GADT regression
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 (closed), 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:
- 8.1 : OK
- 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
- 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
Trac metadata
Trac field | Value |
---|---|
Version | 6.8.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | zunino@di.unipi.it |
Operating system | Unknown |
Architecture | Unknown |