Misleading error message when unboxed tuples used in sums without -XUnboxedTuples

$ cat lib.hs
{-# LANGUAGE UnboxedSums #-}

module Lib where

type Foo = (# (# Int, Int #) | Int #)

$ ghc lib.hs
[1 of 1] Compiling Lib              ( lib.hs, lib.o )

lib.hs:5:1: error:
    • Illegal unboxed tuple type as function argument: (# Int, Int #)
    • In the type synonym declaration for ‘Foo’
  |
5 | type Foo = (# (# Int, Int #) | Int #)
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

But there's nothing illegal about this type, we just need to enable UnboxedTuples. The error message is misleading.

(We should also check using unboxed sums inside tuples)