Missing Proxy instances, make U1 instance more Proxy-like

This accomplishes three things:

* Adds missing `Alternative`, `MonadPlus`, and `MonadZip` instances for
  `Proxy`
* Adds a missing `MonadPlus` instance for `U1`
* Changes several existing `U1` instances to use lazy pattern-matching,
  exactly how `Proxy` does it (in case we ever replace `U1` with
  `Proxy`). This is technically a breaking change (albeit an extremely
  minor one).

Test Plan: ./validate

Reviewers: austin, ekmett, hvr, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1962

GHC Trac Issues: #11650

1 files changed, 17 insertions, 1 deletions

diff --git a/libraries/base/Data/Foldable.hs b/libraries/base/Data/Foldable.hs
index 5d758ae691..0defe6c07c 100644
--- a/libraries/base/Data/Foldable.hs
+++ b/libraries/base/Data/Foldable.hs
@@ -425,8 +425,24 @@ instance Ord a => Monoid (Min a) where
     | otherwise = Min n
 
 -- Instances for GHC.Generics
+instance Foldable U1 where
+    foldMap _ _ = mempty
+    {-# INLINE foldMap #-}
+    fold _ = mempty
+    {-# INLINE fold #-}
+    foldr _ z _ = z
+    {-# INLINE foldr #-}
+    foldl _ z _ = z
+    {-# INLINE foldl #-}
+    foldl1 _ _ = errorWithoutStackTrace "foldl1: U1"
+    foldr1 _ _ = errorWithoutStackTrace "foldr1: U1"
+    length _   = 0
+    null _     = True
+    elem _ _   = False
+    sum _      = 0
+    product _  = 1
+
 deriving instance Foldable V1
-deriving instance Foldable U1
 deriving instance Foldable Par1
 deriving instance Foldable f => Foldable (Rec1 f)
 deriving instance Foldable (K1 i c)