[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:  new
        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):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * keywords:   => deriving


Comment:

 I suspect that `DeriveGeneric` is to blame here. Here is what `-ddump-
 deriv` tells us about the derived `Rep1` associated type family instance
 for `Generic1`:

 {{{#!hs
 Derived type family instances:
   type GHC.Generics.Rep1 (Foo.TyFamily a_a2ta) = GHC.Generics.D1
                                                    ('GHC.Generics.MetaData
                                                       "TyFamily" "Foo"
 "main" 'GHC.Types.False)
                                                    (GHC.Generics.C1
 ('GHC.Generics.MetaCons
                                                          "TyFamily"
 'GHC.Generics.PrefixI
                                                          'GHC.Types.False)
                                                       (GHC.Generics.S1
 ('GHC.Generics.MetaSel
 'GHC.Base.Nothing
 'GHC.Generics.NoSourceUnpackedness
 'GHC.Generics.NoSourceStrictness
 'GHC.Generics.DecidedLazy)
 (GHC.Generics.Rec0 GHC.Types.Int)
                                                        GHC.Generics.:*:
 GHC.Generics.S1
 ('GHC.Generics.MetaSel
 'GHC.Base.Nothing
 'GHC.Generics.NoSourceUnpackedness
 'GHC.Generics.NoSourceStrictness
 'GHC.Generics.DecidedLazy)
 (GHC.Generics.Rec0
 (Foo.FakeOut b_a2q1))))
 }}}

 Notice that `b_a2q1` is unbound in this definition! Perhaps we should be
 turning this into `Any`?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15012#comment:1>
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