Breaking errors in D11.2 or D11.x

This looks good to me. I think given 3x is beta, release notes should just make it clear in case anyone is extending any constraints, that they should add the return types + types to their messages in order to remain compatible.

E.g.

class Class1 {
	public string $message = 'Great message.';
}

class Class2 extends Class1 {
	public $message = 'Great message.';
}
$class = new Class2();

Will result in

Fatal error: Type of Class2::$message must be string (as in class Class1)

And

class Class1 {
	public function myFunction(): string {
		return 'hello1';
	}
}

class Class2 extends Class1 {
	public function myFunction() {
		return 'hello2';
	}
}
$class = new Class2();
print $class->myFunction();

Will result in

Fatal error: Declaration of Class2::myFunction() must be compatible with Class1::myFunction(): string

Patch worked for me as well.
Was able to run update hooks successfully.

Here's the log while upgrading to Drupal 11.2.0.rc2

  Unable to decode output into JSON: Syntax error                                              
                                                                                               
  Fatal error: Declaration of Drupal\field_validation\Plugin\Validation\Constraint\JsonConstr  
  aint::validatedBy() must be compatible with Symfony\Component\Validator\Constraint::validat  
  edBy(): string in /var/www/html/d11site/web/modules/contrib/field_validation/src/Plugin/Valid  
  ation/Constraint/JsonConstraint.php on line 21  

MR !15 solved the problem.
+1 for RTBC

tested this patch when upgrading, from core 11.1.7 to 11.2.0, updates were failing with

PHP Fatal error: Declaration of Drupal\field_validation\Plugin\Validation\Constraint\TimeConstraint::validatedBy() must be compatible with Symfony\Component\Validator\Constraint::validatedBy(): string in /var/www/sites/drupal/multisite/web/modules/contrib/field_validation/src/Plugin/Validation/Constraint/TimeConstraint.php on line 21

after patch updates all ran fine.

Tested patch from #9 while upgrading from d10 to D11.2.2. This patched worked great, and eliminated the error I had been seeing upon trying update the database via drush:

Fatal error: Type of Drupal\field_validation\Plugin\Validation\Constraint\LessThanOrEqualConstraint::$message must
be string (as in class Symfony\Component\Validator\Constraints\LessThanOrEqual) in /var/www/html/web/modules/contri
b/field_validation/src/Plugin/Validation/Constraint/LessThanOrEqualConstraint.php on line 16

Definitely major, this breaks projects and updates. Any chance to tag a new release?

This works as expected, and this is a major blocker, can anyone tag a new release please?

Thank you all for moving this forward. I have created a new release: field_validation 3.0.0-beta5.