use conditional return type for is_a() via stub (#1310)

Author: staabm

stubs/core.stub

@@ -1,5 +1,13 @@
 <?php
 
+/**
+ * @param mixed $object_or_class
+ * @param class-string $class
+ * @param bool $allow_string
+ * @return ($allow_string is false ? ($object_or_class is object ? bool : false) : bool)
+ */
+function is_a($object_or_class, string $class, $allow_string = false): bool{}
+
 /**
  * @param mixed $var
  * @param bool $return

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -885,6 +885,7 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/pr-1244.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-7144.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-7144-composer-integration.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-4371.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/bug-4371.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Bug4371;
+
+use function PHPStan\Testing\assertType;
+
+class HelloWorld
+{
+	/**
+	 * @param Exception $exception
+	 * @param class-string $classString
+	 */
+	public function sayHello(Exception $exception, $classString): void
+	{
+		assertType('bool', is_a($exception, $classString));
+		assertType('bool', is_a($exception, $classString, true));
+		assertType('bool', is_a($exception, $classString, false));
+
+		$exceptionClass = get_class($exception);
+		assertType('false', is_a($exceptionClass, $classString));
+		assertType('bool', is_a($exceptionClass, $classString, true));
+		assertType('false', is_a($exceptionClass, $classString, false));
+	}
+}