[GHC] #16209: GHC behaves differently when binding has separate or inlined type declaration

[GHC]

#16209: GHC behaves differently when binding has separate or inlined type
declaration
-------------------------------------+-------------------------------------
        Reporter:  fghibellini       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by fghibellini:

Old description:

> {{{#!hs
> {-# LANGUAGE RankNTypes #-}
> {-# LANGUAGE ScopedTypeVariables #-}
>
> main = do
>     let a :: forall t. Functor t => t () = undefined
>     let g :: forall u. Functor u => u () = a
>     putStrLn "success"
> }}}
>
> fails with:
>
> {{{
> source_file.hs:6:44:
>     Couldn't match expected type ‘forall (u :: * -> *).
>                                   Functor u =>
>                                   u ()’
>                 with actual type ‘t0 ()’
>     When checking that: t0 ()
>       is more polymorphic than: forall (u :: * -> *). Functor u => u ()
>     In the expression: a
> }}}
>
> but the following compiles successfully
>
> {{{#!hs
> {-# LANGUAGE RankNTypes #-}
> {-# LANGUAGE ScopedTypeVariables #-}
>
> main = do
>     let a :: forall t. Functor t => t () = undefined
>     let g :: forall u. Functor u => u ()
>         g = a
>     putStrLn "success"
> }}}
>
> On GHC 7.10 it works as described above. On newer ones it fails even on
> the "functioning code".
>
> {{{
> a.hs:5:44: error:
>     • Cannot instantiate unification variable ‘a0’
>       with a type involving foralls:
>         forall (t :: * -> *). Functor t => t ()
>         GHC doesn't yet support impredicative polymorphism
>     • In the expression: undefined
>       In a pattern binding:
>         a :: forall (t :: * -> *). Functor t => t () = undefined
>       In the expression:
>         do let a :: forall t. Functor t => t () = undefined
>            let g :: forall u. Functor u => u ()
>                g = a
>            putStrLn "success"
> }}}

New description:

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

 main = do
     let a :: forall t. Functor t => t () = undefined
     let g :: forall u. Functor u => u () = a
     putStrLn "success"
 }}}

 fails with:

 {{{
 source_file.hs:6:44:
     Couldn't match expected type ‘forall (u :: * -> *).
                                   Functor u =>
                                   u ()’
                 with actual type ‘t0 ()’
     When checking that: t0 ()
       is more polymorphic than: forall (u :: * -> *). Functor u => u ()
     In the expression: a
 }}}

 but the following compiles successfully

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

 main = do
     let a :: forall t. Functor t => t () = undefined
     let g :: forall u. Functor u => u ()
         g = a
     putStrLn "success"
 }}}

 On GHC 7.10 it works as described above. On 8.4.4 it fails even on the
 "functioning code".

 {{{
 a.hs:5:44: error:
     • Cannot instantiate unification variable ‘a0’
       with a type involving foralls:
         forall (t :: * -> *). Functor t => t ()
         GHC doesn't yet support impredicative polymorphism
     • In the expression: undefined
       In a pattern binding:
         a :: forall (t :: * -> *). Functor t => t () = undefined
       In the expression:
         do let a :: forall t. Functor t => t () = undefined
            let g :: forall u. Functor u => u ()
                g = a
            putStrLn "success"
 }}}

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16209#comment:1>
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