[GHC] #12046: AllowAmbiguousTypes doesn't work with UndecidableSuperClasses

12 May 2016


      #12046: AllowAmbiguousTypes doesn't work with UndecidableSuperClasses
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           Reporter:  MikeIzbicki    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc3
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Given the code below, `test1` and `test2` are the same except the former
 has an ambiguous type and the latter does not.  The compiler rejects the
 former and accepts the latter.

 {{{

 {-# LANGUAGE UndecidableSuperClasses #-}
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE TypeFamilies #-}

 class A (T a) => A a where
     type T a

 test1 :: forall a. A a => ()
 test1 = ()

 test2 :: A a => proxy a -> ()
 test2 _ = ()
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12046
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