`dMatchUpSAKWithDecl` erroneously rejects invisible `forall` without corresponding invisible type variable binder

[Ryan Scott]

This is a perfectly valid data type:

type D :: forall (a :: Type). Type
data D

Although there is no explicit @a binder in the declaration for D, this is perfectly fine, as GHC is capable of inferring it. th-desugar's dMatchUpSAKWithDecl function, on the other hand, rejects this. I originally encountered this issue in the context of singletons-th, but here is a way to trigger the issue using only the th-desugar API:

{-# LANGUAGE TemplateHaskell #-}
module Bug where

import Data.Kind (Type)
import Language.Haskell.TH hiding (Type)
import Language.Haskell.TH.Desugar

$(do let sak = DForallT (DForallInvis [DKindedTV (mkName "a") SpecifiedSpec (DConT ''Type)]) (DConT ''Type)
     bndrs <- dMatchUpSAKWithDecl sak []
     runIO $ print $ length bndrs
     pure [])

I believe this should work.