Add ClassStringFactoryContainer and corresponding tests for class-string<Countable> validation

Author: rcalicdan

tests/Fixtures/Types/ClassStringFactoryContainer.php

@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace TypePHP\Tests\Fixtures\Types;
+
+use Countable;
+
+class ClassStringFactoryContainer
+{
+    /**
+     * @param class-string<Countable> $class
+     */
+    public static function makeCountable(string $class): string
+    {
+        return $class;
+    }
+}

tests/TypeChecking/Scalars/ClassStringInterfaceTest.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+use TypePHP\Tests\Fixtures\Domain\Car;
+use TypePHP\Tests\Fixtures\Types\ClassStringFactoryContainer;
+use TypePHP\Tests\Fixtures\Types\CountableArrayAccess;
+
+describe('class-string<Interface> Subtype Validation', function () {
+
+    test('accepts concrete class string implementing the target interface', function () {
+        expect(ClassStringFactoryContainer::makeCountable(CountableArrayAccess::class))
+            ->toBe(CountableArrayAccess::class)
+        ;
+    });
+
+    test('accepts the interface string itself', function () {
+        expect(ClassStringFactoryContainer::makeCountable(Countable::class))
+            ->toBe(Countable::class)
+        ;
+    });
+
+    test('throws TypeError when class string does not implement the target interface', function () {
+        expect(fn () => ClassStringFactoryContainer::makeCountable(Car::class))
+            ->toThrow(TypeError::class, 'must be a class-string of Countable')
+        ;
+    });
+});