Options element corrupts integer keys of allowed values for list fields

Updated: Comment #0

Problem/Motivation

Because drupal_array_merge_deep() has no option for preserving integer keys, the call in line 294 of options_element.module can corrupt the #options key of the options element.

Steps to reproduce

1. Add a list field to a content type with integer keys for the allowed values, that do not represent the natural array order, i.e. use 10, 100, 1000, instead of 0, 1, 2, ...
2. Add a node of that content type and fill out the field
3. Go back to the edit page of the field
4. Without changing anything, submit the form
   

   Expected result

   The form submission is successful

   Actual result

   You get a validation error saying
   

   Allowed values list: some values are being removed while currently in use.
   

5. To investigate the problem, look closer now at the keys that the options element is presenting.
   

   Expected result

   10, 100, 100

   Actual result

   0, 1, 2

Proposed resolution

It would be nice if drupal_array_merge_deep_array() had an option for preserving integer keys, just like the equivalent method in D8 has. I re-opened #1825466: Allow NestedArray::mergeDeepArray() to preserve integer keys for the purpose of backporting, but I don't think we need to wait for that.

The problem occurs only, because in _options_element_add_allowed_values_element() $default_element declares the #options key as an empty array. This causes drupal_array_merge_deep(_array)() to merge array() with the passed-in $element['#options'], if that exists. In all 2 cases where the function is called, however $element['#options'] is set. So we can just remove the default empty array and get rid of this problem.

This is an behavioral change in the _options_element_add_allowed_values_element() function, but since that is marked private by the leading underscore I'm not sure that should be considered an API change. One could, of course, also find 100%-BC-compatible solutions to this problem.

Remaining tasks

Review patch.

User interface changes

-

API changes

- (Maybe, see above)