Topmenu template is not loaded from the original module

[digitalpianism]

Steps to reproduce

1. Install Magento from 2.0 branch.
2. Create a module with a preference for the Magento\Theme\Block\Html\Topmenu

<preference for="Magento\Theme\Block\Html\Topmenu" type="MyCompany\Theme\Block\Html\Topmenu" />

1. Create your custom class:

<?php

namespace MyCompany\Theme\Block\Html;

class Topmenu extends \Magento\Theme\Block\Html\Topmenu
{
    protected function _addSubMenu($child, $childLevel, $childrenWrapClass, $limit)
    {
        // my stuff
    }
}

1. Reload your page.

Expected result

1. Custom class is used
2. No error

Actual result

1. Custom class is used
2. An error is triggered:

main.CRITICAL: Invalid template file: 'html/topmenu.phtml' in module: 'MyCompany_Theme' block's name: 'catalog.topnav' [] []

Cause

The template of this block is not prefixed with the module name: https://github.com/magento/magento2/blob/develop/app/code/Magento/Theme/view/frontend/layout/default.xml#L69

Instead of :

<block class="Magento\Theme\Block\Html\Topmenu" name="catalog.topnav" template="html/topmenu.phtml" ttl="3600" before="-"/>

It should be:

<block class="Magento\Theme\Block\Html\Topmenu" name="catalog.topnav" template="Magento_Theme::html/topmenu.phtml" ttl="3600" before="-"/>

Could you please confirm that is a bug so I can make a PR to fix that.

The bug was discovered in this Magento SE post: http://magento.stackexchange.com/questions/115988/are-we-forced-to-rewrite-a-template-in-magento2-when-rewritting-a-block

[digitalpianism]

Thanks to @fooman here's a PR that is fixing the issue: https://github.com/magento/magento2/pull/1895/files

[magento-engcom-team]

Closing, as issue seems to be fixed.