diff --git a/class.c b/class.c
index 4dddf08c67af38..5e50a281d9a9fe 100644
--- a/class.c
+++ b/class.c
@@ -102,7 +102,7 @@ rb_class_classext_free(VALUE klass, rb_classext_t *ext, bool is_prime)
         rb_id_table_free(tbl);
     }
 
-    rb_class_classext_free_subclasses(ext, klass);
+    rb_class_classext_free_subclasses(ext, klass, false);
 
     if (RCLASSEXT_SUPERCLASSES_WITH_SELF(ext)) {
         RUBY_ASSERT(is_prime); // superclasses should only be used on prime
@@ -126,13 +126,30 @@ rb_iclass_classext_free(VALUE klass, rb_classext_t *ext, bool is_prime)
         rb_id_table_free(RCLASSEXT_CALLABLE_M_TBL(ext));
     }
 
-    rb_class_classext_free_subclasses(ext, klass);
+    rb_class_classext_free_subclasses(ext, klass, false);
 
     if (!is_prime) { // the prime classext will be freed with RClass
         xfree(ext);
     }
 }
 
+static void
+iclass_free_orphan_classext(VALUE klass, rb_classext_t *ext)
+{
+    if (RCLASSEXT_ICLASS_IS_ORIGIN(ext) && !RCLASSEXT_ICLASS_ORIGIN_SHARED_MTBL(ext)) {
+        /* Method table is not shared for origin iclasses of classes */
+        rb_id_table_free(RCLASSEXT_M_TBL(ext));
+    }
+
+    if (RCLASSEXT_CALLABLE_M_TBL(ext) != NULL) {
+        rb_id_table_free(RCLASSEXT_CALLABLE_M_TBL(ext));
+    }
+
+    rb_class_classext_free_subclasses(ext, klass, true); // replacing this classext with a newer one
+
+    xfree(ext);
+}
+
 struct rb_class_set_namespace_classext_args {
     VALUE obj;
     rb_classext_t *ext;
@@ -144,11 +161,11 @@ rb_class_set_namespace_classext_update(st_data_t *key_ptr, st_data_t *val_ptr, s
     struct rb_class_set_namespace_classext_args *args = (struct rb_class_set_namespace_classext_args *)a;
 
     if (existing) {
-        if (BUILTIN_TYPE(args->obj) == T_ICLASS) {
-            rb_iclass_classext_free(args->obj, (rb_classext_t *)*val_ptr, false);
+        if (LIKELY(BUILTIN_TYPE(args->obj) == T_ICLASS)) {
+            iclass_free_orphan_classext(args->obj, (rb_classext_t *)*val_ptr);
         }
         else {
-            rb_class_classext_free(args->obj, (rb_classext_t *)*val_ptr, false);
+            rb_bug("Updating existing classext for non-iclass never happen");
         }
     }
 
@@ -375,6 +392,8 @@ class_duplicate_iclass_classext(VALUE iclass, rb_classext_t *mod_ext, const rb_n
 
     RCLASSEXT_SET_INCLUDER(ext, iclass, RCLASSEXT_INCLUDER(src));
 
+    VM_ASSERT(FL_TEST_RAW(iclass, RCLASS_NAMESPACEABLE));
+
     first_set = RCLASS_SET_NAMESPACE_CLASSEXT(iclass, ns, ext);
     if (first_set) {
         RCLASS_SET_PRIME_CLASSEXT_WRITABLE(iclass, false);
@@ -454,6 +473,7 @@ rb_class_duplicate_classext(rb_classext_t *orig, VALUE klass, const rb_namespace
                 if (RBASIC_CLASS(iclass) == klass) {
                     // Is the subclass an ICLASS including this module into another class
                     // If so we need to re-associate it under our namespace with the new ext
+                    VM_ASSERT(FL_TEST_RAW(iclass, RCLASS_NAMESPACEABLE));
                     class_duplicate_iclass_classext(iclass, ext, ns);
                 }
             }
@@ -574,7 +594,7 @@ void
 rb_class_remove_subclass_head(VALUE klass)
 {
     rb_classext_t *ext = RCLASS_EXT_WRITABLE(klass);
-    rb_class_classext_free_subclasses(ext, klass);
+    rb_class_classext_free_subclasses(ext, klass, false);
 }
 
 static struct rb_subclass_entry *
@@ -610,17 +630,18 @@ remove_class_from_subclasses(struct st_table *tbl, VALUE ns_id, VALUE klass)
                 next->prev = prev;
             }
 
-            xfree(entry);
-
             if (first_entry) {
                 if (next) {
                     st_update(tbl, ns_id, remove_class_from_subclasses_replace_first_entry, (st_data_t)next);
                 }
                 else {
-                    // no subclass entries in this ns
+                    // no subclass entries in this ns after the deletion
                     st_delete(tbl, &ns_id, NULL);
                 }
             }
+
+            xfree(entry);
+
             break;
         }
         else if (first_entry) {
@@ -655,7 +676,7 @@ rb_class_remove_from_module_subclasses(VALUE klass)
 }
 
 void
-rb_class_classext_free_subclasses(rb_classext_t *ext, VALUE klass)
+rb_class_classext_free_subclasses(rb_classext_t *ext, VALUE klass, bool replacing)
 {
     rb_subclass_anchor_t *anchor = RCLASSEXT_SUBCLASSES(ext);
     struct st_table *tbl = anchor->ns_subclasses->tbl;
@@ -674,12 +695,12 @@ rb_class_classext_free_subclasses(rb_classext_t *ext, VALUE klass)
     rb_ns_subclasses_ref_dec(anchor->ns_subclasses);
     xfree(anchor);
 
-    if (RCLASSEXT_NS_SUPER_SUBCLASSES(ext)) {
+    if (!replacing && RCLASSEXT_NS_SUPER_SUBCLASSES(ext)) {
         rb_ns_subclasses_t *ns_sub = RCLASSEXT_NS_SUPER_SUBCLASSES(ext);
         remove_class_from_subclasses(ns_sub->tbl, ns_id, klass);
         rb_ns_subclasses_ref_dec(ns_sub);
     }
-    if (RCLASSEXT_NS_MODULE_SUBCLASSES(ext)) {
+    if (!replacing && RCLASSEXT_NS_MODULE_SUBCLASSES(ext)) {
         rb_ns_subclasses_t *ns_sub = RCLASSEXT_NS_MODULE_SUBCLASSES(ext);
         remove_class_from_subclasses(ns_sub->tbl, ns_id, klass);
         rb_ns_subclasses_ref_dec(ns_sub);
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 965355df67d87c..0dbf1fa3ec0918 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -614,8 +614,16 @@ strio_close_write(VALUE self)
  * call-seq:
  *   closed? -> true or false
  *
- * Returns +true+ if +self+ is closed for both reading and writing,
- * +false+ otherwise.
+ * Returns whether +self+ is closed for both reading and writing:
+ *
+ *   strio = StringIO.new
+ *   strio.closed?     # => false  # Open for reading and writing.
+ *   strio.close_read
+ *   strio.closed?     # => false  # Still open for writing.
+ *   strio.close_write
+ *   strio.closed?     # => true   # Now closed for both.
+ *
+ * Related: StringIO.closed_read?, StringIO.closed_write?.
  */
 static VALUE
 strio_closed(VALUE self)
diff --git a/internal/class.h b/internal/class.h
index 5d843e58da3923..138620dd6f0469 100644
--- a/internal/class.h
+++ b/internal/class.h
@@ -490,7 +490,7 @@ void rb_class_classext_foreach(VALUE klass, rb_class_classext_foreach_callback_f
 void rb_class_subclass_add(VALUE super, VALUE klass);
 void rb_class_remove_from_super_subclasses(VALUE);
 void rb_class_remove_from_module_subclasses(VALUE);
-void rb_class_classext_free_subclasses(rb_classext_t *, VALUE);
+void rb_class_classext_free_subclasses(rb_classext_t *, VALUE, bool);
 void rb_class_foreach_subclass(VALUE klass, void (*f)(VALUE, VALUE), VALUE);
 void rb_class_detach_subclasses(VALUE);
 void rb_class_detach_module_subclasses(VALUE);
diff --git a/test/ruby/test_namespace.rb b/test/ruby/test_namespace.rb
index 5661a98ca2b5f1..bfbbe38e051ab2 100644
--- a/test/ruby/test_namespace.rb
+++ b/test/ruby/test_namespace.rb
@@ -538,51 +538,6 @@ def test_load_path_and_loaded_features
     assert !$LOADED_FEATURES.include?(File.join(namespace_dir, 'blank2.rb'))
   end
 
-  def test_prelude_gems_and_loaded_features
-    assert_in_out_err([ENV_ENABLE_NAMESPACE, "--enable=gems"], "#{<<-"begin;"}\n#{<<-'end;'}") do |output, error|
-      begin;
-        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
-        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
-
-        require "error_highlight"
-
-        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
-        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
-      end;
-
-      # No additional warnings except for experimental warnings
-      assert_includes error.join("\n"), EXPERIMENTAL_WARNINGS
-      assert_equal 2, error.size
-
-      assert_includes output.grep(/^before:/).join("\n"), '/bundled_gems.rb'
-      assert_includes output.grep(/^before:/).join("\n"), '/error_highlight.rb'
-      assert_includes output.grep(/^after:/).join("\n"), '/bundled_gems.rb'
-      assert_includes output.grep(/^after:/).join("\n"), '/error_highlight.rb'
-    end
-  end
-
-  def test_prelude_gems_and_loaded_features_with_disable_gems
-    assert_in_out_err([ENV_ENABLE_NAMESPACE, "--disable=gems"], "#{<<-"begin;"}\n#{<<-'end;'}") do |output, error|
-      begin;
-        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
-        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
-
-        require "error_highlight"
-
-        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
-        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
-      end;
-
-      assert_includes error.join("\n"), EXPERIMENTAL_WARNINGS
-      assert_equal 2, error.size
-
-      refute_includes output.grep(/^before:/).join("\n"), '/bundled_gems.rb'
-      refute_includes output.grep(/^before:/).join("\n"), '/error_highlight.rb'
-      refute_includes output.grep(/^after:/).join("\n"), '/bundled_gems.rb'
-      assert_includes output.grep(/^after:/).join("\n"), '/error_highlight.rb'
-    end
-  end
-
   def test_eval_basic
     pend unless Namespace.enabled?
 
@@ -664,4 +619,76 @@ def test_eval_error_handling
       assert_equal 4, result
     end
   end
+
+  # Tests which run always (w/o RUBY_NAMESPACE=1 globally)
+
+  def test_prelude_gems_and_loaded_features
+    assert_in_out_err([ENV_ENABLE_NAMESPACE, "--enable=gems"], "#{<<-"begin;"}\n#{<<-'end;'}") do |output, error|
+      begin;
+        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
+        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
+
+        require "error_highlight"
+
+        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
+        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
+      end;
+
+      # No additional warnings except for experimental warnings
+      assert_includes error.join("\n"), EXPERIMENTAL_WARNINGS
+      assert_equal 2, error.size
+
+      assert_includes output.grep(/^before:/).join("\n"), '/bundled_gems.rb'
+      assert_includes output.grep(/^before:/).join("\n"), '/error_highlight.rb'
+      assert_includes output.grep(/^after:/).join("\n"), '/bundled_gems.rb'
+      assert_includes output.grep(/^after:/).join("\n"), '/error_highlight.rb'
+    end
+  end
+
+  def test_prelude_gems_and_loaded_features_with_disable_gems
+    assert_in_out_err([ENV_ENABLE_NAMESPACE, "--disable=gems"], "#{<<-"begin;"}\n#{<<-'end;'}") do |output, error|
+      begin;
+        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
+        puts ["before:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
+
+        require "error_highlight"
+
+        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/bundled_gems.rb") }&.first].join
+        puts ["after:", $LOADED_FEATURES.select{ it.end_with?("/error_highlight.rb") }&.first].join
+      end;
+
+      assert_includes error.join("\n"), EXPERIMENTAL_WARNINGS
+      assert_equal 2, error.size
+
+      refute_includes output.grep(/^before:/).join("\n"), '/bundled_gems.rb'
+      refute_includes output.grep(/^before:/).join("\n"), '/error_highlight.rb'
+      refute_includes output.grep(/^after:/).join("\n"), '/bundled_gems.rb'
+      assert_includes output.grep(/^after:/).join("\n"), '/error_highlight.rb'
+    end
+  end
+
+  def test_root_and_main_methods
+    assert_separately([ENV_ENABLE_NAMESPACE], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      pend unless Namespace.respond_to?(:root) and Namespace.respond_to?(:main) # for RUBY_DEBUG > 0
+
+      assert Namespace.root.respond_to?(:root?)
+      assert Namespace.main.respond_to?(:main?)
+
+      assert Namespace.root.root?
+      assert Namespace.main.main?
+      assert_equal Namespace.main, Namespace.current
+
+      $a = 1
+      $LOADED_FEATURES.push("/tmp/foobar")
+
+      assert_equal 2, Namespace.root.eval('$a = 2; $a')
+      assert !Namespace.root.eval('$LOADED_FEATURES.push("/tmp/barbaz"); $LOADED_FEATURES.include?("/tmp/foobar")')
+      assert "FooClass", Namespace.root.eval('class FooClass; end; Object.const_get(:FooClass).to_s')
+
+      assert_equal 1, $a
+      assert !$LOADED_FEATURES.include?("/tmp/barbaz")
+      assert !Object.const_defined?(:FooClass)
+    end;
+  end
 end