Implement the -XUnliftedNewtypes extension.

GHC Proposal: 0013-unlifted-newtypes.rst
Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/98
Issues: #15219, #1311, #13595, #15883
Implementation Details:
  Note [Implementation of UnliftedNewtypes]
  Note [Unifying data family kinds]
  Note [Compulsory newtype unfolding]

This patch introduces the -XUnliftedNewtypes extension. When this
extension is enabled, GHC drops the restriction that the field in
a newtype must be of kind (TYPE 'LiftedRep). This allows types
like Int# and ByteArray# to be used in a newtype. Additionally,
coerce is made levity-polymorphic so that it can be used with
newtypes over unlifted types.

The bulk of the changes are in TcTyClsDecls.hs. With -XUnliftedNewtypes,
getInitialKind is more liberal, introducing a unification variable to
return the kind (TYPE r0) rather than just returning (TYPE 'LiftedRep).
When kind-checking a data constructor with kcConDecl, we attempt to
unify the kind of a newtype with the kind of its field's type. When
typechecking a data declaration with tcTyClDecl, we again perform a
unification. See the implementation note for more on this.

Co-authored-by: Richard Eisenberg <rae@richarde.dev>

5 files changed, 13 insertions, 4 deletions

diff --git a/libraries/base/Control/Category.hs b/libraries/base/Control/Category.hs
index e8184956f2..96f0c33aed 100644
--- a/libraries/base/Control/Category.hs
+++ b/libraries/base/Control/Category.hs
@@ -23,7 +23,7 @@ module Control.Category where
 import qualified GHC.Base (id,(.))
 import Data.Type.Coercion
 import Data.Type.Equality
-import GHC.Prim (coerce)
+import Data.Coerce (coerce)
 
 infixr 9 .
 infixr 1 >>>, <<<
diff --git a/libraries/base/Data/Coerce.hs b/libraries/base/Data/Coerce.hs
index 9bd7f9a41a..3785b8a104 100644
--- a/libraries/base/Data/Coerce.hs
+++ b/libraries/base/Data/Coerce.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE Unsafe #-}
 {-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE MagicHash #-}
 
 -----------------------------------------------------------------------------
 -- |
@@ -21,9 +22,11 @@
 
 module Data.Coerce
         ( -- * Safe coercions
-          coerce, Coercible,
+          coerce, Coercible
         ) where
 import GHC.Prim (coerce)
 import GHC.Types (Coercible)
 
-import GHC.Base () -- for build ordering; see Notes in GHC.Base for more info
+-- The import of GHC.Base is for build ordering; see Notes in GHC.Base for
+-- more info.
+import GHC.Base ()
diff --git a/libraries/base/Data/Type/Coercion.hs b/libraries/base/Data/Type/Coercion.hs
index b757682a62..694bedec01 100644
--- a/libraries/base/Data/Type/Coercion.hs
+++ b/libraries/base/Data/Type/Coercion.hs
@@ -8,6 +8,7 @@
 {-# LANGUAGE NoImplicitPrelude   #-}
 {-# LANGUAGE PolyKinds           #-}
 {-# LANGUAGE RankNTypes          #-}
+{-# LANGUAGE MagicHash           #-}
 
 -----------------------------------------------------------------------------
 -- |
diff --git a/libraries/base/GHC/Base.hs b/libraries/base/GHC/Base.hs
index 6fd2ff735b..2649146816 100644
--- a/libraries/base/GHC/Base.hs
+++ b/libraries/base/GHC/Base.hs
@@ -1156,7 +1156,6 @@ The rules for map work like this.
 
 {-# RULES "map/coerce" [1] map coerce = coerce #-}
 
-
 ----------------------------------------------
 --              append
 ----------------------------------------------
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index a9f6fcf272..3c12e7c511 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -9,6 +9,12 @@
     to detect values `<= maxBound::Word` that were incorrectly encoded using
     the `NatJ#` constructor.
 
+  * The type of `coerce` has been generalized. It is now runtime-representation
+    polymorphic:
+    `forall {r :: RuntimeRep} (a :: TYPE r) (b :: TYPE r). Coercible a b => a -> b`.
+    The type argument `r` is marked as `Inferred` to prevent it from
+    interfering with visible type application.
+
 ## 4.13.0.0 *TBA*
   * Bundled with GHC *TBA*