patch applied (ghc): Improve unboxing of strict fields

[Simon Peyton Jones]

Wed Oct  4 08:27:05 PDT 2006  simonpj at microsoft.com
  * Improve unboxing of strict fields
  
  Note [Recursive unboxing]
  ~~~~~~~~~~~~~~~~~~~~~~~~~
  Be careful not to try to unbox this!
  	data T = MkT !T Int
  But it's the *argument* type that matters. This is fine:
  	data S = MkS S !Int
  because Int is non-recursive.
  
  
  Before this patch, we were only doing the unboxing if the *parent*
  data type was non-recursive (eg that meant S was not unboxed), but
  that is over-conservative. 
  
  This showed up with indexed data types (thanks to Roman for finding it)
  because indexed data types are conservatively regarded as always recursive.
  

    M ./compiler/typecheck/TcTyClsDecls.lhs -7 +15