Export liftA2 from Prelude

Changes:
In order to be warning free and compatible, we hide Applicative(..)
from Prelude in a few places and instead import it directly from
Control.Applicative.
Please see the migration guide at
https://github.com/haskell/core-libraries-committee/blob/main/guides/export-lifta2-prelude.md
for more details.

This means that Applicative is now exported in its entirety from
Prelude.

Motivation:

This change is motivated by a few things:
* liftA2 is an often used function, even more so than (<*>) for some
  people.
* When implementing Applicative, the compiler will prompt you for either
  an implementation of (<*>) or of liftA2, but trying to use the latter
  ends with an error, without further imports. This could be confusing
  for newbies.
* For teaching, it is often times easier to introduce liftA2 first,
  as it is a natural generalisation of fmap.
* This change seems to have been unanimously and enthusiastically
  accepted by the CLC members, possibly indicating a lot of love for it.
* This change causes very limited breakage, see the linked issue below
  for an investigation on this.

See https://github.com/haskell/core-libraries-committee/issues/50
for the surrounding discussion and more details.

4 files changed, 7 insertions, 7 deletions

diff --git a/compiler/GHC/Hs/Doc.hs b/compiler/GHC/Hs/Doc.hs
index 209f9608eb..39d477be52 100644
--- a/compiler/GHC/Hs/Doc.hs
+++ b/compiler/GHC/Hs/Doc.hs
@@ -26,7 +26,7 @@ module GHC.Hs.Doc
   , emptyDocs
   ) where
 
-import GHC.Prelude
+import GHC.Prelude hiding (Applicative(..))
 
 import GHC.Utils.Binary
 import GHC.Types.Name
@@ -38,7 +38,7 @@ import GHC.Types.Avail
 import GHC.Types.Name.Set
 import GHC.Driver.Flags
 
-import Control.Applicative (liftA2)
+import Control.Applicative (Applicative(..))
 import Data.Data
 import Data.IntMap (IntMap)
 import qualified Data.IntMap as IntMap
diff --git a/compiler/GHC/Types/SrcLoc.hs b/compiler/GHC/Types/SrcLoc.hs
index 306fde89fa..3f68db0c30 100644
--- a/compiler/GHC/Types/SrcLoc.hs
+++ b/compiler/GHC/Types/SrcLoc.hs
@@ -115,7 +115,7 @@ module GHC.Types.SrcLoc (
 
     ) where
 
-import GHC.Prelude
+import GHC.Prelude hiding (Applicative(..))
 
 import GHC.Utils.Misc
 import GHC.Utils.Json
@@ -125,7 +125,7 @@ import GHC.Data.FastString
 import qualified GHC.Data.Strict as Strict
 
 import Control.DeepSeq
-import Control.Applicative (liftA2)
+import Control.Applicative (Applicative(..))
 import Data.Data
 import Data.List (sortBy, intercalate)
 import Data.Function (on)
diff --git a/compiler/GHC/Utils/Misc.hs b/compiler/GHC/Utils/Misc.hs
index 3824f4ad5a..0ddf057ffc 100644
--- a/compiler/GHC/Utils/Misc.hs
+++ b/compiler/GHC/Utils/Misc.hs
@@ -127,7 +127,7 @@ module GHC.Utils.Misc (
         HasDebugCallStack,
     ) where
 
-import GHC.Prelude
+import GHC.Prelude hiding (Applicative(..))
 
 import GHC.Utils.Exception
 import GHC.Utils.Panic.Plain
@@ -141,7 +141,7 @@ import Data.List.NonEmpty  ( NonEmpty(..) )
 import GHC.Exts
 import GHC.Stack (HasCallStack)
 
-import Control.Applicative ( liftA2 )
+import Control.Applicative (Applicative(..))
 import Control.Monad    ( liftM, guard )
 import Control.Monad.IO.Class ( MonadIO, liftIO )
 import System.IO.Error as IO ( isDoesNotExistError )
diff --git a/compiler/GHC/Utils/Monad.hs b/compiler/GHC/Utils/Monad.hs
index 3bf8737990..f9fc02b338 100644
--- a/compiler/GHC/Utils/Monad.hs
+++ b/compiler/GHC/Utils/Monad.hs
@@ -27,7 +27,7 @@ module GHC.Utils.Monad
 -- Imports
 -------------------------------------------------------------------------------
 
-import GHC.Prelude
+import GHC.Prelude hiding (Applicative(..))
 
 import Control.Applicative
 import Control.Monad