use conditional parameter for is_a() via stub

src/Analyser/DirectScopeFactory.php

@@ -23,6 +23,9 @@
 class DirectScopeFactory implements ScopeFactory
 {
 
+	/**
+	 * @param class-string $scopeClass
+	 */
 	public function __construct(
 		private string $scopeClass,
 		private ReflectionProvider $reflectionProvider,

src/Analyser/LazyScopeFactory.php

@@ -24,6 +24,9 @@ class LazyScopeFactory implements ScopeFactory
 
 	private bool $explicitMixedInUnknownGenericNew;
 
+	/**
+	 * @param class-string $scopeClass
+	 */
 	public function __construct(
 		private string $scopeClass,
 		private Container $container,

stubs/core.stub

@@ -1,7 +1,7 @@
 <?php
 
 /**
- * @param mixed $object_or_class
+ * @param ($allow_string is false ? object : object|string) $object_or_class
  * @param class-string $class
  * @param bool $allow_string
  * @return ($allow_string is false ? ($object_or_class is object ? bool : false) : bool)

tests/PHPStan/Rules/Functions/CallToFunctionParametersRuleTest.php

@@ -949,4 +949,34 @@ public function testBug7017(): void
 		$this->analyse([__DIR__ . '/data/bug-7017.php'], $errors);
 	}
 
+	public function testBug4371(): void
+	{
+		$errors = [
+			[
+				'Parameter #1 $object_or_class of function is_a expects object, string given.',
+				14,
+			],
+			[
+				'Parameter #1 $object_or_class of function is_a expects object, string given.',
+				22,
+			],
+		];
+
+		if (PHP_VERSION_ID < 80000) {
+			// php 7.x had different parameter names
+			$errors = [
+				[
+					'Parameter #1 $object_or_string of function is_a expects object, string given.',
+					14,
+				],
+				[
+					'Parameter #1 $object_or_string of function is_a expects object, string given.',
+					22,
+				],
+			];
+		}
+
+		$this->analyse([__DIR__ . '/data/bug-4371.php'], $errors);
+	}
+
 }

tests/PHPStan/Rules/Functions/data/bug-4371.php

@@ -0,0 +1,34 @@
+<?php declare(strict_types = 1);
+
+namespace Bug4371;
+
+class Foo {
+}
+
+class Bar extends Foo {
+
+}
+
+class Hello {
+	public function foo() {
+		if(is_a(Bar::class, Foo::class)) { // should error
+			echo "This will never be true";
+		} else {
+			echo "NO";
+		}
+	}
+
+	public function bar() {
+		if(is_a(Bar::class, Foo::class, false)) { // should error
+			echo "This will never be true";
+		} else {
+			echo "NO";
+		}
+	}
+
+	public function allFine() {
+		if(is_a(Bar::class, Foo::class, true)) { // no error
+			echo "All good";
+		}
+	}
+}