Problem/Motivation
When a form embeds a subform (see https://www.drupal.org/node/2774077 for a background), the parent form should explicitly call the subform validators. That means the parent form must be aware about subform details. But it's possible that, as the code evolves, a 3rd-party module is adding more validators to the subform, by implementing form alters. Then the parent form is out-of-sync and will perform only some validations on subform.
The attached test shows that, when calling the main subform validation, the other validators are ignored.
Proposed resolution
Implement a method to rule all subform validations once. Where? To be defined...
Remaining tasks
None.
User interface changes
None.
API changes
TBD.
Data model changes
None.
Release notes snippet
TBD.