Generic abstract classes don't respect type annotations

[nikeee]

I tried to build a generic interface that has a parse method that returns an instance of T. However, the implementation seems to expect to have T in the place of the return type. This is not possible because it doesn't exist at run time and will throw an error.

Examples:

/** @template T */
interface SomeInterface {
	/** @return ?T */
	function parse(string $value): mixed;
}
/** @implements SomeInterface<bool> */
class SomeClass0 implements SomeInterface {
	function parse(string $value): ?bool { return false; }
}
/** @implements SomeInterface<int> */
class SomeClass1 implements SomeInterface {
	function parse(string $value): ?int { return 0; }
}

a.php PhanTypeMismatchReturn Returning false of type false but parse(string $value) is declared to return ?T
a.php PhanTypeMismatchReturn Returning 0 of type 0 but parse(string $value) is declared to return ?T

...did not work, so I suspected that phan has issues with the nullability here. That doesn't seem to be the case either:

/** @template T */
interface SomeInterface {
	/** @return T */
	function parse(string $value): mixed;
}
/** @implements SomeInterface<?bool> */
class SomeClass0 implements SomeInterface {
	/** @return ?bool */
	function parse(string $value): ?bool { return false; }
}
/** @implements SomeInterface<?int> */
class SomeClass1 implements SomeInterface {
	/** @return ?int */
	function parse(string $value): ?int { return 0; }
}

a.php PhanTypeMismatchReturn Returning false of type false but parse(string $value) is declared to return T
a.php PhanTypeMismatchReturn Returning 0 of type 0 but parse(string $value) is declared to return T

Next, I've read in the documentation that generic interfaces don't seem to be supported, so I tried using an abstract class. It seems to yield the same result:

/** @template T */
abstract class SomeClass {
	/** @return T */
	abstract function doWork();
}
/** @inherits SomeClass<?bool> */
class SomeClass0 extends SomeClass {
	public function __construct() { }
	function doWork(): ?bool { return false; }
}
/** @inherits SomeClass<?int> */
class SomeClass1 extends SomeClass {
	public function __construct() { }
	/** @return ?int */
	function doWork(): ?int { return 0; }
}

a.php:17 PhanParamSignatureMismatch Declaration of function doWork() : T should be compatible with function doWork() : T defined in a.php:12
a.php:17 PhanTypeMismatchReturn Returning false of type false but doWork() is declared to return T

It seems that SomeClass1 works without any issues here, the error only applies to ?bool. When adding an additional /** @return ?bool */, it seems to work. My guess is that phan sometimes ignores the types that are written in actual PHP code, not in the PHPDoc.

Is this the intended behavior? How to I type this correctly?

$ ./vendor/bin/phan --version
Phan 5.4.3
php-ast version 1.1.1
PHP version used to run Phan: 8.3.3-1+ubuntu22.04.1+deb.sury.org+1