title: Cannot derive (show) from this expression
body: <p>This code doesn't work</p>
  
  <pre><code>data Expression = Atom String
    | Sequence [Expression]
    deriving (show)
  </code></pre>
  
  <p>I get this error: </p>
  
  <pre><code>$ runghc bug.hs
  ghc: panic! (the 'impossible' happened)
    (GHC version 7.4.1 for x86_64-apple-darwin):
      nameModule show{tv a9J}
  
  Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
  </code></pre>
  
  <p>Am I doing something wrong, or is it indeed a bug?</p>