Conditionally define Dry.Equalizer

So that it gets defined if one explicitly depends on dry/core/equalizer

lib/dry/core.rb

@@ -28,14 +28,17 @@ def self.loader
     loader.setup
   end
 
-  # Build an equalizer module for the inclusion in other class
-  #
-  # ## Credits
-  #
-  # Equalizer has been originally imported from the equalizer gem created by Dan Kubb
-  #
-  # @api public
-  def self.Equalizer(*keys, **options)
-    Dry::Core::Equalizer.new(*keys, **options)
+  # See dry/core/equalizer.rb
+  unless singleton_class.method_defined?(:Equalizer)
+    # Build an equalizer module for the inclusion in other class
+    #
+    # ## Credits
+    #
+    # Equalizer has been originally imported from the equalizer gem created by Dan Kubb
+    #
+    # @api public
+    def self.Equalizer(*keys, **options)
+      Dry::Core::Equalizer.new(*keys, **options)
+    end
   end
 end

lib/dry/core/equalizer.rb

@@ -138,4 +138,19 @@ def ==(other)
       end
     end
   end
+
+  # Old modules that depend on dry/core/equalizer may miss
+  # this method if dry/core is not required explicitly
+  unless singleton_class.method_defined?(:Equalizer)
+    # Build an equalizer module for the inclusion in other class
+    #
+    # ## Credits
+    #
+    # Equalizer has been originally imported from the equalizer gem created by Dan Kubb
+    #
+    # @api public
+    def self.Equalizer(*keys, **options)
+      Dry::Core::Equalizer.new(*keys, **options)
+    end
+  end
 end