COMPLETE sets don't work at all with data family instances

Here's some code that uses a pattern synonym for a data family GADT constructor, along with a corresponding COMPLETE set:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
{-# OPTIONS_GHC -Wincomplete-patterns #-}
module Bug where

data family Sing (a :: k)

data instance Sing (z :: Bool) where
  SFalse :: Sing False
  STrue  :: Sing True

pattern STooGoodToBeTrue :: forall (z :: Bool). ()
                         => z ~ True
                         => Sing z
pattern STooGoodToBeTrue = STrue
{-# COMPLETE SFalse, STooGoodToBeTrue #-}

wibble :: Sing (z :: Bool) -> Bool
wibble STrue = True

wobble :: Sing (z :: Bool) -> Bool
wobble STooGoodToBeTrue = True

However, if you compile this, you might be surprised to find out that GHC only warns about wibble being non-exhaustive:

$ ghci Bug.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:23:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘wibble’: Patterns not matched: SFalse
   |
23 | wibble STrue = True
   | ^^^^^^^^^^^^^^^^^^^

I believe the use of data families is the culprit here, since a variant of this program which doesn't use data families correctly warns for both wibble and wobble:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
{-# OPTIONS_GHC -Wincomplete-patterns #-}
module Foo where

data SBool (z :: Bool) where
  SFalse :: SBool False
  STrue  :: SBool True

pattern STooGoodToBeTrue :: forall (z :: Bool). ()
                         => z ~ True
                         => SBool z
pattern STooGoodToBeTrue = STrue
{-# COMPLETE SFalse, STooGoodToBeTrue #-}

wibble :: SBool z -> Bool
wibble STrue = True

wobble :: SBool z -> Bool
wobble STooGoodToBeTrue = True

$ ghci Foo.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Foo              ( Foo.hs, interpreted )

Foo.hs:20:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘wibble’: Patterns not matched: SFalse
   |
20 | wibble STrue = True
   | ^^^^^^^^^^^^^^^^^^^

Foo.hs:23:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘wobble’: Patterns not matched: SFalse
   |
23 | wobble STooGoodToBeTrue = True
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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