Report always false usages of is_a

[b1rdex]

Fixes phpstan/phpstan#4371

[ondrejmirtes]

Hi, could you achieve the same behaviour by modifying IsAFunctionTypeSpecifyingExtension instead? The ImpossibleCheckTypeHelper is there to course-correct mainly "weird" information from extensions, so making the fix in IsAFunctionTypeSpecifyingExtension might be a little harder, but much more correct. Thanks.

[b1rdex]

If I understand IsAFunctionTypeSpecifyingExtension correctly, it's used to set a type of $object var. Should I implement DynamicFunctionReturnTypeExtension also or what?

[ondrejmirtes]

@b1rdex Nope, just the logic of IsAFunctionTypeSpecifyingExtension needs to be modified, there's a general dynamic return type extension for type-specifying functions.

[b1rdex]

So I finally learned the thing:

return $this->typeSpecifier->create(
	$node,
	new ConstantBooleanType(false),
	$context
);

sets the function return type to false. That's what we need... But it won't generate any error message. How do I test it? I tried

if (is_a(Bar::class, Foo::class)) {
	dumpType(is_a(Bar::class, Foo::class));
}

and it dumps false w/o any errors reported on max level. Also, looks like the type specifying works only in conditional context – I tried

if ($a = is_a(Bar::class, Foo::class)) {
	dumpType($a);
}

and it dumps true which is impossible.

[staabm]

@ondrejmirtes I think we can close this one as well then

[staabm]

If I understand IsAFunctionTypeSpecifyingExtension correctly, it's used to set a type of $object var. Should I implement DynamicFunctionReturnTypeExtension also or what?

So I finally learned the thing:

return $this->typeSpecifier->create(
	$node,
	new ConstantBooleanType(false),
	$context
);

just posting the anwser here, since I also worked thru this recently:

the cited code is not correct. instead it works like this:

• the actual type needs to be set like in

  phpstan-src/src/Type/Php/IsAFunctionTypeSpecifyingExtension.php

  Lines 49 to 55 in 9d9dcd8

  	return $this->typeSpecifier->create(
  	$node->getArgs()[0]->value,
  	$this->isAFunctionTypeSpecifyingHelper->determineType($objectOrClassType, $classType, $allowString, true),
  	$context,
  	false,
  	$scope,
  	);

• the returned type is then combined with the previous type in

  phpstan-src/src/Analyser/TypeSpecifier.php

  Lines 1019 to 1025 in 9d9dcd8

  	if ($scope !== null) {
  	if ($context->true()) {
  	$resultType = TypeCombinator::intersect($scope->getType($expr), $type);
  	} elseif ($context->false()) {
  	$resultType = TypeCombinator::remove($scope->getType($expr), $type);
  	}
  	}

• and the combination of the scope-type with the type-specified type is taken into account in

  phpstan-src/src/Type/Php/TypeSpecifyingFunctionsDynamicReturnTypeExtension.php

  Lines 72 to 80 in 9d9dcd8

  	$isAlways = $this->getHelper()->findSpecifiedType(
  	$scope,
  	$functionCall,
  	);
  	if ($isAlways === null) {
  	return ParametersAcceptorSelector::selectSingle($functionReflection->getVariants())->getReturnType();
  	}
  	return new ConstantBooleanType($isAlways);

[ondrejmirtes]

Implemented a different way here: #1311