patch applied (ghc): Add ad-hoc typing checks for tagToEnum#

[Simon Peyton Jones]

Wed Aug 16 13:31:56 PDT 2006  simonpj at microsoft.com
  * Add ad-hoc typing checks for tagToEnum#
  
  The problem with tagToEnum# is that it is not overloaded (in the
  Haskell sense) but you are only supposed to apply it to a TyCon
  that is an enumeration (isEnumerationTyCon).
  
  The Real Way to do this is to have some special kind of type constraint
  for the purpose, but that is wild overkill. So this patch adds a small
  rather ad-hoc check to TcExpr.instFun.  Crude, simple, but it works fine.
  
  Fixes Trac #786
  Test is tcfail164
  

    M ./compiler/prelude/PrelRules.lhs -2 +2
    M ./compiler/prelude/PrimOp.lhs +10
    M ./compiler/typecheck/TcExpr.lhs -10 +55