Will this even work when $value is an array and $target is no ChoiceFormField?

Yes. I didn't change this behaviour. For example, the following is allowed:

// for a field named foo[bar][baz]
$registry->set('foo[bar]', ['baz' => 'fbb']);

Btw, notice I have changed ChoiceFormField to FormField in the condition.

Yeah, that's why I'm asking. :) Previously, if the field was no ChoiceFormField and the input was an array, the code that would have been executed was:

$fields = self::create($name, $value);
foreach ($fields->all() as $k => $v) {
    $this->set($k, $v);
}

Now, it will simply be $target->setValue($value).

@xabbuh there was an assumption that if the value is not an array, the $target is always an object. ChoiceFormField is the only implementation of FormField that works with arrays, therefore the special condition was there. FormFieldRegistry::create() expects a string as its first argument, and it would never worked with any of the FormField classes.

I think how conditions are currently organised makes it easier to understand.

@jakzal I mean something like the following:

$registry = new FormFieldRegistry();
$registry->add($this->getFormFieldMock('bar'));
$registry->set('bar', array('baz'));

Before, you got an error message like InvalidArgumentException: Unreachable field "0". Now, the value will be implicitly cast to string. Is that really the desired behaviour?

I don't think it's cast to string, it simply sets the array as a value. I'll add this test case. Cheers!

You're right, it is cast to string ;)

$fbb is never used.