title: Haskell/GHC UndecidableInstances - example for non-terminating type check?
body: <p>I've written some Haskell code which needs -XUndecidableInstances to compile. I do understand why that happens, that there is a certain condition which is violated and therefore GHC yells.</p>
  
  <p>However, I've never come across a situation where the type checker would actually hang or wind up in an endless loop.</p>
  
  <p><strong>What does a non-terminating instance definition look like - can you give an example?</strong></p>