[GHC] #1158: Problem with GADTs and explicit type signatures

[GHC]

#1158: Problem with GADTs and explicit type signatures
-------------------------+--------------------------------------------------
    Reporter:  guest     |       Owner:         
        Type:  bug       |      Status:  new    
    Priority:  normal    |   Milestone:         
   Component:  Compiler  |     Version:  6.6    
    Severity:  normal    |    Keywords:         
  Difficulty:  Unknown   |    Testcase:         
Architecture:  Unknown   |          Os:  Unknown
-------------------------+--------------------------------------------------
{{{

 {-# OPTIONS_GHC -fglasgow-exts #-}

 module Main where

 data Exp a where
     Val :: a -> Exp b
     App :: Exp a -> Exp b

 instance Show (Exp a) where
     show (Val _) = "Val"
     show (App _) = "App"

 class LiftToExp a b where
     liftToExp :: a -> Exp b

 instance LiftToExp (Exp a) a where
     liftToExp = id

 instance Floating a => LiftToExp a b where
     liftToExp v = Val v :: Exp b

 {-

 Uncommenting the type signature below causes GHCi to fail to load the
 file:

 test.hs:45:14:
     Overlapping instances for LiftToExp a a11
       arising from use of `liftToExp' at test.hs:45:14-24
     Matching instances:
       instance (Floating a) => LiftToExp a b -- Defined at test.hs:19:0
       instance LiftToExp (Exp a) a -- Defined at test.hs:16:0
     (The choice depends on the instantiation of `a, a11'
      Use -fallow-incoherent-instances to use the first choice above)
     In the first argument of `App', namely `(liftToExp x)'
     In the expression: App (liftToExp x)
     In the definition of `test': test x = App (liftToExp x)

 However typing :t test at the GHCi prompt gives this exact signature.

 Tested with GHC 6.6 (compiler and interpreter) under OS X 10.4.8 on an
 iMac G5.

 -}

 --test :: (LiftToExp a a1) => a -> Exp b
 test x = App (liftToExp x)

 main = putStrLn $ show (test (3.0::Float)::Exp Int)

 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1158>
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