[GHC] #15431: Coercible and Existential types don't play nicely

[GHC]

#15431: Coercible and Existential types don't play nicely
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        Reporter:  NioBium           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  Roles
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14333            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * keywords:   => Roles
 * related:   => #14333


Comment:

 Hm, this is interesting. This can be minimized to the following examples,
 which are slight variations of each other:

 {{{#!hs
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE GADTs #-}
 module Bug where

 import Data.Coerce
 import Data.Functor.Identity

 g1 :: Coercible (t a) Int => t a -> Int
 g1 = coerce

 g2 :: Coercible Int (t a) => t a -> Int
 g2 = coerce
 }}}

 `g1` typechecks, but `g2` doesn't!

 {{{
 $ /opt/ghc/8.4.3/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:12:6: error:
     • Couldn't match representation of type ‘t a’ with that of ‘Int’
         arising from a use of ‘coerce’
     • In the expression: coerce
       In an equation for ‘g2’: g2 = coerce
     • Relevant bindings include g2 :: t a -> Int (bound at Bug.hs:12:1)
    |
 12 | g2 = coerce
    |      ^^^^^^
 }}}

 I'm not sure if this is related to #14333 or not.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15431#comment:1>
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