TH silently drops quantification in instance heads

Summary

Here's a program I was writing, the exact program doesn't matter except that it uses a forall in the instance head to bind a variable (and give it an explicit kind, which is what I care about).

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE StandaloneKindSignatures #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}

{-# OPTIONS_GHC -ddump-splices -ddump-to-file -dsuppress-uniques -dsuppress-coercions -dsuppress-type-applications -dsuppress-unfoldings -dsuppress-idinfo -dppr-cols=200 -dumpdir /tmp/dumps #-}
module Repro where

import Data.Kind
$([d|
        data P = L | R
        data T (a :: P) where
                A :: T a
                B :: T R
        type TConstraint = forall a . T a -> Constraint
        type ForAllA :: TConstraint -> Constraint
        class (forall a . constr @a A) => ForAllA constr
        instance forall (constr :: TConstraint) . (forall a . constr @a A) => ForAllA constr

   |])

The splice output is this:

Repro.hs:(17,2)-(27,6): Splicing declarations
    [d| type ForAllA :: TConstraint -> Constraint
        
        data P = L | R
        data T (a :: P)
          where
            A :: T a
            B :: T R
        type TConstraint = forall a. T a -> Constraint
        class (forall a. constr @a A) => ForAllA constr
        
        instance forall (constr :: TConstraint). (forall a. constr @a A) => ForAllA constr |]
  ======>
    data P = L | R
    data T (a :: P)
      where
        A :: T a
        B :: T 'R
    type TConstraint = forall a. T a -> Constraint
    type ForAllA :: TConstraint -> Constraint
    class (forall a. constr @a 'A) => ForAllA constr
    instance (forall a. constr @a 'A) => ForAllA constr

Note the absence of the forall. And indeed, the program fails to compile in exactly the way it would if the forall was missing (which happens to be #21793).

Steps to reproduce

The above file should compile standalone and show the problem.

Expected behavior

Actually include all the syntax!

Environment

• GHC version used: 9.2.3