[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
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              Reporter:  glguy       |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1-rc1
  (Type checker)                     |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  GHC rejects
          Architecture:              |  valid program
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 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 ())
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10079>
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