Problem/Motivation
I was trying to form alter a field widget from the theme using hook_field_widget_form_alter(), which is apparently supposed to work. however it seems to me that it definitely is not possible as WidgetBase only invokes module handlers.
Proposed resolution
I believe that altering field widgets is really important for the theme layer, it's akin to traditional form alters. My proposal is to add a theme callout to the widget base.
Remaining tasks
Patch
User interface changes
N/A
API changes
No, this should bring core in line with the documented API.
Data model changes
N/A
Release notes snippet
TBA