Re-export GHC.Types from GHC.Exts

  Several times in the past, it has happened that things from GHC.Types
  were not re-exported from GHC.Exts, forcing users to import either
  GHC.Types or GHC.Prim, which are subject to internal change without
  notice.

  We now re-export GHC.Types from GHC.Exts, which should avoid this
  happening again in the future.
  In particular, we now re-export `Multiplicity` and `MultMul`,
  which we didn't before.

  Fixes #20695

4 files changed, 63 insertions, 58 deletions

diff --git a/libraries/base/GHC/Exts.hs b/libraries/base/GHC/Exts.hs
index b9d632095c..c03b397601 100755
--- a/libraries/base/GHC/Exts.hs
+++ b/libraries/base/GHC/Exts.hs
@@ -24,23 +24,28 @@
 
 module GHC.Exts
        (
-        -- * Representations of some basic types
-        Int(..),Word(..),Float(..),Double(..),
-        Char(..),
+        -- * Primitive types
+
+        FUN, TYPE, -- See https://gitlab.haskell.org/ghc/ghc/issues/18302
+
+        -- ** Pointer types
         Ptr(..), FunPtr(..),
 
-        -- * The maximum tuple size
-        maxTupleSize,
+        -- ** Other primitive types
+        module GHC.Types,
 
         -- * Primitive operations
-        FUN, -- See https://gitlab.haskell.org/ghc/ghc/issues/18302
+
         module GHC.Prim,
         module GHC.Prim.Ext,
+
+        -- ** Running 'RealWorld' state thread
+        runRW#,
+
+        -- ** Bit shift operations
         shiftL#, shiftRL#, iShiftL#, iShiftRA#, iShiftRL#,
-        isTrue#,
-        Void#, -- Previously exported by GHC.Prim
 
-        -- * Pointer comparison operations
+        -- ** Pointer comparison operations
         -- See `Note [Pointer comparison operations]` in primops.txt.pp
         reallyUnsafePtrEquality,
         eqStableName#,
@@ -57,10 +62,10 @@ module GHC.Exts
         sameTVar#,
         sameIOPort#,
 
-        -- * Compat wrapper
+        -- ** Compat wrapper
         atomicModifyMutVar#,
 
-        -- * Resize functions
+        -- ** Resize functions
         --
         -- | Resizing arrays of boxed elements is currently handled in
         -- library space (rather than being a primop) since there is not
@@ -68,13 +73,20 @@ module GHC.Exts
         -- may become primops in a future release of GHC.
         resizeSmallMutableArray#,
 
-        -- * Fusion
+        -- ** Fusion
         build, augment,
 
-        -- * Overloaded string literals
+        -- * Overloaded lists
+        IsList(..),
+
+        -- * Transform comprehensions
+        Down(..), groupWith, sortWith, the,
+
+        -- * Strings
+        -- ** Overloaded string literals
         IsString(..),
 
-        -- * CString
+        -- ** CString
         unpackCString#,
         unpackAppendCString#,
         unpackFoldrCString#,
@@ -83,59 +95,46 @@ module GHC.Exts
         cstringLength#,
 
         -- * Debugging
+        -- ** Breakpoints
         breakpoint, breakpointCond,
 
-        -- * Ids with special behaviour
-        inline, noinline, lazy, oneShot, considerAccessible, SPEC (..),
+        -- ** Event logging
+        traceEvent,
 
-        -- * Running 'RealWorld' state thread
-        runRW#,
+        -- ** The call stack
+        currentCallStack,
 
-        -- * Casting class dictionaries with single methods
-        withDict,
+        -- * Ids with special behaviour
+        inline, noinline, lazy, oneShot, considerAccessible,
 
-        -- * Safe coercions
+        -- * SpecConstr annotations
+        SpecConstrAnnotation(..), SPEC (..),
+
+        -- * Coercions
+        -- ** Safe coercions
         --
-        -- | These are available from the /Trustworthy/ module "Data.Coerce" as well
+        -- | These are available from the /Trustworthy/ module "Data.Coerce" as well.
         --
         -- @since 4.7.0.0
-        Data.Coerce.coerce, Data.Coerce.Coercible,
+        Data.Coerce.coerce,
 
-        -- * Very unsafe coercion
+        -- ** Very unsafe coercion
         unsafeCoerce#,
 
-        -- * Equality
-        type (~~),
-
-        -- * Representation polymorphism
-        GHC.Prim.TYPE, RuntimeRep(..), Levity(..),
-        LiftedRep, UnliftedRep, UnliftedType,
-        VecCount(..), VecElem(..),
-
-        -- * Transform comprehensions
-        Down(..), groupWith, sortWith, the,
-
-        -- * Event logging
-        traceEvent,
-
-        -- * SpecConstr annotations
-        SpecConstrAnnotation(..),
-
-        -- * The call stack
-        currentCallStack,
-
-        -- * The Constraint kind
-        Constraint,
-
-        -- * The Any type
-        Any,
+        -- ** Casting class dictionaries with single methods
+        withDict,
 
-        -- * Overloaded lists
-        IsList(..)
+        -- * The maximum tuple size
+        maxTupleSize,
        ) where
 
-import GHC.Prim hiding ( coerce, TYPE )
-import qualified GHC.Prim
+import GHC.Prim hiding ( coerce )
+import GHC.Types
+  hiding ( IO   -- Exported from "GHC.IO"
+         , Type -- Exported from "Data.Kind"
+
+           -- GHC's internal representation of 'TyCon's, for 'Typeable'
+         , Module, TrName, TyCon, TypeLitSort, KindRep, KindBndr )
 import qualified GHC.Prim.Ext
 import GHC.Base hiding ( coerce )
 import GHC.Ptr
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 7ddc112af4..52f6a56d0c 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -27,6 +27,8 @@
 
     These new operators have the same fixity as the originals.
 
+  * `GHC.Exts` now re-exports `Multiplicity` and `MultMul`.
+
 ## 4.16.0.0 *Nov 2021*
 
   * The unary tuple type, `Solo`, is now exported by `Data.Tuple`.
diff --git a/libraries/ghc-prim/GHC/Types.hs b/libraries/ghc-prim/GHC/Types.hs
index d40e7a8f38..22e637af6e 100644
--- a/libraries/ghc-prim/GHC/Types.hs
+++ b/libraries/ghc-prim/GHC/Types.hs
@@ -27,6 +27,7 @@ module GHC.Types (
         --    Lists:          []( [], (:) )
         --    Type equality:  (~)( Eq# )
 
+        -- * Built-in types
         Bool(..), Char(..), Int(..), Word(..),
         Float(..), Double(..),
         Ordering(..), IO(..),
@@ -34,7 +35,11 @@ module GHC.Types (
         SPEC(..),
         Symbol,
         Any,
+
+        -- * Type equality
         type (~~), Coercible,
+
+        -- * Representation polymorphism
         TYPE, Levity(..), RuntimeRep(..),
         LiftedRep, UnliftedRep,
         Type, UnliftedType, Constraint,
@@ -45,12 +50,12 @@ module GHC.Types (
         VecCount(..), VecElem(..),
         Void#,
 
+        -- * Multiplicity types
+        Multiplicity(..), MultMul,
+
         -- * Runtime type representation
         Module(..), TrName(..), TyCon(..), TypeLitSort(..),
         KindRep(..), KindBndr,
-
-        -- * Multiplicity Types
-        Multiplicity(..), MultMul
     ) where
 
 import GHC.Prim
diff --git a/testsuite/tests/typecheck/should_run/KindInvariant.stderr b/testsuite/tests/typecheck/should_run/KindInvariant.stderr
index fb4dd69779..ac31936e39 100644
--- a/testsuite/tests/typecheck/should_run/KindInvariant.stderr
+++ b/testsuite/tests/typecheck/should_run/KindInvariant.stderr
@@ -1,7 +1,6 @@
 
 <interactive>:1:3: error:
     • Couldn't match a lifted type with an unlifted type
-      Expected kind ‘* -> *’,
-        but ‘State#’ has kind ‘* -> GHC.Types.ZeroBitType’
+      Expected kind ‘* -> *’, but ‘State#’ has kind ‘* -> ZeroBitType’
     • In the first argument of ‘T’, namely ‘State#’
       In the type ‘T State#’