Re-memoize methods in subclasses (fix #70)

This should fix some corner cases when inheritance is in play.

Author: flash-gordon

lib/dry/core/memoizable.rb

@@ -13,6 +13,13 @@ module Base
           def memoize(*names)
             prepend(Memoizer.new(self, names))
           end
+
+          def inherited(base)
+            super
+
+            memoizer = base.ancestors.find { _1.is_a?(Memoizer) }
+            base.prepend(memoizer.dup)
+          end
         end
 
         module BasicObject
@@ -79,7 +86,7 @@ def define_memoizable(method:)
           kernel = KERNEL
 
           if parameters.empty?
-            key = method.name.hash.abs
+            key = "#{object_id}:#{method.name}".hash.abs
 
             define_method(method.name) do
               value = super()

spec/dry/core/memoizable_spec.rb

@@ -206,5 +206,29 @@ def initialize(*args, **kwargs, &block)
         expect(counter.value).to eql(3)
       end
     end
+
+    context "inheritance" do
+      let(:subclass) { Class.new(klass) }
+
+      before do
+        klass.define_method(:nodes) { [:root] }
+        klass.define_method(:path) { [*nodes, :leaf] }
+        klass.memoize(:nodes, :path)
+
+        subclass.define_method(:nodes) { [*super(), :node] }
+      end
+
+      let(:subclass_instance) do
+        subclass.new
+      end
+
+      it "memoizes results separately" do
+        expect(instance.nodes).to eql([:root])
+        expect(instance.path).to eql([:root, :leaf])
+
+        expect(subclass_instance.nodes).to eql([:root, :node])
+        expect(subclass_instance.path).to eql([:root, :node, :leaf])
+      end
+    end
   end
 end