decomposeRuleLhs reports bogus error

Consider

f :: Int -> Int
f x = x

{-# RULES "foo" forall a (x::a) (g :: a -> Int). f (g x) = 2 #-}

Currently compiling with -O gives

Foo.hs:8:11: error:
    Rule "foo": Forall'd variable ‘a’ does not appear on left hand side
  |
8 | {-# RULES "foo" forall a (x::a) (g :: a -> Int). f (g x) = 2 #-}
  |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Diagnosis

GHC.HsToCore.Binds.decomposeRuleLhs takes the free vars of the LHS and checks that all of those free vars are among the binders. But the (shallow) free vars of f (g x) are are just {f,g,x}, but not a. Yet a is a binder.

Clearly we want the deep free vars of the LHS for this error check.