[GHC] #12587: InstanceSigs doesn't work with ambigous types

[GHC]

#12587: InstanceSigs doesn't work with ambigous types
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           Reporter:  vagarenko      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 Why this code:
 {{{#!hs
 {-# LANGUAGE ScopedTypeVariables, InstanceSigs, AllowAmbiguousTypes #-}

 module Bug where

 class Foo a

 class Bar a where
     bar :: forall b. (Foo b) => a

 instance Bar Int where
     bar :: forall b. (Foo b) => Int  -- error here
     bar = undefined
         where
             x :: b
             x = undefined
 }}}
 is rejected with message:
 {{{
 Error: * Could not deduce (Foo b0)
     from the context: Foo b
       bound by the type signature for:
                  bar :: Foo b => Int
       at Bug.hs:11:12-35
     The type variable `b0' is ambiguous
   * When checking that: forall b. Foo b => Int
       is more polymorphic than: forall b. Foo b => Int
     When checking that instance signature for `bar'
       is more general than its signature in the class
       Instance sig: forall b. Foo b => Int
          Class sig: forall b. Foo b => Int
     In the instance declaration for `Bar Int'
 }}}
 ?
 Where does `b0` type var come from?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12587>
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