If there is a compilation error with the less files, the only exception we get is

Warning: array_map(): An error occurred while invoking the map callback in /home/santhosh/dev/wiki/core/includes/resourceloader/ResourceLoaderFileModule.php

This is not helpful. Because array maps cannot properly report errors from callbacks.

Roan already reported this issue at PHP with a patch. https://bugs.php.net/bug.php?id=55416 but not accepted at upstream.

Can't we modify the code in readStyleFiles of ResourceLoaderFileModule a bit so that we get sensible stack trace and saves time in debugging?

instead of array map, it can be something like

$style= "";
foreach ( $files as $file ) {

$style = $style . "\n".  $this->readStyleFile( $file,  $flip );

}
$styles[$media] = $style;

What I expect is a helpful error message like:

exception 'Exception' with message '.mw-ui-one-third is undefined: failed at .mw-ui-one-third; /home/santhosh/dev/wiki/ContentTranslation/modules/base/styles/base.less on line 27' in /home/santhosh/dev/wiki/core/includes/libs/lessc.inc.php:3527

Version: 1.23.0
Severity: normal