diff --git a/CHANGES b/CHANGES
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,20 @@
+* 0.5: 14 May 2018
+
+- Modernize Data.Monoid.WithSemigroup
+
+  It used to export a type class Monoid' with no methods and a single
+  instance, for use as a "poor man's constraint synonym" for the
+  combination of Monoid and Semigroup.  Now Monoid':
+
+    - Is a real constraint synonym, using ConstraintKinds.
+    - Is simply a synonym for Monoid under base-4.11 and later, in
+      which case Semigroup is already a superclass of Monoid.
+
+  This technically necessitates a major version bump but should not
+  cause any issues for packages that depend on monoid-extras, other
+  than potentially requiring the addition of a ConstraintKinds pragma
+  under GHC 7.8.
+
 * 0.4.4: 8 April 2018
 
 - Fix build on older (< 7.10) GHCs (thanks to George Wilson for the fix)
diff --git a/monoid-extras.cabal b/monoid-extras.cabal
--- a/monoid-extras.cabal
+++ b/monoid-extras.cabal
@@ -1,5 +1,5 @@
 name:                monoid-extras
-version:             0.4.4
+version:             0.5
 synopsis:            Various extra monoid-related definitions and utilities
 description:         Various extra monoid-related definitions and utilities,
                      such as monoid actions, monoid coproducts, semi-direct
@@ -14,7 +14,7 @@
 category:            Data
 build-type:          Simple
 cabal-version:       >=1.10
-tested-with:         GHC == 7.6.3, GHC == 7.8.4, GHC == 7.10.2, GHC == 8.0.1, GHC == 8.2.2, GHC == 8.4.1
+tested-with:         GHC == 7.8.4, GHC == 7.10.3, GHC == 8.0.2, GHC == 8.2.2, GHC == 8.4.2
 
 source-repository head
   type: git
@@ -53,5 +53,6 @@
   main-is: SemiDirectProduct.hs
   type: exitcode-stdio-1.0
   build-depends: base          >= 4.3 &&  < 4.12
+               , semigroups
                , criterion
                , monoid-extras
diff --git a/src/Data/Monoid/Recommend.hs b/src/Data/Monoid/Recommend.hs
--- a/src/Data/Monoid/Recommend.hs
+++ b/src/Data/Monoid/Recommend.hs
@@ -3,6 +3,8 @@
 {-# LANGUAGE DeriveFoldable     #-}
 {-# LANGUAGE DeriveFunctor      #-}
 {-# LANGUAGE DeriveTraversable  #-}
+
+{-# OPTIONS_GHC -fno-warn-unused-imports #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Monoid.Recommend
diff --git a/src/Data/Monoid/WithSemigroup.hs b/src/Data/Monoid/WithSemigroup.hs
--- a/src/Data/Monoid/WithSemigroup.hs
+++ b/src/Data/Monoid/WithSemigroup.hs
@@ -1,7 +1,9 @@
-{-# LANGUAGE FlexibleInstances
-           , UndecidableInstances
-  #-}
+{-# LANGUAGE ConstraintKinds      #-}
+{-# LANGUAGE CPP                  #-}
+{-# LANGUAGE FlexibleInstances    #-}
+{-# LANGUAGE UndecidableInstances #-}
 
+{-# OPTIONS_GHC -fno-warn-unused-imports #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Monoid.WithSemigroup
@@ -17,16 +19,15 @@
        ( Monoid'
        ) where
 
-import Data.Semigroup
-
--- Poor man's constraint synonym.  Eventually, once it becomes
--- standard, we can make this a real constraint synonym and get rid of
--- the UndecidableInstances flag.  Better yet, hopefully the Monoid
--- class will eventually have a Semigroup superclass.
+import           Data.Semigroup
 
--- | The @Monoid'@ class is a synonym for things which are instances
---   of both 'Semigroup' and 'Monoid'.  Ideally, the 'Monoid' class
---   itself will eventually include a 'Semigroup' superclass and we
---   can get rid of this.
-class (Semigroup m, Monoid m) => Monoid' m
-instance (Semigroup m, Monoid m) => Monoid' m
+-- | For base < 4.11, the @Monoid'@ constraint is a synonym for things
+--   which are instances of both 'Semigroup' and 'Monoid'.  For base
+--   version 4.11 and onwards, @Monoid@ has @Semigroup@ as a
+--   superclass already, so for backwards compatibility @Monoid'@ is
+--   provided as a synonym for @Monoid@.
+#if MIN_VERSION_base(4,11,0)
+type Monoid' = Monoid
+#else
+type Monoid' m = (Semigroup m, Monoid m)
+#endif