[GHC] #8745: GeneralizedNewtypeDeriving is still not Safe

[GHC]

#8745: GeneralizedNewtypeDeriving is still not Safe
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        Reporter:  goldfire                      |            Owner:
            Type:  bug                           |           Status:  new
        Priority:  normal                        |        Milestone:
       Component:  Compiler                      |          Version:
      Resolution:                                |  7.8.1-rc1
Operating System:  Unknown/Multiple              |         Keywords:  Safe
 Type of failure:  None/Unknown                  |     Architecture:
       Test Case:                                |  Unknown/Multiple
  should_fail/TcCoercibleFailSafe                |       Difficulty:
        Blocking:                                |  Unknown
                                                 |       Blocked By:
                                                 |  Related Tickets:
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Changes (by nomeata):

 * testcase:   => should_fail/TcCoercibleFailSafe


Comment:

 > With this in place (is it tested by anything in the testsuite?)

 One simple test case is in place
 (`typecheck/should_fail/TcCoercibleFailSafe`).

 > class Foo a where ...
 > type role Foo representational

 Why is that even accepted? I thought role annotation can only stricten
 roles (i.e. P→N, P→R, R→N), and we have decided that classes have role
 nominal by default, so I would expect `type role Foo representational` to
 be rejected even in non-Safe Haskell.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8745#comment:6>
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