Add Constraint stub (#167)

src/Stubs/common/Constraint.stubphp

@@ -0,0 +1,13 @@
+<?php
+
+namespace Symfony\Component\Validator;
+
+use Symfony\Component\Validator\Context\ExecutionContextInterface;
+
+abstract class Constraint
+{
+    /**
+     * @var array<string, string>
+     */
+    protected static $errorNames = [];
+}

tests/acceptance/acceptance/Constraint.feature

@@ -0,0 +1,37 @@
+@symfony-common
+Feature: Constraint
+
+  Background:
+    Given I have the following config
+      """
+      <?xml version="1.0"?>
+      <psalm errorLevel="1">
+        <projectFiles>
+          <directory name="."/>
+          <ignoreFiles> <directory name="../../vendor"/> </ignoreFiles>
+        </projectFiles>
+        <plugins>
+          <pluginClass class="Psalm\SymfonyPsalmPlugin\Plugin"/>
+        </plugins>
+      </psalm>
+      """
+
+  Scenario: NonInvariantDocblockPropertyType error about $errorNames is not raised
+    Given I have the following code
+      """
+      <?php
+
+      use Symfony\Component\Validator\Constraint;
+
+      class CustomConstraint extends Constraint
+      {
+        /**
+         * @var array<string, string>
+         */
+        protected static $errorNames = [
+          'test' => 'test',
+        ];
+      }
+      """
+    When I run Psalm
+    Then I see no errors