Fix method caching bug when including/prepend module A that prepends module B

Fix by always adding the generated iclass to the subclasses list,
otherwise the method cache for the iclass is not cleared when
the method in the module is overwritten.

Fixes [Bug #20716]

2 files changed, 32 insertions, 7 deletions

diff --git a/class.c b/class.c
index 5cce99e334..20086d15d5 100644
--- a/class.c
+++ b/class.c
@@ -1336,7 +1336,6 @@ do_include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super
         iclass = rb_include_class_new(module, super_class);
         c = RCLASS_SET_SUPER(c, iclass);
         RCLASS_SET_INCLUDER(iclass, klass);
-        add_subclass = TRUE;
         if (module != RCLASS_ORIGIN(module)) {
             if (!origin_stack) origin_stack = rb_ary_hidden_new(2);
             VALUE origin[2] = {iclass, RCLASS_ORIGIN(module)};
@@ -1347,14 +1346,11 @@ do_include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super
             RCLASS_SET_ORIGIN(RARRAY_AREF(origin_stack, (origin_len -= 2)), iclass);
             RICLASS_SET_ORIGIN_SHARED_MTBL(iclass);
             rb_ary_resize(origin_stack, origin_len);
-            add_subclass = FALSE;
         }
 
-        if (add_subclass) {
-            VALUE m = module;
-            if (BUILTIN_TYPE(m) == T_ICLASS) m = METACLASS_OF(m);
-            rb_module_add_to_subclasses_list(m, iclass);
-        }
+        VALUE m = module;
+        if (BUILTIN_TYPE(m) == T_ICLASS) m = METACLASS_OF(m);
+        rb_module_add_to_subclasses_list(m, iclass);
 
         if (BUILTIN_TYPE(klass) == T_MODULE && FL_TEST(klass, RMODULE_IS_REFINEMENT)) {
             VALUE refined_class =
diff --git a/test/ruby/test_super.rb b/test/ruby/test_super.rb
index aa09f9e1b9..418f293f66 100644
--- a/test/ruby/test_super.rb
+++ b/test/ruby/test_super.rb
@@ -633,6 +633,35 @@ class TestSuper < Test::Unit::TestCase
     }
   end
 
+  def test_super_with_included_prepended_module_method_caching_bug_20716
+    a = Module.new do
+      def test(*args)
+        super
+      end
+    end
+
+    b = Module.new do
+      def test(a)
+        a
+      end
+    end
+
+    c = Class.new
+
+    b.prepend(a)
+    c.include(b)
+
+    assert_equal(1, c.new.test(1))
+
+    b.class_eval do
+      def test
+        :test
+      end
+    end
+
+    assert_equal(:test, c.new.test)
+  end
+
   class TestFor_super_with_modified_rest_parameter_base
     def foo *args
       args