[GHC] #12473: Ambiguous type var with DefaultSignatures and FunctionalDependencies

[GHC]

#12473: Ambiguous type var with DefaultSignatures and FunctionalDependencies
-------------------------------------+-------------------------------------
           Reporter:  dylex          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  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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 Consider:
 {{{
 {-# LANGUAGE DefaultSignatures, FunctionalDependencies,
 ScopedTypeVariables #-}
 class Foo a b where
   foo :: a -> b -> Int
 class Bar a b | b -> a where
   get :: b -> a
   bar :: b -> Int
   default bar :: Foo a b => b -> Int
   bar b = foo (get b :: a) b
 }}}
 This works fine with ghc 7.10.3 (even without the explicit :: a type
 spec), but with 8.0.1, produces:
 {{{
     - Could not deduce (Foo a0 b)
       from the context: Foo a b
         bound by the type signature for:
                    bar :: Foo a b => b -> Int
         at t.hs:6:3-17
       The type variable `a0'€™ is ambiguous
     - In the ambiguity check for `bar'™
       To defer the ambiguity check to use sites, enable
 AllowAmbiguousTypes
       When checking the class method:
         bar :: forall a b. Bar a b => b -> Int
       In the class declaration for `Bar'€™
 }}}
 Not sure if this is intentional or some good way to get around it, but I
 couldn't find anything related to it in the release notes or any similar
 looking bugs.  (Real-life case: https://github.com/dylex/postgresql-
 typed/blob/master/Database/PostgreSQL/Typed/Dynamic.hs#L48 )

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12473>
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