Function f() should return T of A but returns A

[javaDeveloperKid]

Bug report

PHP version: 7.2.

As I need to support both \DateTime and \DateTimeImmutable, at first I used @template T of \DateTimeInterface but method modify does not exist on this interface so I moved to @template T of \DateTime|\DateTimeImmutable.
I'm not sure if @template supports union types. I couldn't determine this from docs but if it doesn't then maybe PHPStan should throw when such a notation is used?

Code snippet that reproduces the problem

https://phpstan.org/r/5a4f44a1-4266-4c04-a0db-30674b25053a

Expected output

No error.

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

Yes, it did.

[Ahummeling]

this works though.
Phpstan doesn't know that DateTime::modify returns the an instance of itself. That's why you get this error message.

[ondrejmirtes]

I'm surprised it doesn't work because these methods are already marked as returning static in functionMap. Needs more investigation.

[phpstan-bot]

@javaDeveloperKid After the latest commit in 1.7.x, PHPStan now reports different result with your code snippet:

@@ @@
-15: Function modify() should return T of DateTime|DateTimeImmutable but returns DateTime|DateTimeImmutable.
+12: Function modify() never returns DateTimeImmutable so it can be removed from the return type.

Full report

Line	Error
12	Function modify() never returns DateTimeImmutable so it can be removed from the return type.

[ondrejmirtes]

FYI @VincentLanglet we broke something :)

[VincentLanglet]

FYI @VincentLanglet we broke something :)

Yes, but I dunno why. I did

return new StaticType($methodReflection->getDeclaringClass());

But IMHO this code is not recommended

$date = $date->modify('+1 day');

let believe that it returns a new instance of DateTime which is not.

this https://phpstan.org/r/5e05cad2-18d4-4b6d-8cdf-8f54c4ae05ed might be better.
and it gives a different error message.

[ondrejmirtes]

Fixed: phpstan/phpstan-src@b5da7ce

[github-actions]

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