[GHC] #12616: type synonyms confuse generalized newtype deriving role checking

[GHC]

#12616: type synonyms confuse generalized newtype deriving role checking
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        Reporter:  daviddarais       |                Owner:  goldfire
            Type:  bug               |               Status:  merge
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  generalized
                                     |  newtype deriving roles rankntypes
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  deriving/should_compile/T12616
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * status:  new => merge
 * testcase:   => deriving/should_compile/T12616


Comment:

 Great bug report thank you.

 Although the patch looks large it isn't really.

 But it ''does'' change the behaviour of GHC with `-XImpredicativeTypes`.
 Since we don't advertise any particular behaviour, that might seem OK.  Or
 someone might complain. Still, I'm inclined to merge the change to the 8.0
 branch because it makes the compiler more stable and predictable.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12616#comment:3>
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