[Haskell-cafe] Surprising behavior with wildcard pattern and unlifted type

[Tom Ellis]

Is there supposed to be this surprising difference between ex1 and ex2?

    {-# LANGUAGE UnliftedDatatypes #-}
    
    {-# OPTIONS_GHC -Wall #-}
    
    module Main where
    
    import GHC.Exts (UnliftedType)
    
    type T :: UnliftedType
    data T = T
    
    ex1 :: ()
    ex1 = let _ = undefined :: T in ()
    
    ex2 :: ()
    ex2 = let _a = undefined :: T in ()
    

    ghci> ex1
    ()
    ghci> ex2
    *** Exception: Prelude.undefined
    CallStack (from HasCallStack):
      undefined, called at test17.hs:53:16 in fake-package-0-inplace:Main