patch applied (ghc): Signature type variables must not be instantiated with tycons

[Simon Peyton Jones]

Mon Feb 19 09:52:48 PST 2007  simonpj at microsoft.com
  * Signature type variables must not be instantiated with tycons
  
  An egregious bug in the type checker meant that it was possible for a 
  "signature type variable" (a MetaTv of SigTv form) to be instantatiated 
  with a type-constructor application.  This destroys the invariant for
  SigTv.
  
  The fix is easy; adding the predicate TcType.isTyConableTyVar
  
  Fixes Trac #1153
  

    M ./compiler/typecheck/TcSimplify.lhs -1 +1
    M ./compiler/typecheck/TcType.lhs -2 +18
    M ./compiler/typecheck/TcUnify.lhs -3 +3