Incorrect incomplete pattern warning with GADTs and classes

Here is the test case:

{-# LANGUAGE DataKinds, KindSignatures, GADTs, ScopedTypeVariables,
             TypeApplications, AllowAmbiguousTypes #-}

{-# OPTIONS_GHC -Wincomplete-patterns #-}

module Bug where

import Data.Kind

data SBool :: Bool -> Type where
  SFalse :: SBool False
  STrue :: SBool True

class IBool b where
  bool :: SBool b

instance IBool False where
  bool = SFalse
instance IBool True where
  bool = STrue

foo :: forall b. IBool b => SBool b -> ()
foo _sb = case bool @b of
           STrue -> ()
           SFalse -> ()

I get

Bug.hs:23:11: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In a case alternative:
        Patterns not matched: _ :: IBool b => SBool b
   |
23 | foo _sb = case bool @b of
   |           ^^^^^^^^^^^^

But I'm pretty sure my pattern match is complete there. The class seems necessary: if I just use _sb as the scrutinee, all is well.