Fix false positive for invariant generic types when checkNullables is off

- At rule levels 3-7, RuleLevelHelper strips null from accepted type arguments
- This caused Trap<int|null, A|null> to become Trap<int, A> on the accepted side
- Invariant template check uses equals(), so int|null ≠ int → false positive
- Added fallback in GenericObjectType::isSuperTypeOfInternal() to detect when
  the only difference is null stripping and accept the type in that case
- New regression test in tests/PHPStan/Rules/Properties/data/bug-13876.php

Closes phpstan/phpstan#13876

Author: ondrejmirtes

src/Type/Generic/GenericObjectType.php

@@ -20,6 +20,7 @@
 use PHPStan\Type\IsSuperTypeOfResult;
 use PHPStan\Type\ObjectType;
 use PHPStan\Type\Type;
+use PHPStan\Type\TypeCombinator;
 use PHPStan\Type\TypeWithClassName;
 use PHPStan\Type\UnionType;
 use PHPStan\Type\VerbosityLevel;
@@ -186,7 +187,17 @@ private function isSuperTypeOfInternal(Type $type, bool $acceptsContext): IsSupe
 			if (!$thisVariance->invariant()) {
 				$results[] = $thisVariance->isValidVariance($templateType, $this->types[$i], $ancestor->types[$i]);
 			} else {
-				$results[] = $templateType->isValidVariance($this->types[$i], $ancestor->types[$i]);
+				$result = $templateType->isValidVariance($this->types[$i], $ancestor->types[$i]);
+				if ($acceptsContext && !$result->yes()) {
+					$thisWithoutNull = TypeCombinator::removeNull($this->types[$i]);
+					if (
+						$this->types[$i]->isSuperTypeOf($ancestor->types[$i])->yes()
+						&& $ancestor->types[$i]->isSuperTypeOf($thisWithoutNull)->yes()
+					) {
+						$result = IsSuperTypeOfResult::createYes();
+					}
+				}
+				$results[] = $result;
 			}
 
 			$results[] = IsSuperTypeOfResult::createFromBoolean($thisVariance->validPosition($ancestorVariance));

tests/PHPStan/Rules/Properties/TypesAssignedToPropertiesRuleTest.php

@@ -13,13 +13,15 @@
 class TypesAssignedToPropertiesRuleTest extends RuleTestCase
 {
 
+	private bool $checkNullables = true;
+
 	private bool $checkExplicitMixed = false;
 
 	private bool $checkImplicitMixed = false;
 
 	protected function getRule(): Rule
 	{
-		return new TypesAssignedToPropertiesRule(new RuleLevelHelper(self::createReflectionProvider(), true, false, true, $this->checkExplicitMixed, $this->checkImplicitMixed, false, true), new PropertyReflectionFinder());
+		return new TypesAssignedToPropertiesRule(new RuleLevelHelper(self::createReflectionProvider(), $this->checkNullables, false, true, $this->checkExplicitMixed, $this->checkImplicitMixed, false, true), new PropertyReflectionFinder());
 	}
 
 	public function testTypesAssignedToProperties(): void
@@ -1038,4 +1040,10 @@ public function testBug4525(): void
 		$this->analyse([__DIR__ . '/data/bug-4525.php'], []);
 	}
 
+	public function testBug13876(): void
+	{
+		$this->checkNullables = false;
+		$this->analyse([__DIR__ . '/data/bug-13876.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Properties/data/bug-13876.php

@@ -0,0 +1,68 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13876;
+
+/**
+ * @template BAIT
+ * @template PROMISED
+ */
+class Trap
+{
+
+	/**
+	 * @var BAIT
+	 */
+	private $bait;
+
+	/**
+	 * @var \Closure(BAIT):PROMISED
+	 */
+	private $switch;
+
+	/**
+	 * @param BAIT $bait
+	 * @param \Closure(BAIT):PROMISED $switch
+	 */
+	public function __construct($bait, \Closure $switch)
+	{
+		$this->bait = $bait;
+		$this->switch = $switch;
+	}
+
+	/**
+	 * @return PROMISED
+	 */
+	public function fall()
+	{
+		return ($this->switch)($this->bait);
+	}
+}
+
+class A {}
+
+class B {
+
+	/**
+	 * @var Trap<int|null, A|null>
+	 */
+	private Trap $b;
+
+	public function __construct() {
+
+		/**
+		 * @var Trap<int|null, A|null>
+		 */
+		$nullPerson = new Trap(null, function (): ?A {
+			return null;
+		});
+
+		$this->b = $nullPerson;
+	}
+
+	/**
+	 * @return ?A
+	 */
+	public function getB() {
+		return $this->b->fall();
+	}
+}