Add Stub for Trait to make phpstan happy

- it's not advised to test traits isolated from the code that actually uses the trait
sebastianbergmann/phpunit#5958
sebastianbergmann/phpunit#5244
- lots of code around mocking traits for that matter got deprecated in phpunit because of that

- that's why we try to use a stub class that uses the trait artificially to test its private method

src/CanValidateField.php

@@ -5,8 +5,8 @@
 namespace WMDE\FunValidators;
 
 /**
- * @license GPL-2.0-or-later
- * @author Gabriel Birke < [email protected] >
+ * @deprecated this trait is used in other repositories and cannot be tested here properly,
+ * thus it should be turned into a helper class instead or get the behaviour extracted in another way
  */
 trait CanValidateField {
 

tests/Unit/CanValidateFieldTest.php

@@ -0,0 +1,38 @@
+<?php
+
+declare( strict_types = 1 );
+
+namespace WMDE\FunValidators\Tests\Unit;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+use WMDE\FunValidators\CanValidateField;
+use WMDE\FunValidators\ConstraintViolation;
+use WMDE\FunValidators\ValidationResult;
+
+#[CoversClass( CanValidateField::class )]
+#[CoversClass( CanValidateFieldTraitStub::class )]
+class CanValidateFieldTest extends TestCase {
+
+	public function testGivenValidResult_returnsNoValidations(): void {
+		$canValidateFieldTraitStub = new CanValidateFieldTraitStub();
+
+		$validValidationResult = new ValidationResult();
+
+		$this->assertNull(
+			$canValidateFieldTraitStub->publicWrapperMethodForGetFieldViolationMethod( $validValidationResult )
+		);
+	}
+
+	public function testGivenResultWithConstraints_returnsConstraintViolations(): void {
+		$canValidateFieldTraitStub = new CanValidateFieldTraitStub();
+
+		$constraintViolation = new ConstraintViolation( "a", "b" );
+		$validValidationResult = new ValidationResult( $constraintViolation );
+
+		$this->assertEquals(
+			$constraintViolation,
+			$canValidateFieldTraitStub->publicWrapperMethodForGetFieldViolationMethod( $validValidationResult )
+		);
+	}
+}

tests/Unit/CanValidateFieldTraitStub.php

@@ -0,0 +1,22 @@
+<?php
+
+declare( strict_types = 1 );
+
+namespace WMDE\FunValidators\Tests\Unit;
+
+use WMDE\FunValidators\CanValidateField;
+use WMDE\FunValidators\ConstraintViolation;
+use WMDE\FunValidators\ValidationResult;
+
+class CanValidateFieldTraitStub {
+
+	use CanValidateField;
+
+	public function publicWrapperMethodForGetFieldViolationMethod( ValidationResult $validationResult ): ?ConstraintViolation {
+		return $this->getFieldViolation(
+			$validationResult,
+			"a field name"
+		);
+	}
+
+}