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

[David Feuer]

Ouch ... that looks ... not great. I don't know if that was intentional,
but it's weird enough to deserve a bug report.

On Mon, Jan 2, 2023, 2:01 PM Tom Ellis <
tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:

> 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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