patch applied (ghc): Take account of GADTs when reporting patterm-match overlap

[Simon Peyton Jones]

Tue Jul 21 22:09:33 PDT 2009  simonpj at microsoft.com
  * Take account of GADTs when reporting patterm-match overlap
  Ignore-this: 7dcbdcb91021e83e6e6208a2e68c50c9
  
  When matching against a GADT, some of the constructors may be impossible.
  For example
  	data T a where
            T1 :: T Int
            T2 :: T Bool
            T3 :: T a
  
          f :: T Int -> Int
          f T1 = 3
          f T3 = 4
  
  Here, does not have any missing cases, despite omittting T2, because
  T2 :: T Bool.
  
  This patch teaches the overlap checker about GADTs, which happily
  turned out to be rather easy even though the overlap checker needs
  a serious rewrite.
  

    M ./compiler/deSugar/Check.lhs -6 +9
    M ./compiler/typecheck/TcTyClsDecls.lhs -1 +8

View patch online:
http://darcs.haskell.org/ghc/_darcs/patches/20090722050933-1287e-d07a71da1e5456f4c1ada3fc15adf183e532fff1.gz