When I say

{-# LANGUAGE RoleAnnotations #-}

module Bug (N, twiddle) where

import Data.Coerce

newtype N a = MkN Int
type role N representational

twiddle :: N a -> N b
twiddle = coerce

I get

Bug.hs:7:15: warning: Defined but not used: data constructor ‘MkN’

But the constructor is used, in order to do the coercion. The role annotation is to force GHC to use the constructor to do the coercion -- otherwise, it might not have.

This bug exists in 7.10 and in HEAD.

I will fix.