[GHC] #10079: Coercible solver regression: Couldn't match rep of () with Const () b

[GHC]

#10079: Coercible solver regression: Couldn't match rep of () with Const () b
-------------------------------------+-------------------------------------
        Reporter:  glguy             |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.1-rc1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:                    |
-------------------------------------+-------------------------------------
Description changed by glguy:

Old description:

> Hello, I ran into what appears to be a regression in the Coercible solver
> since 7.8.4. This is as small as I've managed to get my example case.
>
> {{{
> {-# LANGUAGE MultiParamTypeClasses #-}
> {-# LANGUAGE FunctionalDependencies #-}
> {-# LANGUAGE FlexibleContexts #-}
> module Bug where
>
> import Control.Applicative
> import Data.Coerce
>
> broken :: Bizarre (->) w => w a b t -> ()
> broken = getConst #. bazaar (Const #. const ())
>
> class Profunctor p where
>   (#.) :: Coercible c b => (b -> c) -> p a b -> p a c
>
> class Bizarre p w | w -> p where
>   bazaar :: Applicative f => p a (f b) -> w a b t -> f t
> }}}
>
> {{{
> Bug.hs:10:36:
>     Couldn't match representation of type ‘()’
>                              with that of ‘Const () b’
>     Relevant role signatures: type role Const representational phantom
>     Relevant bindings include
>       broken :: w a b t -> () (bound at Bug.hs:10:1)
>     In the first argument of ‘bazaar’, namely ‘(Const #. const ())’
>     In the second argument of ‘(#.)’, namely
>       ‘bazaar (Const #. const ())’
>     In the expression: getConst #. bazaar (Const #. const ())
> }}}

New description:

 Hello, I ran into what appears to be a regression in the Coercible solver
 since 7.8.4. This is as small as I've managed to get my example case.

 {{{
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE FlexibleContexts #-}
 module Bug where

 import Control.Applicative
 import Data.Coerce

 broken :: Bizarre (->) w => w a b t -> ()
 broken = getConst #. bazaar (Const #. const ())

 class Profunctor p where
   (#.) :: Coercible c b => (b -> c) -> p a b -> p a c

 instance Profunctor (->) where
   (#.) = (.)

 class Bizarre p w | w -> p where
   bazaar :: Applicative f => p a (f b) -> w a b t -> f t
 }}}

 {{{
 Bug.hs:10:36:
     Couldn't match representation of type ‘()’
                              with that of ‘Const () b’
     Relevant role signatures: type role Const representational phantom
     Relevant bindings include
       broken :: w a b t -> () (bound at Bug.hs:10:1)
     In the first argument of ‘bazaar’, namely ‘(Const #. const ())’
     In the second argument of ‘(#.)’, namely
       ‘bazaar (Const #. const ())’
     In the expression: getConst #. bazaar (Const #. const ())
 }}}

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10079#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler