bug #36894 [Validator] never directly validate Existence (Required/Optional) constraints (xabbuh)

This PR was merged into the 3.4 branch.

Discussion
----------

[Validator] never directly validate Existence (Required/Optional) constraints

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #36637 #36723
| License       | MIT
| Doc PR        |

Using `Optional` or `Required` like "regular" constraints does not make any sense, but doing so didn't break before #36365. I suggest to ignore them for now and deprecate using them outside the `Collection` constraint in 5.2.

Commits
-------

d333aae187 never directly validate Existence (Required/Optional) constraints

Author: nicolas-grekas

Tests/Validator/RecursiveValidatorTest.php

@@ -19,6 +19,8 @@
 use Symfony\Component\Validator\Constraints\Length;
 use Symfony\Component\Validator\Constraints\NotBlank;
 use Symfony\Component\Validator\Constraints\NotNull;
+use Symfony\Component\Validator\Constraints\Optional;
+use Symfony\Component\Validator\Constraints\Required;
 use Symfony\Component\Validator\ConstraintValidatorFactory;
 use Symfony\Component\Validator\Context\ExecutionContextFactory;
 use Symfony\Component\Validator\Mapping\ClassMetadata;
@@ -157,4 +159,18 @@ public function testAllConstraintValidateAllGroupsForNestedConstraints()
         $this->assertInstanceOf(NotBlank::class, $violations->get(0)->getConstraint());
         $this->assertInstanceOf(Length::class, $violations->get(1)->getConstraint());
     }
+
+    public function testRequiredConstraintIsIgnored()
+    {
+        $violations = $this->validator->validate([], new Required());
+
+        $this->assertCount(0, $violations);
+    }
+
+    public function testOptionalConstraintIsIgnored()
+    {
+        $violations = $this->validator->validate([], new Optional());
+
+        $this->assertCount(0, $violations);
+    }
 }

Validator/RecursiveContextualValidator.php

@@ -13,6 +13,7 @@
 
 use Symfony\Component\Validator\Constraint;
 use Symfony\Component\Validator\Constraints\Composite;
+use Symfony\Component\Validator\Constraints\Existence;
 use Symfony\Component\Validator\Constraints\GroupSequence;
 use Symfony\Component\Validator\Constraints\Valid;
 use Symfony\Component\Validator\ConstraintValidatorFactoryInterface;
@@ -790,6 +791,10 @@ private function validateInGroup($value, $cacheKey, MetadataInterface $metadata,
         $context->setGroup($group);
 
         foreach ($metadata->findConstraints($group) as $constraint) {
+            if ($constraint instanceof Existence) {
+                continue;
+            }
+
             // Prevent duplicate validation of constraints, in the case
             // that constraints belong to multiple validated groups
             if (null !== $cacheKey) {