HsPredTy p -> extract_pred p acc
HsOpTy ty1 (L loc tv) ty2 -> extract_tv loc tv (extract_lty ty1 (extract_lty ty2 acc))
HsParTy ty -> extract_lty ty acc
- HsNumTy _ -> acc
+ HsNumTy {} -> acc
+ HsCoreTy {} -> acc -- The type is closed
HsQuasiQuoteTy {} -> acc -- Quasi quotes mention no type variables
HsSpliceTy {} -> acc -- Type splices mention no type variables
HsKindSig ty _ -> extract_lty ty acc