[GHC] #15012: "Iface type variable out of scope" when compiling with -c

[GHC]

#15012: "Iface type variable out of scope" when compiling with -c
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.1
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4602
       Wiki Page:                    |
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Comment (by simonpj):

 I added a suggestion on the Phab, but it is (still, alas) not broadcast.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15012#comment:3>
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