Error Identifier: function.unresolvableReturnType

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Every error reported by PHPStan has an error identifier. Here’s a list of all error identifiers. In PHPStan Pro you can see the error identifier next to each error and filter errors by their identifiers.

Code example #

<?php declare(strict_types = 1);

interface Loggable
{
	public function log(): void;
}

/**
 * @template T of object
 * @param T $value
 * @return T&Loggable
 */
function makeLoggable(object $value): object
{
	return $value;
}

class PlainObject {}

function doFoo(): void
{
	$result = makeLoggable(new PlainObject());
}

Why is it reported? #

PHPStan reports this error when the return type of a function call contains an unresolvable type after generic template substitution. This happens when the template type resolves to a value that makes the return type impossible.

In the example above, makeLoggable returns T&Loggable. When called with new PlainObject(), T resolves to PlainObject. Since PlainObject does not implement Loggable, the intersection PlainObject&Loggable is impossible and PHPStan cannot determine a valid return type.

How to fix it #

Pass an argument whose type satisfies all constraints in the return type:

+class LoggableObject implements Loggable
+{
+	public function log(): void {}
+}
+
 function doFoo(): void
 {
-	$result = makeLoggable(new PlainObject());
+	$result = makeLoggable(new LoggableObject());
 }

Or constrain the template type to require the interface upfront:

 /**
- * @template T of object
+ * @template T of Loggable
  * @param T $value
- * @return T&Loggable
+ * @return T
  */
 function makeLoggable(object $value): object

How to ignore this error #

You can use the identifier function.unresolvableReturnType to ignore this error using a comment:

// @phpstan-ignore function.unresolvableReturnType
codeThatProducesTheError();

You can also use only the identifier key to ignore all errors of the same type in your configuration file in the ignoreErrors parameter:

parameters:
	ignoreErrors:
		-
			identifier: function.unresolvableReturnType

Rules that report this error #

PHPStan\Rules\Classes\ClassAttributesRule [1]
PHPStan\Rules\Classes\ClassConstantAttributesRule [1]
PHPStan\Rules\Classes\InstantiationRule [1]
PHPStan\Rules\Constants\ConstantAttributesRule [1]
PHPStan\Rules\EnumCases\EnumCaseAttributesRule [1]
PHPStan\Rules\Functions\ArrowFunctionAttributesRule [1]
PHPStan\Rules\Functions\CallCallablesRule [1]
PHPStan\Rules\Functions\CallToFunctionParametersRule [1]
PHPStan\Rules\Functions\CallUserFuncRule [1]
PHPStan\Rules\Functions\ClosureAttributesRule [1]
PHPStan\Rules\Functions\FunctionAttributesRule [1]
PHPStan\Rules\Functions\ParamAttributesRule [1]
PHPStan\Rules\Methods\CallMethodsRule [1]
PHPStan\Rules\Methods\CallStaticMethodsRule [1]
PHPStan\Rules\Methods\MethodAttributesRule [1]
PHPStan\Rules\Properties\PropertyAttributesRule [1]
PHPStan\Rules\Properties\PropertyHookAttributesRule [1]
PHPStan\Rules\Traits\TraitAttributesRule [1]

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