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
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