Add non regression test (phpstan#5109)

Author: VincentLanglet

tests/PHPStan/Analyser/nsrt/pr-5108.php

@@ -0,0 +1,43 @@
+<?php declare(strict_types = 1);
+
+namespace Pr5108;
+
+use function PHPStan\Testing\assertType;
+
+class A
+{
+}
+
+interface I
+{
+	function method(): void;
+}
+
+class Testing
+{
+	/**
+	 * @param A|null $arg
+	 * @param-out ($arg is null ? A&I : A) $arg
+	 */
+	public static function testMethod(?A &$arg = null): void
+	{
+		if ($arg === null) {
+			$arg = new A();
+		}
+	}
+
+	public function doTest(A $a): void
+	{
+		Testing::testMethod($a);
+		assertType('Pr5108\A', $a);
+
+		$b = null;
+		Testing::testMethod($b);
+		assertType('Pr5108\A&Pr5108\I', $b);
+
+		$d = $a->getFoo();
+		assertType('*ERROR*', $d);
+		Testing::testMethod($d);
+		assertType('Pr5108\A', $d);
+	}
+}