Add regression test for #10040

Closes phpstan/phpstan#10040

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-10040.php

@@ -0,0 +1,34 @@
+<?php declare(strict_types = 1); // lint >= 8.0
+
+namespace Bug10040;
+
+use function PHPStan\Testing\assertType;
+
+class A
+{
+	public string|null $foo;
+}
+
+/**
+ * @phpstan-assert-if-true !null $a->foo
+ */
+function assertStringNotNull(A $a): bool
+{
+	return true;
+}
+
+$a1 = new A();
+if(assertStringNotNull($a1)){
+	assertType('string', $a1->foo);
+}
+
+$a2 = new A();
+$stringIsNotNull = assertStringNotNull($a2);
+if($stringIsNotNull){
+	assertType('string', $a2->foo);
+}
+
+$a3 = new A();
+if($stringIsNotNull = assertStringNotNull($a3)){
+	assertType('string', $a3->foo);
+}