patch applied (ghc): Fix pre-subsumption and pre-matching

[Simon Peyton Jones]

Tue Aug  8 02:11:08 PDT 2006  simonpj at microsoft.com
  * Fix pre-subsumption and pre-matching
  
  The pre-subsuption and pre-matching functions should NEVER make bogus
  bindings of type variables, although they are free to bale out and make
  too few bindings.
  
  I hadn't been thiking carefully enough about this, and there were two
  separate bugs.  
  
  - Firstly, in pre-subsumption we must ignore the 'theta'
    part of any overloaded type.  
  
  - Second, in pre-matching, we must return the empty subustition 
    on a mis-match, rather than returning the substitution so far.
  
  This bug showed up when compiling Data.Generics.Schemes.hs, and is
  imortalised in test tc206
  

    M ./compiler/typecheck/TcUnify.lhs -6 +24