Consider modified modules initialized [Bug #18185]

class.c

@@ -351,10 +351,22 @@ copy_tables(VALUE clone, VALUE orig)
 
 static bool ensure_origin(VALUE klass);
 
+/**
+ * If this flag is set, that module is allocated but not initialized yet.
+ */
+enum {RMODULE_ALLOCATED_BUT_NOT_INITIALIZED = RUBY_FL_USER5};
+
 static inline bool
 RMODULE_UNINITIALIZED(VALUE module)
 {
-    return RCLASS_SUPER(module) == rb_cBasicObject;
+    return FL_TEST_RAW(module, RMODULE_ALLOCATED_BUT_NOT_INITIALIZED);
+}
+
+void
+rb_module_set_initialized(VALUE mod)
+{
+    FL_UNSET_RAW(mod, RMODULE_ALLOCATED_BUT_NOT_INITIALIZED);
+    /* no more re-initialization */
 }
 
 void
@@ -808,7 +820,7 @@ rb_module_s_alloc(VALUE klass)
 {
     VALUE mod = class_alloc(T_MODULE, klass);
     RCLASS_M_TBL_INIT(mod);
-    RB_OBJ_WRITE(mod, &RCLASS(mod)->super, rb_cBasicObject);
+    FL_SET(mod, RMODULE_ALLOCATED_BUT_NOT_INITIALIZED);
     return mod;
 }
 
@@ -916,10 +928,7 @@ ensure_includable(VALUE klass, VALUE module)
 {
     rb_class_modify_check(klass);
     Check_Type(module, T_MODULE);
-    if (RMODULE_UNINITIALIZED(module)) {
-        RB_OBJ_WRITE(module, &RCLASS(module)->super, 0);
-        /* no more re-initialization */
-    }
+    rb_module_set_initialized(module);
     if (!NIL_P(rb_refinement_module_get_refined_class(module))) {
 	rb_raise(rb_eArgError, "refinement module is not allowed");
     }

eval.c

@@ -424,6 +424,9 @@ rb_class_modify_check(VALUE klass)
     if (SPECIAL_CONST_P(klass)) {
 	Check_Type(klass, T_CLASS);
     }
+    if (RB_TYPE_P(klass, T_MODULE)) {
+        rb_module_set_initialized(klass);
+    }
     if (OBJ_FROZEN(klass)) {
 	const char *desc;
 

internal/class.h

@@ -115,6 +115,7 @@ int rb_singleton_class_internal_p(VALUE sklass);
 VALUE rb_class_boot(VALUE);
 VALUE rb_class_s_alloc(VALUE klass);
 VALUE rb_module_s_alloc(VALUE klass);
+void rb_module_set_initialized(VALUE module);
 void rb_module_check_initializable(VALUE module);
 VALUE rb_make_metaclass(VALUE, VALUE);
 VALUE rb_include_class_new(VALUE, VALUE);

test/ruby/test_module.rb

@@ -446,6 +446,35 @@ def initialize_copy(other)
     end
   end
 
+  class Bug18185 < Module
+    module InstanceMethods
+    end
+    attr_reader :ancestor_list
+    def initialize
+      @ancestor_list = ancestors
+      include InstanceMethods
+    end
+    class Foo
+      attr_reader :key
+      def initialize(key:)
+        @key = key
+      end
+    end
+  end
+
+  def test_module_subclass_initialize
+    mod = Bug18185.new
+    c = Class.new(Bug18185::Foo) do
+      include mod
+    end
+    anc = c.ancestors
+    assert_include(anc, mod)
+    assert_equal(1, anc.count(BasicObject), ->{anc.inspect})
+    b = c.new(key: 1)
+    assert_equal(1, b.key)
+    assert_not_include(mod.ancestor_list, BasicObject)
+  end
+
   def test_dup
     OtherSetup.call