Panic involving unboxed tuples and phantom types

I ran into a bug involving unboxed tuples and the following piece of code is an attempt at isolating the problem:

{-# LANGUAGE UnboxedTuples #-}
module Main where

type Ty a = Int

bug :: Ty a -> (# Ty a, () #)
bug ty = (# ty, () #)

main = do
  let (# a, b #) = bug undefined
  return ()

It seems that the phantom type is necessary to trigger the bug.

The bug is still present in 7.8rc1.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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Panic involving unboxed tuples and phantom types