-fwarn-unused-binds misses a use

[George Russell]

# ghc Unused.hs -fwarn-unused-binds -fglasgow-exts

Unused.hs:10: Warning: Defined but not used: Dummy


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module Unused(Foo(..),ff) where

class Foo (v :: (* -> *) -> *) where
   f :: v a -> Int

ff :: Foo v => v a -> Int
ff (_ :: (v :: (* -> *) -> *) a) = (f (error "bar" :: v Dummy))


data Dummy x = Dummy x