`TypeRepMap` should have role `representational`

[re-xyr]

Currently, TypeRepMap has type role phantom which allows us to conjure Void out of thin air:

k :: TypeRepMap Identity
k = TMap.one (Identity True)

k' :: Void
k' = getConst $ fromJust $ TMap.lookup @Bool $ coerce k

which is not something very desirable.

[int-index]

It’s not just Void, this is unsound

{-# LANGUAGE TypeApplications #-}

module UnsafeTRM where

import Data.Functor.Const
import Data.Coerce
import Data.TypeRepMap as TRM

unsafeCoerceTRM :: a -> b
unsafeCoerceTRM a = b
  where
    Just (Const b) = TRM.lookup @() (coerce (TRM.one @() (Const a)))

ghci> unsafeCoerceTRM "Hello" :: Int
-2305843008106343517