[GHC] #10285: Bug in Coerciible

[GHC]

#10285: Bug in Coerciible
-------------------------------------+-------------------------------------
              Reporter:  simonpj     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  highest     |         Milestone:  7.10.2
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 Richard and I uncovered this
 {{{
 {-# LANGUAGE RoleAnnotations #-}
 module Bar( N ) where   -- Data constructor not exported

 type role N representational
 newtype N a = MkN Int
 }}}
 and
 {{{
 {-# LANGUAGE FlexibleContexts #-}
 module Foo where

 import Data.Coerce
 import Data.Type.Coercion
 import Bar

 f :: Coercible (N a) (N b) => Coercion a b
 f = Coercion
 }}}
 This compiles but it should not!  It wrongly compiles by using `nth (N a
 ~R N b)`; but (as Fig 4 of [http://research.microsoft.com/en-
 us/um/people/simonpj/papers/ext-f/ the Corcible paper] says), we can't
 decompose a newtype at representational role.

 Easy to fix.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10285>
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