Make the pattern match exhaustiveness checker smarter about UnliftedDatatypes

Motivation

In recent versions of GHC, the pattern match exhaustiveness checker has become smarter, and code like

{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
module Main where
import Data.Kind (Type)

type Foo :: Type -> Type -> Type 
data Foo a b = 
      Bar !a
    | Baz b

type MyVoid :: Type
data MyVoid

v :: Foo MyVoid Char
v = Baz 'c'

main :: IO ()
main = case v of
    -- The Baz case is impossible
    Baz c -> putChar c

doesn't emit a "Pattern match(es) are non-exhaustive" warning.

Now that we have UnliftedDatatypes in 9.2.1, it seems to me that the exhaustiveness checker could be equally smart about code like

{-# LANGUAGE UnliftedDatatypes  #-}
{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
module Main where
import Data.Kind (Type)
import GHC.Exts (UnliftedType)

type Foo :: UnliftedType -> Type -> Type 
data Foo a b = 
      Bar a -- no need for strictness annotation
    | Baz b

type MyVoid :: UnliftedType
data MyVoid

v :: Foo MyVoid Char
v = Baz 'c'

main :: IO ()
main = case v of
    -- The Baz case is impossible
    -- MyVoid has no values, and it can't 
    -- be undefined because it's unlifted.
    Baz c -> putChar c

Proposal

Make the pattern match exhaustiveness checker more aware of impossible branches caused by the use of UnliftedDatatypes.