Standalone null with default value null does not make parameter implicitly nullable

Author: ondrejmirtes

src/Rules/FunctionDefinitionCheck.php

@@ -710,6 +710,13 @@ private function checkImplicitlyNullableType(
 			return null;
 		}
 
+		if ($type instanceof Identifier && strtolower($type->name) === 'null') {
+			return null;
+		}
+		if ($type instanceof Name && $type->toLowerString() === 'null') {
+			return null;
+		}
+
 		if ($type instanceof UnionType) {
 			foreach ($type->types as $innerType) {
 				if ($innerType instanceof Identifier && strtolower($innerType->name) === 'null') {

tests/PHPStan/Rules/Methods/ExistingClassesInTypehintsRuleTest.php

@@ -548,4 +548,12 @@ public function testDeprecatedImplicitlyNullableParameterType(): void
 		]);
 	}
 
+	public function testBug12501(): void
+	{
+		if (PHP_VERSION_ID < 80400) {
+			self::markTestSkipped('This test needs PHP 8.4.');
+		}
+		$this->analyse([__DIR__ . '/data/bug-12501.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Methods/data/bug-12501.php

@@ -0,0 +1,11 @@
+<?php // lint >= 8.4
+
+declare(strict_types = 1);
+
+namespace Bug12501;
+
+final readonly class EmptyObject {
+	public function __construct(
+		public null $value1 = null,
+	) {}
+}