Box: allocate classes as boxable when it happens in the root box

Without this change, classes (including iclass) are allocated
as un-boxable classes after initializing user boxes (after starting
script evaluation). Under this situation, iclasses are created as
un-boxabled class when core modules are included by a class in the
root box, then it causes problems because it's in the root box but
it can't have multiple classexts.

This change makes it possible to allocate boxable classes even after
initializing user boxes. Classes create in the root box will be
boxable, and those can have 2 or more classexts.

Author: tagomoris

class.c

@@ -491,6 +491,7 @@ rb_class_duplicate_classext(rb_classext_t *orig, VALUE klass, const rb_box_t *bo
         while (subclass_entry) {
             if (subclass_entry->klass && RB_TYPE_P(subclass_entry->klass, T_ICLASS)) {
                 iclass = subclass_entry->klass;
+                VM_ASSERT(RB_TYPE_P(iclass, T_ICLASS));
                 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 box with the new ext
@@ -819,7 +820,8 @@ class_alloc0(enum ruby_value_type type, VALUE klass, bool boxable)
 static VALUE
 class_alloc(enum ruby_value_type type, VALUE klass)
 {
-    return class_alloc0(type, klass, false);
+    bool boxable = BOX_ROOT_P(rb_current_box());
+    return class_alloc0(type, klass, boxable);
 }
 
 static VALUE

test/ruby/test_box.rb

@@ -855,4 +855,20 @@ def test_loaded_extension_deleted_in_user_box
       assert_empty(Dir.children(tmpdir))
     end
   end
+
+  def test_root_box_iclasses_should_be_boxable
+    assert_separately([ENV_ENABLE_BOX], __FILE__, __LINE__, "#{<<~"begin;"}\n#{<<~'end;'}", ignore_stderr: true)
+    begin;
+      Ruby::Box.root.eval("class IMath; include Math; end") # (*)
+      module Math
+        def foo = :foo
+      end
+      # This test crashes here if iclasses (created at the line (*) is not boxable)
+      class IMath2; include Math; end
+      assert_equal :foo, IMath2.new.foo
+      assert_raise NoMethodError do
+        Ruby::Box.root.eval("IMath.new.foo")
+      end
+    end;
+  end
 end