[GHC] #10722: GHC generates wrong unused type argument warning(ScopedTypeVariables related)

[GHC]

#10722: GHC generates wrong unused type argument warning(ScopedTypeVariables
related)
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              Reporter:  osa1        |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.11
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 {{{#!hs
 {-# LANGUAGE KindSignatures, ScopedTypeVariables #-}

 {-# OPTIONS_GHC -Wall #-}

 module Main where

 import Data.Proxy

 class Blah (a :: * -> *) where
   f :: a b -> b

 instance forall b . Blah [] where
   f a = let (_ :: Proxy b) = undefined in head a

 main :: IO ()
 main = return ()
 }}}

 When I run this using GHC 7.10.1 and GHC HEAD, I'm getting this warnings:

 {{{
 [1 of 1] Compiling Main             ( Main.hs, Main.o )

 Main.hs:12:17: warning:
     Unused quantified type variable ‘b’
     In the type ‘forall b. Blah []’
     In an instance declaration

 Main.hs:13:13: warning:
     This pattern-binding binds no variables: (_ :: Proxy b) = undefined
 Linking Main ...
 }}}

 First warning is wrong. `b` is used, and this program doesn't compile if I
 remove that `b`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10722>
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