Covariance problem with template type with $this and static

[mvorisek]

Bug report

In #9153 (comment) I was advised to use @template instead of @template-covariant. But then I get another phpstan error:

Method MethodMock::methodName() should return MagicMethod<static(MethodMock)> but returns
MagicMethod<$this(MethodMock)>.
Template type TTargetClass on class MagicMethod is not covariant. Learn more:
[What's Up With @template-covariant?](https://phpstan.org/blog/whats-up-with-template-covariant)

I would say the phpstan error (with @template) is wrong, as $this is strictly a subtype of static.

Code snippet that reproduces the problem

https://phpstan.org/r/6897d3da-b2b5-4a31-b740-927310bd6e72 (with @template-covariant)

https://phpstan.org/r/75af0bc1-1122-4734-8b36-b8cc966c32df (with @template)

Expected output

no error

[ondrejmirtes]

as $this is strictly a subtype of static.

And that's exactly why this error is reported. The linked article explains it.

[mvorisek]

@ondrejmirtes Yes, @template-covariant should be probably used, but then I get the Template type TTargetClass is declared as covariant, but occurs in invariant position in property MagicAbstract::$_atk__core__hintable_magic__class. (first repro link).

[ondrejmirtes]

Yes. You can't have everything because that leads to unsafe code.

[mvorisek]

@ondrejmirtes thank you. one phpstan ignore /w @template-covariant fixed the phpstan errors.

Still have one question - why Method MethodMock::methodName() should return MagicMethod<static(MethodMock)> but returns MagicMethod<$this(MethodMock)>. is a problem with invariant template? static vs $this is no different in sense of classes, the only difference is the returned type is asserted for specific object instance, which is strictly stronger/safer assumption.

[ondrejmirtes]

@mvorisek $this(MethodMock) is a subtype of static(MethodMock). Meaning not every static(MethodMock) is also $this(MethodMock). And there's no special-case in the code to allow this even without @template-covariant.

It might start working when this suggestion is implemented #6732 (comment)

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