[GHC] #13512: GHC incorrectly warns that a variable used in a type application is unused

[GHC]

#13512: GHC incorrectly warns that a variable used in a type application is unused
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
  TypeApplications                   |
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This bug is reproducible on GHC 8.0.1, 8.0.2, 8.2, and HEAD.

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeInType #-}

 {-# OPTIONS_GHC -Wunused-foralls #-}
 module Bug where

 import Data.Proxy

 proxy :: forall k (a :: k). Proxy a
 proxy = Proxy

 data SomeProxy where
   SomeProxy :: forall k (a :: k). Proxy a -> SomeProxy

 someProxy :: forall k (a :: k). SomeProxy
 someProxy = SomeProxy (proxy @k @a)
 }}}

 {{{
 $ /opt/ghc/head/bin/ghci Bug.hs
 GHCi, version 8.3.20170327: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:17:23: warning: [-Wunused-foralls]
     Unused quantified type variable ‘(a :: k)’
     In the type ‘forall k (a :: k). SomeProxy’
    |
 17 | someProxy :: forall k (a :: k). SomeProxy
    |                       ^^^^^^^^
 Ok, modules loaded: Bug.
 }}}

 But that `a` is used in `proxy @k @a`!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13512>
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