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I don't understand what's going wrong here.

-- Fishy.hs
{-# language RankNTypes, QuantifiedConstraints, RoleAnnotations #-}
module Fishy (Yeah, yeahCoercible) where
import Data.Coerce

data Yeah_ a = Yeah_ Int
newtype Yeah a = Yeah (Yeah_ a)
type role Yeah representational

yeahCoercible :: ((forall a b. Coercible (Yeah a) (Yeah b)) => r) -> r
yeahCoercible r = r


-- Fishy2.hs

module Fishy2 where

import Fishy
import Data.Type.Coercion
import Data.Coerce

yeah :: Coercion [Yeah a] [Yeah b]
yeah = yeahCoercible Coercion

I get

Fishy2.hs:8:22: error:
    • Couldn't match representation of type ‘a’ with that of ‘b’
        arising from a use of ‘Coercion’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          yeah :: forall a b. Coercion [Yeah a] [Yeah b]
        at Fishy2.hs:7:1-34
      ‘b’ is a rigid type variable bound by
        the type signature for:
          yeah :: forall a b. Coercion [Yeah a] [Yeah b]
        at Fishy2.hs:7:1-34
    • In the first argument of ‘yeahCoercible’, namely ‘Coercion’
      In the expression: yeahCoercible Coercion
      In an equation for ‘yeah’: yeah = yeahCoercible Coercion
    • Relevant bindings include
        yeah :: Coercion [Yeah a] [Yeah b] (bound at Fishy2.hs:8:1)
  |
8 | yeah = yeahCoercible Coercion
  |

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
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Blocking
CC
Operating system
Architecture