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Created by glguy@glguy

Coercible solver regression: Couldn't match rep of () with Const () b

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 ())
Edited by glguy