Unary Unboxed Tuple Type Constructor

The type constructor for a unary unboxed tuple cannot be written in GHC haskell. Consider the following value which is wrapped in by a unary unboxed tuple:

>>> :type (# 5# #)
(# 5# #) :: (# Int# #)

Now consider the type constructor for unboxed 2-tuples:

>>> :set -fprint-explicit-kinds
>>> :set -fprint-explicit-foralls
>>> :kind! (# , #)
(# , #) :: forall (k0 :: RuntimeRep) (k1 :: RuntimeRep).
           TYPE k0
           -> TYPE k1
           -> TYPE
                ('TupleRep
                   ((':) RuntimeRep k0 ((':) RuntimeRep k1 ('[] RuntimeRep))))
= (#,#)

Horrid looking, but undoubtedly correct. But how can we do this for a unary unboxed tuple? The naive approach gives us the nullary tuple instead:

>>> :kind! (# #)
(# #) :: TYPE ('TupleRep ('[] RuntimeRep))
= (# #)

I do actually have a real use-case for this that I can discuss more if needed. For the syntax, I really haven't the faintest idea what I would expect. Maybe something like: (## ##) or (# @1 #), but those both seem pretty bad.

Trac metadata

Trac field	Value
Version	8.4.1-alpha1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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