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+{-# OPTIONS_GHC -O2 -fforce-recomp #-}
+
+-- | From Note [Boxity Analysis] and related Notes
+module T19871 where
+
+data Huge
+  = Huge
+  { f1 :: Bool
+  , f2 :: Bool
+  , f3 :: Bool
+  , f4 :: Bool
+  , f5 :: Bool
+  , f6 :: Bool
+  , f7 :: Bool
+  , f8 :: Bool
+  , f9 :: Bool
+  , f10 :: Bool
+  , f11 :: Bool
+  , f12 :: Bool }
+
+-- | Should not unbox Huge
+ann :: Huge -> (Bool, Huge)
+ann h@(Huge{f1=True}) = (False, h)
+ann h                 = (True,  h)
+{-# NOINLINE ann #-}
+
+-- A few examples demonstrating the lubBoxity = unboxedWins tradeoff
+
+-- | Should unbox 'z'.
+-- We won't with `lubBoxity = boxedWins`.
+-- We will  with `lubBoxity = unboxedWins`.
+sumIO :: Int -> Int -> IO Int
+sumIO 0 !z = return z
+sumIO n !z = sumIO (n-1) (z+n)
+{-# NOINLINE sumIO #-}
+
+-- | Should /not/ unbox 'h'.
+-- We won't with `lubBoxity = boxedWins`.
+-- We will  with `lubBoxity = unboxedWins`.
+update :: Huge -> (Bool, Huge)
+update h@(Huge{f1=True}) = (False, h{f1=False})
+update h                 = (True,  h)
+{-# NOINLINE update #-}
+
+-- | Should /not/ unbox 'h'.
+-- We won't with `lubBoxity = boxedWins`.
+-- We will  with `lubBoxity = unboxedWins`.
+guarded :: (Huge -> Bool) -> Huge -> Bool
+guarded g h | f1 h      = True
+            | otherwise = g h
+{-# NOINLINE guarded #-}
+
+-- | Should /not/ unbox 'h'.
+-- We won't with `lubBoxity = boxedWins`.
+-- We will  with `lubBoxity = unboxedWins`.
+--
+-- This example also demonstrates the usefulness of carrying a Boxity in Poly.
+-- Most absent sub-demands here should be considered Boxed (and of course we
+-- also need Unboxed absent Poly). See Note [Boxity in Poly].
+absent :: Huge -> Int
+absent h = if f1 h || f2 h then g h else 2
+  where
+    g :: a -> Int
+    g a = a `seq` f a True
+    {-# NOINLINE g #-}
+    f :: a -> Bool -> Int
+    f _ True = 1
+    f a False = a `seq` 2
+    {-# NOINLINE f #-}
+{-# NOINLINE absent #-}