-- 警告無し
data TestA = TestA
  { aA :: {-# UNPACK #-} !Int,
    bA :: {-# UNPACK #-} !Int
  }

{- 以下の警告が表示:
Main.hs:6:16: warning:
    • Ignoring unusable UNPACK pragma on the second argument of ‘TestB’
    • In the definition of data constructor ‘TestB’
      In the data type declaration for ‘TestB’
  |
6 | data TestB a = TestB
  |                ^^^^^...

Main.hs:6:16: warning:
    • Ignoring unusable UNPACK pragma on the first argument of ‘TestB’
    • In the definition of data constructor ‘TestB’
      In the data type declaration for ‘TestB’
  |
6 | data TestB a = TestB
-}
data TestB a = TestB
  { aB :: {-# UNPACK #-} !a,
    bB :: {-# UNPACK #-} !a
  }

main :: IO ()
main = putStrLn "hello world"