[GHC] #13734: Code requires ScopedTypeVariables, possibly erroneously

[GHC]

#13734: Code requires ScopedTypeVariables, possibly erroneously
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 This doesn't work without `ScopedTypeVariables`, but shouldn't it? (on
 `8.0.1`, 8.2.0.20170507`)

 {{{#!hs
 {-# Language ScopedTypeVariables, TypeApplications, AllowAmbiguousTypes,
 InstanceSigs #-}

 class Sized f where
   size :: Int

 instance Sized Int where
   size :: Int
   size = 42

 instance (Sized a, Sized b) => Sized (a, b) where
   size :: Int
   size = size @a + size @b
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13734>
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