[GHC] #2151: GADTs in function Patterns

[GHC]

#2151: GADTs in function Patterns
-------------------------+--------------------------------------------------
    Reporter:  hpacheco  |       Owner:          
        Type:  bug       |      Status:  new     
    Priority:  normal    |   Component:  Compiler
     Version:  6.9       |    Severity:  blocker 
    Keywords:            |    Testcase:          
Architecture:  Multiple  |          Os:  Multiple
-------------------------+--------------------------------------------------
 I have found a bug on the compiler (at least ghc >6.8.2). For some module
 (yes, the example does nothing at all):

 module Test where

 data Type a where
     Func :: Type a -> Type b -> Type (a -> b)
     PF :: Type a -> Type (PF a)

 data PF a where
     ID :: PF (a -> a)

 test :: Type a -> a -> a
 test (PF (Func _ _)) ID = ID

 I get the impossible:

 $ ghci Test.hs -fglasgow-exts
 GHCi, version 6.9.20080303: http://www.haskell.org/ghc/  :? for help
 Loading package base ... linking ... done.
 [1 of 1] Compiling Test             ( Test.hs, interpreted )
 ghc-6.9.20080303: panic! (the 'impossible' happened)
   (GHC version 6.9.20080303 for i386-apple-darwin):
     Coercion.splitCoercionKindOf
     $co${tc aog} [tv]
     <pred>t_ao8{tv} [tau] ~ a{tv aob} [sk] -> a{tv aob} [sk]
 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 However, the following implementations of test compiles ok:

 test :: Type a -> a -> a
 test (PF _) ID = ID

 test :: Type a -> a -> a
 test (PF (Func _ _)) x = x

 It has something to do with mixing different GADTs contructors.

 NOTE: This same code used to work fine in ghc 6.6, but I have ben told the
 whole GADT type checing was redone in 6.9. However, with 6.8.2 I still get
 the same error.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2151>
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