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While experimenting with unsafeCoerce# recently, I tried the following:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE TypeApplications #-}
module Bug where

import GHC.Exts
import Unsafe.Coerce

f :: Int -> Int
f x = unsafeCoerce# @LiftedRep @LiftedRep @Int @Int x

Surprisingly, GHC 9.0 and later reject this:

$ /opt/ghc/9.0.1/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:10:7: error:
    Cannot use function with levity-polymorphic arguments:
      unsafeCoerce# :: forall (q :: RuntimeRep) (r :: RuntimeRep)
                              (a :: TYPE q) (b :: TYPE r).
                       a -> b
    (Note that levity-polymorphic primops such as 'coerce' and unboxed tuples
    are eta-expanded internally because they must occur fully saturated.
    Use -fprint-typechecker-elaboration to display the full expression.)
    Levity-polymorphic arguments: a :: TYPE q
   |
10 | f x = unsafeCoerce# @LiftedRep @LiftedRep @Int @Int x
   |       ^^^^^^^^^^^^^

The error message confuses me, since nothing about this is levity polymorphic.

Note that this only occurs if visible type applications are used. If I remove them:

f :: Int -> Int
f x = unsafeCoerce# x

Then GHC accepts it.