I'm calling out for you to decide how to resolve the inconsistency between the Form::isValid() method and the valid variable available in the template:

• isValid() returns false if a form was not submitted. This way it is possible to write concise controller code:

$form = $this->createForm(...);
$form->handleRequest($request);

if ($form->isValid()) {
    // only executed if the form is submitted AND valid
}

• valid contains true if a form was not submitted. This way it is possible to rely on this variable for error styling of a form.

<div{% if not form.vars.valid %} class="error"{% endif %}>

We have two alternatives for resolving this problem:

1. Leave the inconsistency as is.
2. Make isValid() return true if a form was not submitted (consistent with valid)
3. Revert to the 2.2 behavior of throwing an exception if isValid() is called on a non-submitted form (not consistent with valid).

Both 2. and 3. will require additional code in the controller:

$form = $this->createForm(...);
$form->handleRequest($request);

if ($form->isSubmitted() && $form->isValid()) {
    // only executed if the form is submitted AND valid
}

What do you think?