`Module#alias_method` should return `self` in ISO standard

src/class.c

@@ -1843,7 +1843,7 @@ mrb_mod_alias(mrb_state *mrb, mrb_value mod)
 
   mrb_get_args(mrb, "nn", &new_name, &old_name);
   mrb_alias_method(mrb, c, new_name, old_name);
-  return mrb_nil_value();
+  return mod;
 }
 
 void

test/t/module.rb

@@ -25,6 +25,17 @@ assert('Module', '15.2.2') do
   assert_equal Class, Module.class
 end
 
+assert('Module#alias_method', '15.2.2.4.8') do
+  cls = Class.new do
+    def foo
+      "FOO"
+    end
+  end
+
+  assert_same(cls, cls.alias_method(:bar, :foo))
+  assert_equal("FOO", cls.new.bar)
+end
+
 # TODO not implemented ATM assert('Module.constants', '15.2.2.3.1') do
 
 assert('Module#ancestors', '15.2.2.4.9') do