Consider a field that lets a user set a value based on a category, like this:
{{{field|FieldName|input type=text with autocomplete|values from category=Category}}}
Currently, the field will allow the user to save a value that does not exist. I propose that the field can be defined so that the saved value is limited to existing values, like this:
{{{field|FieldName|input type=text with autocomplete|values from category=Category|existing values only}}}
The user could still type in a non-existing page, but existence is verified after the user clicks the "Save page" button. If the value does not exist within the "values from" parameter, an error message displays and the form stays open. That error message behavior would match that of a mandatory field that is empty when submitted.
Most of the text of this report was copied from here:
https://bugzilla.wikimedia.org/show_bug.cgi?id=26088
and based on this discussion:
http://wikimedia.7.n6.nabble.com/quot-one-to-many-quot-validation-td678004.html
Version: unspecified
Severity: enhancement