title: Can't drop composite unique index in MySQL
body: <p>I have a Lumen 5.5 (MySQL 5.7) app where I create a unique index in a database migration:</p>
  
  <pre><code>Schema::table('form_data', function (Blueprint $table){
      $table-&gt;unique(['organisation_id', 'template_id', 'entity_type_id', 'entity_id'], 'form_data_identifier');
      // name of the index is 'form_data_identifier'
  });
  </code></pre>
  
  <p>In the <code>down</code> migration, I try to drop the index:</p>
  
  <pre><code>Schema::table('form_data', function (Blueprint $table){
      $table-&gt;dropUnique('form_data_identifier');
  });
  </code></pre>
  
  <p>But I get the error message:</p>
  
  <blockquote>
    <p>In Connection.php line 664:
    SQLSTATE[HY000]: General error: 1553 Cannot drop index 'form_data_identifier': needed in a foreign key constraint (SQL: alter table <code>form_data</code> drop index <code>form_data_identifier</code>)                                                                                            </p>
  </blockquote>
  
  <p>I've tried to run the query directly on the database myself using both <code>DROP INDEX x ON y</code> and <code>ALTER TABLE y DROP INDEX x</code> syntaxes, and I get the same error. I've checked, and this index <code>form_data_identifier</code> isn't referenced by any other table or key. (Note: some of the keys comprising the unique key <strong>are</strong> foreign keys, but I don't think that should be a problem.) </p>
  
  <p>What could I be missing?</p>