#2040: GADT regression
------------------------+---------------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.8.2 | Severity: normal
Keywords: gadt | Testcase:
Architecture: Unknown | Os: Unknown
------------------------+---------------------------------------------------
The following compiles fine in 6.8.1 but does not in later versions of
GHC.
I'm not sure if this is related to #1823, since there type classes are not
involved.
A workaround is included below.
{{{
{-# OPTIONS_GHC -Wall -fglasgow-exts #-}
module Bug where
data Teq a b where Teq :: Teq a a
class C a b where proof :: Teq a b
data S a = S a
data W b where
-- This would make every version of GHC happy
-- W :: (C a c , c ~ S b) => W a -> W c
W :: C a (S b) => W a -> W (S b)
foo :: W (S ()) -> W (S ()) -> ()
foo (W (_ :: W a1)) (W (_ :: W a2)) =
case proof :: Teq a1 (S ()) of
Teq -> ()
foo2 :: W (S ()) -> W (S ()) -> ()
foo2 (W (_ :: W a1)) (W (_ :: W a2)) =
case proof :: Teq a1 (S ()) of
Teq -> case proof :: Teq a2 (S ()) of
Teq -> ()
}}}
Results:
6.8.1 : OK
6.8.2 : foo OK, foo2 FAIL
{{{
Bug.hs:23:16:
Could not deduce (C a1 (S ())) from the context ()
arising from a use of `proof' at Bug.hs:23:16-20
}}}
6.9.20080108 : FAIL
{{{
Bug.hs:16:7:
Could not deduce (C a (S ()))
from the context (S () ~ S b1, C a1 (S b1))
arising from a use of `proof' at Bug.hs:16:7-11
Bug.hs:21:7:
Could not deduce (C a (S ()))
from the context (S () ~ S b1, C a1 (S b1))
arising from a use of `proof' at Bug.hs:21:7-11
Bug.hs:22:16:
Could not deduce (C a1 (S ())) from the context (S () ~ a)
arising from a use of `proof' at Bug.hs:22:16-20
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2040>
GHC <http://www.haskell.org/ghc/>
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