Fix phpstan/phpstan#13876: Covariance: Error reported on L6, but not L10

[phpstan-bot]

Summary

Fix false positive error "Property B::$b (Trap<int|null, A|null>) does not accept Trap<int|null, A|null>" that occurred at rule levels 3-7 when assigning a generic object with nullable template arguments to a property with the same generic type.

Changes

• Modified src/Type/Generic/GenericObjectType.php: Added a fallback check in isSuperTypeOfInternal() for accepts context when invariant template variance check fails. If the only difference between the accepting and accepted type arguments is null (stripped by RuleLevelHelper::transformAcceptedType()), the type is accepted.
• Added $checkNullables property to tests/PHPStan/Rules/Properties/TypesAssignedToPropertiesRuleTest.php to allow testing with checkNullables = false.
• New regression test data in tests/PHPStan/Rules/Properties/data/bug-13876.php.

Root cause

At rule levels 3-7, RuleLevelHelper::transformAcceptedType() traverses the accepted type and removes null via TypeCombinator::removeNull(). This traversal descends into generic type arguments through GenericObjectType::traverse(), transforming Trap<int|null, A|null> into Trap<int, A> on the accepted side. The accepting type (property type) retains its nulls as Trap<int|null, A|null>.

For invariant template parameters, TemplateTypeVariance::isValidVariance() uses strict Type::equals() checking. Since int|null ≠ int and A|null ≠ A, the check fails and a false positive is reported - even though both types are semantically identical before null stripping.

The fix detects this situation in accepts context: when the accepting type argument is a supertype of the accepted argument AND the accepted argument is a supertype of the accepting argument with null removed, the types differ only by null stripping and should be accepted.

Test

Added testBug13876() in TypesAssignedToPropertiesRuleTest that sets checkNullables = false and verifies no errors are reported when assigning Trap<int|null, A|null> to a property of type Trap<int|null, A|null>.

Fixes phpstan/phpstan#13876

[VincentLanglet]

It might require more work but I think the right fix would be this one #4210