Bug in Coerciible

Richard and I uncovered this

{-# LANGUAGE RoleAnnotations #-}
module Bar( N ) where   -- Data constructor not exported

type role N representational
newtype N a = MkN Int

and

{-# LANGUAGE FlexibleContexts #-}
module Foo where

import Data.Coerce
import Data.Type.Coercion
import Bar

f :: Coercible (N a) (N b) => Coercion a b
f = Coercion

This compiles but it should not! It wrongly compiles by using nth (N a ~R N b); but (as Fig 4 of the Corcible paper says), we can't decompose a newtype at representational role.

Easy to fix.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler
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