TYPO3 Core

The "empty" check in the NotEmpty viewhelper is a bit flawed since it also accepts empty strings.

Thus the value is trimmed in the xclassed viewhelper before checking

public function isValid(mixed $value): void
    {
        $value = \is_string($value) ? trim($value) : $value;

The $acceptsEmptyValues flag is set, but the isEmpty check checks for null and empty strings

final protected function isEmpty(mixed $value): bool
    {
        return $value === null || $value === '';
    }

Maybe there could be some solution by using a distinct custom validator which is also used by the custom viewhelper?

I did not really get the meaning of this. Maybe you mean i add the core NotEmpty and a custom validation? Would take quite some code changes and testing again... And whenever a new form is added, this has to be remembered.

Maybe basing some parts on an interface can be done (instead of instanceof), and wouldn't break the contract between the core and the Validator&ViewHelper (&extbase) for final classes.

That's what i was thinking of, or an annotation to indicate that this validation means required, or maybe call a method on the the viewhelper to let it decide if it it means i am required

I'd also e.g. consider a StringLength Validator to mean "required" for a form if minLength > 0, but currently you have to explicitly set Required in the form module, meaning it adds a NotEmpty validator in the yaml.
The StringLength Validator also does not set acceptsEmptyValues to false, thus it HAS to be combined with NotEmpty

--> The wording and functionality is pretty confusing right now to get it right.

(I'm not mentioning composer patches here of course....)

Well, that's what i will go for since i don't want to patch all the custom code and EXT:form to get a consistent behavior.