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This works:

{-# LANGUAGE TypeFamilies #-}
module M where

import GHC.Exts
import Type.Reflection

type family Id x
type instance Id x = x

cast :: forall a. TypeRep a -> (Typeable a => Int) -> Int
cast = withDict @(TypeRep a) @(Typeable a)

If I replace the type of cast with

cast :: forall a. Id (TypeRep a) -> (Typeable a => Int) -> Int

I get an error:

    Invalid instantiation of ‘withDict’ at type:
        Id (TypeRep a) -> (Typeable a -> Int) -> Int

Two problems:

(1) I understand withDict might not work with casts, but this code is explicitly passing type arguments to withDict. See also #19915 / #20000.

(2) Typeable a -> in the error message should be Typeable a =>.