[GHC] #13069: hs-boot files permit default methods in type class

[GHC]

#13069: hs-boot files permit default methods in type class
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        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |             Keywords:  backpack hs-
      Resolution:                    |  boot
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 It looks like some hs-boot files have come to rely on this behavior:

 {{{
 MiniAgda-0.2014.9.12/Eval.hs-boot:class Reval a where
 MiniAgda-0.2014.9.12/Eval.hs-boot-  reval' :: Valuation -> a -> TypeCheck
 a
 MiniAgda-0.2014.9.12/Eval.hs-boot-  reval  :: a -> TypeCheck a
 MiniAgda-0.2014.9.12/Eval.hs-boot-  reval = reval' emptyVal
 postgresql-simple-0.5.2.1/src/Database/PostgreSQL/Simple/ToRow.hs-
 boot:class ToRow a where
 postgresql-simple-0.5.2.1/src/Database/PostgreSQL/Simple/ToRow.hs-boot-
 toRow :: a -> [Action]
 postgresql-simple-0.5.2.1/src/Database/PostgreSQL/Simple/ToRow.hs-boot-
 default toRow :: (Generic a, GToRow (Rep a)) => a -> [Action]
 postgresql-simple-0.5.2.1/src/Database/PostgreSQL/Simple/ToRow.hs-boot-
 toRow = gtoRow . from
 event-monad-0.0.3/src/Control/Monad/Event/Classes.hs-boot:class
 (MonadEvent m e, MonadTime m t) => ScheduleEvent m t e | m -> t
 event-monad-0.0.3/src/Control/Monad/Event/Classes.hs-boot-    where
 event-monad-0.0.3/src/Control/Monad/Event/Classes.hs-boot-
 scheduleEventAt :: t -> e -> m EventID
 event-monad-0.0.3/src/Control/Monad/Event/Classes.hs-boot-        doNext
 :: e -> m ()
 event-monad-0.0.3/src/Control/Monad/Event/Classes.hs-boot-        doNext e
 = do
 event-monad-0.0.3/src/Control/Monad/Event/Classes.hs-boot-            now
 <- getCurrentTime
 event-monad-0.0.3/src/Control/Monad/Event/Classes.hs-boot-
 scheduleEventAt now e
 }}}

 Maybe less than a dozen I could see on Hackage. So maybe we should just
 update the docs.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13069#comment:1>
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