[GHC] #15639: Surprising failure combining QuantifiedConstraints with Coercible

[GHC]

#15639: Surprising failure combining QuantifiedConstraints with Coercible
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           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.5
  (Type checker)                     |
           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:
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 I don't understand what's going wrong here.

 {{{#!hs
 -- 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
   |
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15639>
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