title: Issue warning on usage of specific instance
body: <p>I have created an instance of a type class:</p>
  
  <pre><code>data Foo = Foo
  
  instance Show Foo where
    show _ = "Foo"
  </code></pre>
  
  <p>Now this is part of a library and I want to issue a warning if a user of the library uses this instance. I have tried various things with the <code>WARNING</code> pragma but I can't get it to work.</p>