added failling test

Author: staabm

tests/PHPStan/Rules/Methods/CallMethodsRuleTest.php

@@ -3667,4 +3667,15 @@ public function testBug12735(): void
 		$this->analyse([__DIR__ . '/data/bug-12735.php'], []);
 	}
 
+	public function testBug12735b(): void
+	{
+		$this->checkThisOnly = false;
+		$this->checkNullables = true;
+		$this->checkUnionTypes = true;
+		$this->checkExplicitMixed = true;
+		$this->checkImplicitMixed = true;
+
+		$this->analyse([__DIR__ . '/data/bug-12735b.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Methods/data/bug-12735b.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Bug12735b;
+
+use DateTimeImmutable;
+
+class HelloWorld
+{
+	public function test(): void
+	{
+		$now = null;
+		if (rand(0,1)) {
+			$now = new DateTimeImmutable();
+		}
+
+		$this->foo(
+			$now ??= new DateTimeImmutable(),
+			$now,
+		);
+
+		$this->foo($now, $now);
+	}
+
+	private function foo(DateTimeImmutable $a, DateTimeImmutable $b): void {}
+}