Big types prevent inlining of their Generic instances. Test case for GHC issue https://gitlab.haskell.org/ghc/ghc/-/issues/18523

i.hs

{-# OPTIONS_GHC -dsuppress-all -O #-}
{-# LANGUAGE
    BangPatterns,
    DeriveFunctor,
    DeriveFoldable,
    DeriveTraversable,
    DeriveGeneric,
    DerivingVia,
    EmptyCase,
    EmptyDataDeriving,
    TemplateHaskell
    #-}

{-# LANGUAGE TypeOperators, TypeFamilies #-}

import GHC.Generics
import Test.Inspection

-- A big sum of stuff
data Big a
  = Big0
  | Big1 a
  | Big2 a a
  | Big4 a a a a
  | Big5 a a a a a
  deriving Generic

{-
-- Handwritten to add INLINE pragmas
instance Generic (Big a) where
  type Rep (Big a) =
        U1
    :+: K1 () a
    :+: (K1 () a :*: K1 () a)
    :+: (K1 () a :*: K1 () a :*: K1 () a :*: K1 () a)
    :+: (   K1 () a :*: K1 () a :*: K1 () a :*: K1 () a :*: K1 () a)
  from Big0 = L1 U1
  from (Big1 x) = R1 (L1 (K1 x))
  from (Big2 x1 x2) = R1 (R1 (L1 (K1 x1 :*: K1 x2)))
  from (Big4 x1 x2 x3 x4) = R1 (R1 (R1 (L1 (K1 x1 :*: K1 x2 :*: K1 x3 :*: K1 x4))))
  from (Big5 x1 x2 x3 x4 x5) = R1 (R1 (R1 (R1 (K1 x1 :*: K1 x2 :*: K1 x3 :*: K1 x4 :*: K1 x5))))
  {-# INLINE from #-}

  to (L1 _) = Big0
  to (R1 (L1 (K1 x))) = Big1 x
  to (R1 (R1 (L1 (K1 x1 :*: K1 x2)))) = Big2 x1 x2
  to (R1 (R1 (R1 (L1 (K1 x1 :*: K1 x2 :*: K1 x3 :*: K1 x4))))) = Big4 x1 x2 x3 x4
  to (R1 (R1 (R1 (R1 (K1 x1 :*: K1 x2 :*: K1 x3 :*: K1 x4 :*: K1 x5))))) = Big5 x1 x2 x3 x4 x5
  {-# INLINE to #-}
-}

testi :: Big a -> Big a
testi = to . from

testj :: Big a -> Big a
testj !x = x

inspect $ 'testi === 'testj

main :: IO ()
main = pure ()