Add Data instance for Const

Summary:
Fixes #12438. As discussed on the Haskell libraries mailing list here:
https://mail.haskell.org/pipermail/libraries/2016-November/027396.html

Reviewers: hvr, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2726

GHC Trac Issues: #12438

2 files changed, 6 insertions, 0 deletions

diff --git a/libraries/base/Data/Data.hs b/libraries/base/Data/Data.hs
index df37ab1a1a..e43ec74e3f 100644
--- a/libraries/base/Data/Data.hs
+++ b/libraries/base/Data/Data.hs
@@ -9,6 +9,7 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE Trustworthy #-}
+{-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 
 -----------------------------------------------------------------------------
@@ -1197,6 +1198,9 @@ deriving instance (Coercible a b, Data a, Data b) => Data (Coercion a b)
 -- | @since 4.9.0.0
 deriving instance Data a => Data (Identity a)
 
+-- | @since 4.10.0.0
+deriving instance (Typeable k, Data a, Typeable (b :: k)) => Data (Const a b)
+
 -- | @since 4.7.0.0
 deriving instance Data Version
 
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index a01c878a53..dcc171979f 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -26,6 +26,8 @@
   * Add `Semigroup` instance for `IO`, as well as for `Event` and `Lifetime`
     from `GHC.Event` (#12464)
 
+  * Add `Data` instance for `Const` (#12438)
+
 ## 4.9.0.0  *May 2016*
 
   * Bundled with GHC 8.0