Add Class#subclasses

Implements [Feature #18273]

Returns an array containing the receiver's direct subclasses without
singleton classes.

core/class/subclasses_spec.rb

@@ -0,0 +1,38 @@
+require_relative '../../spec_helper'
+require_relative '../module/fixtures/classes'
+
+ruby_version_is '3.1' do
+  describe "Class#subclasses" do
+    it "returns a list of classes directly inheriting from self" do
+      assert_subclasses(ModuleSpecs::Parent, [ModuleSpecs::Child, ModuleSpecs::Child2])
+    end
+
+    it "does not return included modules" do
+      parent = Class.new
+      child = Class.new(parent)
+      mod = Module.new
+      parent.include(mod)
+
+      assert_subclasses(parent, [child])
+    end
+
+    it "does not return singleton classes" do
+      a = Class.new
+
+      a_obj = a.new
+      def a_obj.force_singleton_class
+        42
+      end
+
+      a.subclasses.should_not include(a_obj.singleton_class)
+    end
+
+    it "has 1 entry per module or class" do
+      ModuleSpecs::Parent.subclasses.should == ModuleSpecs::Parent.subclasses.uniq
+    end
+
+    def assert_subclasses(mod,subclasses)
+      mod.subclasses.sort_by(&:inspect).should == subclasses.sort_by(&:inspect)
+    end
+  end
+end