[GHC] #10646: Adding GADTs extension makes RankNTypes code fail to compile.

[GHC]

#10646: Adding GADTs extension makes RankNTypes code fail to compile.
-------------------------------------+-------------------------------------
        Reporter:  phadej            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------

Old description:

> The failing example:
>
> {{{#!hs
> {-# LANGUAGE RankNTypes #-}
> {-# LANGUAGE GADTs #-}
>
> data I a = I a
>
> example :: String -> I a -> String
> example str x = withContext x s
>   where
>     s i = "Foo" ++ str
>
> withContext :: I a -> (forall b. I b -> c) -> c
> withContext x f = f x
> }}}
>
> '''Without''' `GADTs`, this compiles and works fine:
>
> {{{
> λ *Main > example "bar" (P 'a' "quux")
> "Foobar"
> }}}
>
> '''With''' `GADTs` the code fails to compile with an error:
>
> {{{
> ex.hs:7:31:
>     Couldn't match type ‘t0’ with ‘I b’
>       because type variable ‘b’ would escape its scope
>     This (rigid, skolem) type variable is bound by
>       a type expected by the context: I b -> String
>       at ex.hs:7:17-31
>     Expected type: I b -> String
>       Actual type: t0 -> [Char]
>     Relevant bindings include s :: t0 -> [Char] (bound at ex.hs:9:5)
>     In the second argument of ‘withContext’, namely ‘s’
>     In the expression: withContext x s
> Failed, modules loaded: none.
> }}}
>
> '''Yet''', if I add type annotation for `s`, everything seems to be fine:
>
> {{{#!hs
> {-# LANGUAGE RankNTypes #-}
> {-# LANGUAGE GADTs #-}
>
> data I a = I a
>
> example :: String -> I a -> String
> example str x = withContext x s
>   where
>     s :: I a -> String
>     s i = "Foo" ++ str
>
> withContext :: I a -> (forall b. I b -> c) -> c
> withContext x f = f x
> }}}
>
> ----
>
> I tried to make the failing example smaller, but seems that every bit
> participates. The use of `str` inside `s` especially.
>
> Is there some bug hiding inside GADTs related stuff?

New description:

 The failing example:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE GADTs #-}

 data I a = I a

 example :: String -> I a -> String
 example str x = withContext x s
   where
     s i = "Foo" ++ str

 withContext :: I a -> (forall b. I b -> c) -> c
 withContext x f = f x
 }}}

 '''Without''' `GADTs`, this compiles and works fine:

 {{{
 λ *Main > example "bar" (P 'a' "quux")
 "Foobar"
 }}}

 '''With''' `GADTs` the code fails to compile with an error:

 {{{
 ex.hs:7:31:
     Couldn't match type ‘t0’ with ‘I b’
       because type variable ‘b’ would escape its scope
     This (rigid, skolem) type variable is bound by
       a type expected by the context: I b -> String
       at ex.hs:7:17-31
     Expected type: I b -> String
       Actual type: t0 -> [Char]
     Relevant bindings include s :: t0 -> [Char] (bound at ex.hs:9:5)
     In the second argument of ‘withContext’, namely ‘s’
     In the expression: withContext x s
 Failed, modules loaded: none.
 }}}

 '''Yet''', if I add type annotation for `s`, everything seems to be fine:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE GADTs #-}

 data I a = I a

 example :: String -> I a -> String
 example str x = withContext x s
   where
     s :: I a -> String
     s i = "Foo" ++ str

 withContext :: I a -> (forall b. I b -> c) -> c
 withContext x f = f x
 }}}

 ----

 I tried to make the failing example smaller, but seems that every bit
 participates. The use of `str` inside `s` especially.

 Is there some bug hiding inside GADTs related stuff?

 Tried with 7.8.4 and 7.10.1

--

Comment (by phadej):

 Happens

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10646#comment:1>
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