As far as I know this is expected behaviour, and it was always like that. There might be even some tooltip in the template name field telling that only alpha-numeric characters and dashes are allowed.

If someone else confirms this (I don't expect myself to be perfect), I will close this issue as expected behaviour.

I have a site using a child template cassiopeia_leadershape
As you can see from the screenshot below I have lots of blogcategory overrides that have been renamed as something else and also have an xml file

@brianteeman ok so the issue is not coming from the underscore? Strange :-/

silly question but in html/com_content/category/mylayout.php did you update the loadTemplate for your new names
$this->loadTemplate('myarticles')

It can NOT come from the underscore because that is how child templates are created by joomla

yes I did - the error dissapears when I move the complete structure
from the child to the main - so something else is interfering

well its something in your code and not a core issue

@richard67

Not in my code but maybe some exception for not child template templates with underscore - i will investigate further

@brianteeman and @richard67 thanks for testing - I will report back what the issue was.

@coolcat-creations if you make a template override for any default and call it mylayout.php
then this script still contains a statement like

echo $this->loadTemplate('item');

it will load mylayout_item.php if yo did not override this statement to make

echo $this->loadTemplate('myitem');

@chmst thats what I wrote #41270 (comment)

@brianteeman @chmst I wrote both correctly in the layout. As said it works as indented when I move it in another template. there must be something else interfering. I am investigating further.

I just repeated it from scratch with no problem at all

I just tested with a not template and named it test_test and it worked too. So the issue is not in the core but something in the maintemplate probably. Thank you all for testing.

The problem is because the child template folder name begins with the same name as the template (e.g. cassiopeia and cassiopeia_child).

Here is a fix for it in libraries/MVC/HtmlView.php on line 377
Add a directory separator to prevent replacing the template path twice.

// Change the template folder if alternative layout is in different template
if (isset($layoutTemplate) && $layoutTemplate !== '_' && $layoutTemplate != $template->template) {
    $this->_path['template'] = str_replace(
        JPATH_THEMES . DIRECTORY_SEPARATOR . $template->template . DIRECTORY_SEPARATOR,
        JPATH_THEMES . DIRECTORY_SEPARATOR . $layoutTemplate . DIRECTORY_SEPARATOR,
        $this->_path['template']
    );
}

so why is it working perfectly for me (and others)

so why is it working perfectly for me (and others)

This depends on how many subtemplates are loaded, therefore the HtmlView::loadTemplate() method is called multiple times.

$this->subtemplatename = 'myarticles';
echo LayoutHelper::render('joomla.content.category_default', $this);

Should be closed as there is a PR

please test #41274