[GHC] #13598: role annotation for newtype (partially?) ignored?

[GHC]

#13598: role annotation for newtype (partially?) ignored?
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           Reporter:  edsko          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 A constraint `Coercible o (T a)` is ambiguous if the role of `a` is
 representational (provided `a` is not otherwise constrained, of course),
 but unambiguous if `a` is a phantom type:

 {{{#!hs
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE RoleAnnotations #-}
 module CoerceTest where

 import Data.Coerce

 type role A phantom
 data A a = MkA Int

 type role B representational
 data B a = MkB Int

 -- accepted by the type checker (as it should be)
 fooA :: Coercible o (A a) => o -> ()
 fooA _ = ()

 -- rejected by the type checker (as it should be) with
 -- "Couldn't match representation of type ‘a0’ with that of ‘a’"
 -- fooB :: Coercible o (B a) => o -> ()
 -- fooB _ = ()
 }}}

 However, for `newtype`s something odd happens:

 {{{#!hs
 type role C representational
 newtype C a = MkC Int

 -- accepted by the type checker (but should not be)
 fooC :: Coercible o (C a) => o -> ()
 fooC _ = ()
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13598>
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