patch applied (ghc): Fix a deriving bug, arising from recent refactoring

[Simon Peyton Jones]

Wed Feb 21 04:47:39 PST 2007  simonpj at microsoft.com
  * Fix a deriving bug, arising from recent refactoring
  
  This one is a hangover from something I did a month or two ago, but
  didn't get quite right.  tcSimplifyDefault should not check for no-instances;
  instead the checkValidInstance in TcDeriv does so.
  
  Conal's DeepArrow needs this fix.  Test is drv015.
  
  

    M ./compiler/typecheck/TcDeriv.lhs +20
    M ./compiler/typecheck/TcSimplify.lhs -28 +4