feature #42593 [Validator] Add the When constraint and validator (wuchen90)

This PR was merged into the 6.2 branch.

Discussion
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[Validator] Add the `When` constraint and validator

| Q             | A
| ------------- | ---
| Branch?       | 6.2
| Bug fix?      | no
| New feature?  | yes <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#15722

This constraint allows you to apply constraints validation only if the provided condition is matched.

Usage:

```php
namespace App\Model;

use Symfony\Component\Validator\Constraints as Assert;

class Discount
{
    private $type; // 'percent' or 'absolute'

    /**
     * `@Assert`\GreaterThan(0)
     * `@Assert`\When(
     *     expression="this.type == 'percent'",
     *     constraints={`@LessThan`(100, message="The value should be between 0 and 100!")}
     * )
     */
    private $value;
    // ...
}
```

See the documentation for details.

Commits
-------

9b7bdc9b18 [Validator] Add the When constraint and validator

Author: xabbuh

DependencyInjection/FrameworkExtension.php

@@ -223,6 +223,7 @@
 use Symfony\Component\Translation\Translator;
 use Symfony\Component\Uid\Factory\UuidFactory;
 use Symfony\Component\Uid\UuidV4;
+use Symfony\Component\Validator\Constraints\WhenValidator;
 use Symfony\Component\Validator\ConstraintValidatorInterface;
 use Symfony\Component\Validator\Mapping\Loader\PropertyInfoLoader;
 use Symfony\Component\Validator\ObjectInitializerInterface;
@@ -1571,6 +1572,10 @@ private function registerValidationConfiguration(array $config, ContainerBuilder
         if (!class_exists(ExpressionLanguage::class)) {
             $container->removeDefinition('validator.expression_language');
         }
+
+        if (!class_exists(WhenValidator::class)) {
+            $container->removeDefinition('validator.when');
+        }
     }
 
     private function registerValidatorMapping(ContainerBuilder $container, array $config, array &$files)

Resources/config/validator.php

@@ -17,6 +17,7 @@
 use Symfony\Component\Validator\Constraints\EmailValidator;
 use Symfony\Component\Validator\Constraints\ExpressionValidator;
 use Symfony\Component\Validator\Constraints\NotCompromisedPasswordValidator;
+use Symfony\Component\Validator\Constraints\WhenValidator;
 use Symfony\Component\Validator\ContainerConstraintValidatorFactory;
 use Symfony\Component\Validator\Mapping\Loader\PropertyInfoLoader;
 use Symfony\Component\Validator\Validation;
@@ -95,6 +96,12 @@
                 'alias' => NotCompromisedPasswordValidator::class,
             ])
 
+        ->set('validator.when', WhenValidator::class)
+            ->args([service('validator.expression_language')->nullOnInvalid()])
+            ->tag('validator.constraint_validator', [
+                'alias' => WhenValidator::class,
+            ])
+
         ->set('validator.property_info_loader', PropertyInfoLoader::class)
             ->args([
                 service('property_info'),