[GHC] #12716: Suggest ScopedTypeVariables when type variable not in scope

[GHC]

#12716: Suggest ScopedTypeVariables when type variable not in scope
-------------------------------------+-------------------------------------
           Reporter:  ezyang         |             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:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I recently wrote this:

 {{{
 {-# LANGUAGE ExplicitForAll #-}
 {-# LANGUAGE TypeApplications #-}
 module B where
 f :: forall a. a -> a
 f x = id @a x
 }}}

 And was hornswoggled for the better part of five minutes for why "a"
 wasn't in scope. Answer: you need `ScopedTypeVariables`. A suggestion
 would be nice.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12716>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler