Use traverseSimultaneously to properly handle null removal in GenericObjectType type arguments

Instead of completely blocking traversal into GenericObjectType, use
traverseSimultaneously to pair up accepted and accepting type arguments.
Only remove null from accepted type arguments where the corresponding
accepting type argument does not contain null.

This ensures that:
- Container<string|null> accepted where Container<string|null> is expected
  does not produce a false positive (null not removed when accepting also has null)
- Container<string|null> accepted where Container<string> is expected
  correctly has null removed (accepted matches after transformation)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

src/Rules/RuleLevelHelper.php

@@ -128,6 +128,16 @@ private function transformAcceptedType(Type $acceptingType, Type $acceptedType):
 			}
 
 			if ($acceptedType instanceof GenericObjectType) {
+				if (!$this->checkNullables && $acceptingType instanceof GenericObjectType) {
+					return $acceptedType->traverseSimultaneously($acceptingType, static function (Type $acceptedInner, Type $acceptingInner): Type {
+						if (TypeCombinator::containsNull($acceptingInner)) {
+							return $acceptedInner;
+						}
+
+						return TypeCombinator::removeNull($acceptedInner);
+					});
+				}
+
 				return $acceptedType;
 			}
 

tests/PHPStan/Rules/Methods/data/bug-12490.php

@@ -61,3 +61,29 @@ protected function durationMs(): Attribute
 		return Attribute::get(fn () => $this->id);
 	}
 }
+
+/**
+ * @template T
+ */
+class Container
+{
+	/** @var T */
+	public $value;
+
+	/**
+	 * @param T $value
+	 */
+	public function __construct($value)
+	{
+		$this->value = $value;
+	}
+}
+
+class Bar
+{
+	/** @return Container<string> */
+	public function test(?string $val): Container
+	{
+		return new Container($val);
+	}
+}