`self` and `static` return types don't work if parent class has a constrained template type

[janopae]

Consider the following code: https://psalm.dev/r/c676544a45

It works as expected: The parent class get method returns static – therefore, when called on the child, we can be sure that the self return type is correct.

Now, let's constrain the template parameter on the parent class (but in a way where the child is still valid). Urgh! https://psalm.dev/r/6f35cf8914

Now Psalm complains that "The type 'ParentClass<object>' is more general than the declared return type 'ChildClass' for ChildClass::myGet".

We can't fix that by using static on the child return type: https://psalm.dev/r/091f9485a7

Seems like we can't fix it at all in our application code, since Psalm seems to override the type constraint over the actual known type of the template.

[psalm-github-bot[bot]]

I found these snippets:

https://psalm.dev/r/c676544a45

<?php

class A {}

/**
* @template T
*/
class ParentClass
{
	public function get(): static
    {
    	return $this;
    }
}

/**
* @template-extends ParentClass<A>
*/
final class ChildClass extends ParentClass
{
	public function myGet(): self
    {
    	return $this->get();
    } 
}

Psalm output (using commit a75d26a):

No issues!

https://psalm.dev/r/6f35cf8914

<?php

class A {}

/**
* @template T of object
*/
class ParentClass
{
	public function get(): static
    {
    	return $this;
    }
}

/**
* @template-extends ParentClass<A>
*/
final class ChildClass extends ParentClass
{
	public function myGet(): self
    {
    	return $this->get();
    } 
}

Psalm output (using commit a75d26a):

INFO: LessSpecificReturnStatement - 23:13 - The type 'ParentClass<object>' is more general than the declared return type 'ChildClass' for ChildClass::myGet

INFO: MoreSpecificReturnType - 21:27 - The declared return type 'ChildClass' for ChildClass::myGet is more specific than the inferred return type 'ParentClass<object>'

https://psalm.dev/r/091f9485a7

<?php

class A {}

/**
* @template T of object
*/
class ParentClass
{
	public function get(): static
    {
    	return $this;
    }
}

/**
* @template-extends ParentClass<A>
*/
final class ChildClass extends ParentClass
{
	public function myGet(): static
    {
    	return $this->get();
    } 
}

Psalm output (using commit a75d26a):

INFO: LessSpecificReturnStatement - 23:13 - The type 'ParentClass<object>' is more general than the declared return type 'ChildClass' for ChildClass::myGet

INFO: MoreSpecificReturnType - 21:27 - The declared return type 'ChildClass' for ChildClass::myGet is more specific than the inferred return type 'ParentClass<object>'

[weirdan]

The fix seems to be rather simple: https://psalm.dev/r/872aced924

[psalm-github-bot[bot]]

I found these snippets:

https://psalm.dev/r/872aced924

<?php

class A {}

/**
* @template T of object
*/
class ParentClass
{
    /** @return static<T> */
	public function get(): static
    {
    	return $this;
    }
}

/**
* @template-extends ParentClass<A>
*/
final class ChildClass extends ParentClass
{
	public function myGet(): self
    {
    	return $this->get();
    } 
}

Psalm output (using commit b940c7e):

No issues!

[janopae]

Thanks, @weirdan! I didn't think of a solution like this, because in my case, the parent class lives in a library.

Do you think Psalm should change its behaviour in this regard? After all, the resolved static type doesn't even support template types, so static<T> doesn't make so much sense. Otherwise, feel free to close this issue.