From f453452940a81d106f5e6fbf29992b994fb33bd0 Mon Sep 17 00:00:00 2001
From: Vladislav Zavialov <vlad.z.4096@gmail.com>
Date: Mon, 8 Nov 2021 19:17:35 +0300
Subject: [PATCH] Role annotation for TypeRepMap (#113)

Before this patch, GHC inferred the phantom role and allowed
the user to implement unsafeCoerce via TypeRepMap:

  unsafeCoerceTRM :: a -> b
  unsafeCoerceTRM a = b
    where
      Just (Const b) = TRM.lookup @() (coerce (TRM.one @() (Const a)))
---
 src/Data/TypeRepMap/Internal.hs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/Data/TypeRepMap/Internal.hs b/src/Data/TypeRepMap/Internal.hs
index 1eb82d7..bd7e3a9 100644
--- a/src/Data/TypeRepMap/Internal.hs
+++ b/src/Data/TypeRepMap/Internal.hs
@@ -9,6 +9,7 @@
 {-# LANGUAGE MagicHash             #-}
 {-# LANGUAGE PolyKinds             #-}
 {-# LANGUAGE Rank2Types            #-}
+{-# LANGUAGE RoleAnnotations       #-}
 {-# LANGUAGE TypeFamilies          #-}
 {-# LANGUAGE TypeInType            #-}
 {-# LANGUAGE ViewPatterns          #-}
@@ -92,6 +93,7 @@ The runtime representation of 'TypeRepMap' is an array, not a tree. This makes
 'lookup' significantly more efficient.
 
 -}
+type role TypeRepMap representational
 data TypeRepMap (f :: k -> Type) =
   TypeRepMap
     { fingerprintAs :: {-# UNPACK #-} !(PrimArray Word64) -- ^ first components of key fingerprints