The following code runs fine in Smarty 4, but gives a "Too many shorthand attributes" error in Smarty 5:

$this->smarty->registerPlugin(Smarty::PLUGIN_COMPILER, 'testcompilerfunction', 'foo');
$this->assertEquals('1', $this->smarty->fetch('eval:{testcompilerfunction 1}'));

function foo($params, $smarty)
{
    return "<?php echo '{$params[0]}';?>";
}

It's unclear to me if this was an intentional change in Smarty 5, I cannot find any documentation saying so. We unfortunately have a large number of templates relying on the old syntax, so this regression/change is currently a blocker for us moving to Smarty 5.

Somewhat related to #1005, though I interpret that issue as being for function plugins, where the equivalent syntax didn't work in Smarty 4, as far as I know.