This is e.g. usefull to merge a global configuration file with a stage specific configuration.

Where do you want to use this specifically?

I worry this won't be sufficient in the general case. If both configuration files have lists, sometimes those lists should be combined instead of overwritten, and other times they should be overwritten entirely.

For example, if you're merging .arclint files like this:

{
  "exclude" : ["(^globalstuff/)"]
}

And:

{
  "exclude" : ["(^localstuff/)"]
}

The best way to merge these files is probably to produce ["(^globalstuff/)", "(^localstuff/)"], but this won't do that.

Or, suppose you're merging .arcconfig files like this:

{
  "load": ["a", "b"],
}

And:

{
  "load" : ["z"],
}

...doesn't that produce ["z", "b"], a nonsensical/bizarre result?

In D9135#6, @epriestley wrote:

This is e.g. usefull to merge a global configuration file with a stage specific configuration.

Where do you want to use this specifically?

I worry this won't be sufficient in the general case. If both configuration files have lists, sometimes those lists should be combined instead of overwritten, and other times they should be overwritten entirely.

Yes, it would not work with arrays that have no or numerical keys like in you example.

We have a global configuration file for phabricator to configure stuff. That is located in conf/custom/global.conf.php. For each stage of our deployment we have a seperate config file to adjust settings like conf/custom/integration.conf.php which is loaded by the envirement.

return array(
  'phabricator.show-beta-applications' => true,
  'phabricator.uninstalled-applications' => array(
    'PhabricatorApplicationUIExamples' => false,
    'PhabricatorApplicationXHProf' => false
  )
);

We want to load the global.conf.php and overrule some settings for a stage. Currently we use the + operator to merge the arrays but this will not return the expected result here.
The stage config currently looks like that:

return array(
  'phabricator.show-beta-applications' => false,
  'phabricator.uninstalled-applications' => array(
    'PhabricatorApplicationUIExamples' => true,
  )
) + phabricator_read_config_file('custom/global.conf.php');

I've just run the unit test cases against +, array_merge, array_merge_recursive and array_merge_recursive_overrule to show the different result. P1142
Do you have a better idea how to handle these configurations?