Narrowing a type by extending loses information of parent

[BackEndTea]

Bug report

When a method has a return type, which gets narrowed in the child class, any information from the parent seems to be lost.

I think that this could be solved by making a child return type an intersection between its return type and that of the parent?

Code snippet that reproduces the problem

https://phpstan.org/r/9d725389-4e11-4b53-aa3f-df765c811e21

Expected output

I'd expect the dumped types to be array<string> for the Fail class, and array<string>|(callable(): array<string>) for the Good class (which already works).

And there should be no error for Fail::check.

Did PHPStan help you today? Did it make you happy in any way?

No response

[staabm]

I think you should use generics: https://phpstan.org/r/c3562717-43a9-4f1c-ad23-416386d1afe2

[BackEndTea]

At that point i could also just add the @return array<string> myself. That also works, but seems unnecessary when all i'm doing is narrowing the type.

[ondrejmirtes]

Right now you need to repeat the @return. It's a known issue and already is present here on GitHub.

[BackEndTea]

@ondrejmirtes Could you link me to the issue? I can't seem to find it. I'd love to take a stab at this if i can, and that might be easier with more context from the other issue.

[ondrejmirtes]

I'll be damned, can't find it either. Alright, this one will be the canonical one :)

[phpstan-bot]

@staabm After the latest push in 2.0.x, PHPStan now reports different result with your code snippet:

@@ @@
 29: Dumped type: array
 30: Dumped type: array|(callable(): mixed)
 
-PHP 7.2 – 7.4 (3 errors)
+PHP 7.4 (7 errors)
 ==========
 
+ 9: Method Base::check() has invalid return type mixed.
+ 9: PHPDoc tag @return with type T is not subtype of native type mixed.
+15: Return type array of method Fail::check() is not covariant with return type mixed|null of method Base<array>::check().
 24: Method Good::check() uses native union types but they're supported only on PHP 8.0 and later.
+24: Return type array|(callable) of method Good::check() is not covariant with return type mixed|null of method Base<array|(callable)>::check().
+29: Dumped type: array
+30: Dumped type: array|(callable(): mixed)
+
+PHP 7.2 – 7.3 (7 errors)
+==========
+
+ 9: Method Base::check() has invalid return type mixed.
+ 9: PHPDoc tag @return with type T is not subtype of native type mixed.
+15: Return type array of method Fail::check() is not compatible with return type mixed|null of method Base<array>::check().
+24: Method Good::check() uses native union types but they're supported only on PHP 8.0 and later.
+24: Return type array|(callable) of method Good::check() is not compatible with return type mixed|null of method Base<array|(callable)>::check().
 29: Dumped type: array
 30: Dumped type: array|(callable(): mixed)

Full report

PHP 8.0 – 8.3 (2 errors)

Line	Error
29	Dumped type: array
30	`Dumped type: array

PHP 7.4 (7 errors)

Line	Error
9	Method Base::check() has invalid return type mixed.
9	PHPDoc tag @return with type T is not subtype of native type mixed.
15	`Return type array of method Fail::check() is not covariant with return type mixed
24	Method Good::check() uses native union types but they're supported only on PHP 8.0 and later.
24	`Return type array
29	Dumped type: array
30	`Dumped type: array

PHP 7.2 – 7.3 (7 errors)

Line	Error
9	Method Base::check() has invalid return type mixed.
9	PHPDoc tag @return with type T is not subtype of native type mixed.
15	`Return type array of method Fail::check() is not compatible with return type mixed
24	Method Good::check() uses native union types but they're supported only on PHP 8.0 and later.
24	`Return type array
29	Dumped type: array
30	`Dumped type: array

[phpstan-bot]

@staabm After the latest push in 2.0.x, PHPStan now reports different result with your code snippet:

@@ @@
 PHP 8.0 – 8.3 (2 errors)
 ==========
 
-29: Dumped type: array
-30: Dumped type: array|(callable(): mixed)
+29: Dumped type: array<mixed>
+30: Dumped type: array<mixed>|(callable(): mixed)
 
-PHP 7.2 – 7.4 (3 errors)
+PHP 7.4 (7 errors)
 ==========
 
+ 9: Method Base::check() has invalid return type mixed.
+ 9: PHPDoc tag @return with type T is not subtype of native type mixed.
+15: Return type array of method Fail::check() is not covariant with return type mixed|null of method Base<array<mixed>>::check().
 24: Method Good::check() uses native union types but they're supported only on PHP 8.0 and later.
-29: Dumped type: array
-30: Dumped type: array|(callable(): mixed)
+24: Return type array|(callable) of method Good::check() is not covariant with return type mixed|null of method Base<array<mixed>|(callable)>::check().
+29: Dumped type: array<mixed>
+30: Dumped type: array<mixed>|(callable(): mixed)
+
+PHP 7.2 – 7.3 (7 errors)
+==========
+
+ 9: Method Base::check() has invalid return type mixed.
+ 9: PHPDoc tag @return with type T is not subtype of native type mixed.
+15: Return type array of method Fail::check() is not compatible with return type mixed|null of method Base<array<mixed>>::check().
+24: Method Good::check() uses native union types but they're supported only on PHP 8.0 and later.
+24: Return type array|(callable) of method Good::check() is not compatible with return type mixed|null of method Base<array<mixed>|(callable)>::check().
+29: Dumped type: array<mixed>
+30: Dumped type: array<mixed>|(callable(): mixed)

Full report

PHP 8.0 – 8.3 (2 errors)

Line	Error
29	Dumped type: array<mixed>
30	`Dumped type: array

PHP 7.4 (7 errors)

Line	Error
9	Method Base::check() has invalid return type mixed.
9	PHPDoc tag @return with type T is not subtype of native type mixed.
15	`Return type array of method Fail::check() is not covariant with return type mixed
24	Method Good::check() uses native union types but they're supported only on PHP 8.0 and later.
24	`Return type array
29	Dumped type: array<mixed>
30	`Dumped type: array

PHP 7.2 – 7.3 (7 errors)

Line	Error
9	Method Base::check() has invalid return type mixed.
9	PHPDoc tag @return with type T is not subtype of native type mixed.
15	`Return type array of method Fail::check() is not compatible with return type mixed
24	Method Good::check() uses native union types but they're supported only on PHP 8.0 and later.
24	`Return type array
29	Dumped type: array<mixed>
30	`Dumped type: array