patch applied (ghc): Improve consistency checking for derived instances

[Simon Peyton Jones]

Mon Jun 26 03:00:34 PDT 2006  simonpj at microsoft.com
  * Improve consistency checking for derived instances
  
  This patch arranges that derived instances use the same instance-decl
  checking code as user-defined instances.  That gives greater consistency
  in error messages.
  
  Furthermore, the error description if this consistency check fails is now
  much more explicit.  For example, drvfail003 now says
       Variable occurs more often in a constraint than in the instance head
         in the constraint: Show (v (v a))
       (Use -fallow-undecidable-instances to permit this)
       In the derived instance
         instance (Show (v (v a))) => Show (Square_ v w a)
  

    M ./compiler/typecheck/TcDeriv.lhs -13 +19
    M ./compiler/typecheck/TcSimplify.lhs -11 +1