[Validator] Encourage type safety

[kiler129]

Example provided at https://symfony.com/doc/current/validation/custom_constraint.html#creating-the-validator-itself shows example validation as:

    public function validate($value, Constraint $constraint)
    {
        if (!preg_match('/^[a-zA-Z0-9]+$/', $value, $matches)) {
            $this->context->buildViolation($constraint->message)
                ->setParameter('{{ string }}', $value)
                ->addViolation();
        }
    }

This is a perfectly fine example, but sets a trap creating assumption that $value is always a string. I think at minimum we should include:

        if (!is_string($value)) {
            throw new UnexpectedTypeException($value, 'string');
        }

if not even something like DateTimeValidator has:

        if (!is_scalar($value) && !(is_object($value) && method_exists($value, '__toString'))) {
            throw new UnexpectedTypeException($value, 'string');
        }

        $value = (string) $value;

WDYT?

[xabbuh]

👍 from my side. I (almost) always do this in custom constraint validators. Usually, this should never be triggered, but in case you misconfigure some validation it may greatly help debugging because you end up with much more meaningful error messages.

[javiereguiluz]

@kiler129 thanks for reporting this issue. It was fixed in #9993.