Capturing type variables in TemplateHaskell

Problem.hs

import Language.Haskell.TH.Lib
import Language.Haskell.TH.Syntax

module Problem where

data PP a where
  PP :: forall a. Typeable a => PP a

instance Lift (PP a) where
  lift PP = [|PP|]
  -- lift PP = unTypeQ ([||PP||] :: Q (TExp (PP a)))

Problem2.hs

import Language.Haskell.TH.Lib
import Language.Haskell.TH.Syntax
import Problem

module Problem2 where

exp = $(lift (PP @Int))

--    • No instance for (Typeable a0) arising from a use of ‘PP’
--    • In the expression: (PP)
--      In an equation for ‘exp’: exp = (PP)