Template covariance false positive with nullable type on level < 8 #12008

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1. ondrejmirtes

   added

   bug

   on Nov 12, 2024
2. ondrejmirtes

   added this to the Generics milestone on Nov 12, 2024
3. ondrejmirtes commented on Nov 12, 2024

   

   ondrejmirtes

   on Nov 12, 2024

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   A solution would be to somehow employ SimultaneousTypeTraverser instead of TypeTraverser which keeps the context of both $acceptingType and $acceptedType. Then we'd be able to check the @template of the accepting side and decide if null should be removed or not.
4. piotr-cz commented on Nov 13, 2024

   

   piotr-cz

   on Nov 13, 2024

   Last edited by piotr-cz

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   Thanks for reporting, I could not understand what's wrong here after updating to v2.0.1:

   Method Tests\TestCase::getAppInstance() should return Slim\App<Psr\Container\ContainerInterface|null> but returns Slim\App<Psr\Container\ContainerInterface|null>.
   🪪  return.type
   💡 Template type TContainerInterface on class Slim\App is not covariant. Learn more: https://phpstan.org/blog/whats-up-with-template-covariant
   

   By the way, it's referring to Slim/App
5. VincentLanglet commented on Aug 3, 2025

   

   VincentLanglet

   on Aug 3, 2025

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   Just discovered this, and it seems like there is at least 5 issues open for this (maybe you'll want to close some)
   cf #9096 (comment)

   A solution would be to somehow employ SimultaneousTypeTraverser instead of TypeTraverser which keeps the context of both $acceptingType and $acceptedType. Then we'd be able to check the @template of the accepting side and decide if null should be removed or not.

   I tried, but as soon as TypeTraverser is replaced by SimultaneousTypeTraverser, some tests are failing.

   For instance when comparing array<string> with list<string|null>, since
   traverseSimultaneously with Array&List, Array the early IntersectionType check

   if (!$right instanceof self) {
         return $this;
   }
   

   prevents to keep traversing the array keys/values.

   I dunno how to handle this...

   The only workaround I found so far would be to have an early check

   $acceptingType->accepts($acceptedType)
   

   to remove lot of false positive (is it ok enough for you ?), but it won't fully solve the general issue.
6. ondrejmirtes commented on Aug 5, 2025

   

   ondrejmirtes

   on Aug 5, 2025

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   @VincentLanglet Please try to do a SimultaneousTypeTraverser proof-of-concept that would solve the GenericObjectType problem with levels.

   If it works then we can look into these issues, probably by fixing IntersectionType::traverseSimultaneously() for lists/arrays etc.
7. VincentLanglet commented on Aug 6, 2025

   

   VincentLanglet

   on Aug 6, 2025

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   @VincentLanglet Please try to do a SimultaneousTypeTraverser proof-of-concept that would solve the GenericObjectType problem with levels.

   If it works then we can look into these issues, probably by fixing IntersectionType::traverseSimultaneously() for lists/arrays etc.

   Poc opened phpstan/phpstan-src#4210

   👍React with 👍1Reacted by Piotr
8. phpstan-bot commented on Sep 10, 2026

   

   phpstan-bot

   on Sep 10, 2026

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   Closed via merging phpstan/phpstan-src#6392
9. phpstan-bot

   closed this as completedon Sep 10, 2026
10. github-actions commented on Oct 12, 2026

    

    github-actionsBot

    on Oct 12, 2026 – with GitHub Actions

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    This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.
11. github-actions

    locked as resolved and limited conversation to collaborators on Oct 12, 2026