patch applied (ghc): Fix Trac #3965: tighten conditions when deriving Data

[Simon Peyton Jones]

Fri Apr  9 11:44:20 PDT 2010  simonpj at microsoft.com
  * Fix Trac #3965: tighten conditions when deriving Data
  
  It's tricky to set up the context for a Data instance.  I got it wrong
  once, and fixed it -- hence the "extra_constraints" in
  TcDeriv.inferConstraints.  
  
  But it still wasn't right!  The tricky bit is that dataCast1 is only
  generated when T :: *->*, and dataCast2 when T :: *->*->*. (See
  the code in TcGenDeriv for dataCastX.

    M ./compiler/typecheck/TcDeriv.lhs -8 +14

View patch online:
http://darcs.haskell.org/cgi-bin/darcsweb.cgi?r=ghc;a=darcs_commitdiff;h=20100409184420-1287e-797ef851921f64e3563eae5626faa8836a8f315e.gz