src/Plugin/migrate/source/MigrateMissingDatabaseSource.php
Query.php in core/lib/Drupal/Core/Entity/Query/Sql/Query.php
SelectionInterface.php in core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionInterface.php
SelectionPluginBase.php in core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php
Tables.php in core/lib/Drupal/Core/Entity/Query/Sql/Tables.php
UserSelection.php in core/modules/user/src/Plugin/EntityReferenceSelection/UserSelection.php

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