id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
1746,GADT bug with -O2,igloo,igloo,"Reported by Daniel Gorín in
http://www.haskell.org/pipermail/glasgow-haskell-users/2007-September/013178.html

This module:
{{{
{-# OPTIONS_GHC -fglasgow-exts #-}
module T where

data T a where T :: T a -> T [a]

class C a where
  f :: a -> ()

instance C (T [a]) where
  f (T x@(T _)) = f x
}}}
gives a compiler panic when compiled with `-O2`. With `-dcore-lint` we get:
{{{
$ ghc -c -O2 e.hs -dcore-lint
rm: cannot remove `e.o': No such file or directory
rm: cannot remove `e.hi': No such file or directory
*** Core Lint Errors: in result of SpecConstr ***
<no location info>:
    [in body of lambda with binder co_a5Z :: [a_a5t] ~ [a_a5X]]
    a_a5X is out of scope
*** Offending Program ***
Rec {
$sa_s6U :: forall a_a5t a_a60.
           ([a_a5t] ~ [a_a5X], a_a5t ~ [a_a60]) =>
           T.T a_a60 -> ()
[]
$sa_s6U =
  \ (@ a_a5t) (@ co_a5Z) (@ a_a60) (@ co_a62) (ds_d69 [Just B] :: T.T a_a60) ->
    case (T.T @ a_a5t @ a_a60 @ co_a62 ds_d69)
         `cast` (trans
                   (T.T (right co_a5Z)) (trans (T.T (right (sym co_a5Z))) (T.T co_a62))
                 :: T.T a_a5t ~ T.T [a_a60])
    of wild_B1 [Just B] { T.T @ a_a5X @ co_X68 ds_d67 [Just X] ->
    case ds_d67 `cast` (T.T (right (sym co_X68)) :: T.T a_a5X ~ T.T a_a60)
    of wild_Xc [Just X] { T.T @ a_X6c @ co_X6f ds_X6n [Just B] ->
    a_s6j
      @ a_X6c
      (wild_Xc
       `cast` (trans
                 (T.T (right co_X68)) (trans (T.T (right (sym co_X68))) (T.T co_X6f))
               :: T.T a_a60 ~ T.T [a_X6c]))
    }
    }
a_s6j [LoopBreaker Nothing] :: forall a_a5t. T.T [a_a5t] -> ()
[Arity 1
 Str: DmdType Sb
 RULES: ""SC:a_s6j0"" [0]
            forall {@ a_a5t @ co_a5Z @ a_a60 @ co_a62 ds_d69 [Just B] :: T.T a_a60}
              a_s6j @ a_a60
                    ((T.T @ a_a5t @ a_a60 @ co_a62 ds_d69)
                     `cast` (trans
                               (T.T (right co_a5Z)) (trans (T.T (right (sym co_a5Z))) (T.T co_a62))
                             :: T.T a_a5t ~ T.T [a_a60]))
              = $sa_s6U @ a_a5t @ co_a5Z @ a_a60 @ co_a62 ds_d69]
a_s6j =
  \ (@ a_a5t) (ds_X6c :: T.T [a_a5t]) ->
    case ds_X6c of wild_B1 [Just B] { T.T @ a_a5X @ co_a5Z ds_d67 [Just X] ->
    case ds_d67 `cast` (T.T (right (sym co_a5Z)) :: T.T a_a5X ~ T.T a_a5t)
    of wild_Xc [Just X] { T.T @ a_a60 @ co_a62 ds_d69 [Just B] ->
    a_s6j
      @ a_a60
      (wild_Xc
       `cast` (trans
                 (T.T (right co_a5Z)) (trans (T.T (right (sym co_a5Z))) (T.T co_a62))
               :: T.T a_a5t ~ T.T [a_a60]))
    }
    }
end Rec }

T.$f1 :: forall a_a5t. T.C (T.T [a_a5t])
[Exported]
[Arity 1
 Str: DmdType Sb]
T.$f1 =
  \ (@ a_a5t) ->
    (a_s6j @ a_a5t)
    `cast` (sym ((T.:Co:TC) (T.T [a_a5t]))
            :: T.T [a_a5t] -> () ~ (T.:TC) (T.T [a_a5t]))

*** End of Offense ***


<no location info>: 
Compilation had errors
}}}

This is a regression since 6.6.
",merge,closed,normal,6.8.1,Compiler,6.8,fixed,,,Unknown/Multiple,Unknown/Multiple,,Unknown,simpl019,,,