Add Semigroup instances for base>=4.9

This makes unordered-containers `-Wcompat`-clean on GHC HEAD

See also ekmett/semigroups#56

Author: hvr

Data/HashMap/Base.hs

@@ -95,6 +95,9 @@ import Data.Monoid (Monoid(mempty, mappend))
 import Data.Traversable (Traversable(..))
 import Data.Word (Word)
 #endif
+#if __GLASGOW_HASKELL__ >= 711
+import Data.Semigroup (Semigroup((<>)))
+#endif
 import Control.DeepSeq (NFData(rnf))
 import Control.Monad.ST (ST)
 import Data.Bits ((.&.), (.|.), complement)
@@ -163,10 +166,20 @@ instance Functor (HashMap k) where
 instance Foldable.Foldable (HashMap k) where
     foldr f = foldrWithKey (const f)
 
+#if __GLASGOW_HASKELL__ >= 711
+instance (Eq k, Hashable k) => Semigroup (HashMap k v) where
+  (<>) = union
+  {-# INLINE (<>) #-}
+#endif
+
 instance (Eq k, Hashable k) => Monoid (HashMap k v) where
   mempty = empty
   {-# INLINE mempty #-}
+#if __GLASGOW_HASKELL__ >= 711
+  mappend = (<>)
+#else
   mappend = union
+#endif
   {-# INLINE mappend #-}
 
 instance (Data k, Data v, Eq k, Hashable k) => Data (HashMap k v) where

Data/HashSet.hs

@@ -76,7 +76,9 @@ import Control.DeepSeq (NFData(..))
 import Data.Data hiding (Typeable)
 import Data.HashMap.Base (HashMap, foldrWithKey)
 import Data.Hashable (Hashable(hashWithSalt))
-#if __GLASGOW_HASKELL__ < 709
+#if __GLASGOW_HASKELL__ >= 711
+import Data.Semigroup (Semigroup(..), Monoid(..))
+#elif __GLASGOW_HASKELL__ < 709
 import Data.Monoid (Monoid(..))
 #endif
 import GHC.Exts (build)
@@ -114,10 +116,20 @@ instance Foldable.Foldable HashSet where
     foldr = Data.HashSet.foldr
     {-# INLINE foldr #-}
 
+#if __GLASGOW_HASKELL__ >= 711
+instance (Hashable a, Eq a) => Semigroup (HashSet a) where
+    (<>) = union
+    {-# INLINE (<>) #-}
+#endif
+
 instance (Hashable a, Eq a) => Monoid (HashSet a) where
     mempty = empty
     {-# INLINE mempty #-}
+#if __GLASGOW_HASKELL__ >= 711
+    mappend = (<>)
+#else
     mappend = union
+#endif
     {-# INLINE mappend #-}
 
 instance (Eq a, Hashable a, Read a) => Read (HashSet a) where