6 files declare their use of Merge

DatabaseLegacyTest.php in core/tests/Drupal/KernelTests/Core/Database/DatabaseLegacyTest.php
DatabaseStorage.php in core/lib/Drupal/Core/KeyValueStore/DatabaseStorage.php
Merge.php in core/lib/Drupal/Core/Database/Driver/sqlite/Merge.php
Merge.php in core/lib/Drupal/Core/Database/Driver/mysql/Merge.php
Merge.php in core/lib/Drupal/Core/Database/Driver/pgsql/Merge.php
MergeTest.php in core/tests/Drupal/KernelTests/Core/Database/MergeTest.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.