[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):

 Actually, the bug is worse: the following A.hs-boot file is accepted by
 GHC.

 {{{
 module A where
 class C a where
     f :: a -> a
     f = True
 }}}

 This doesn't seem to be a soundness problem because what this actually
 seems to translate into "there is a default method for f", so this code
 never gets compiled / used for anything.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13069#comment:2>
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