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Error Identifier: callable.unresolvableReturnType

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);

/**
 * @template T
 * @param T $value
 * @return T
 */
function identity(mixed $value): mixed
{
	return $value;
}

$fn = identity(...);
$result = $fn('hello');

Why is it reported? #

PHPStan detected that the return type of a callable, closure, or first-class callable contains an unresolvable template (generic) type. This happens when creating a first-class callable reference to a generic function — the template type cannot be resolved because there is no calling context with concrete arguments at the point where the callable is created.

Learn more: Solving PHPStan error “Unable to resolve the template type”

How to fix it #

Call the function directly instead of via a first-class callable so that PHPStan can resolve the template types from the arguments:

 <?php declare(strict_types = 1);
 
-$fn = identity(...);
-$result = $fn('hello');
+$result = identity('hello');

If you need a callable reference, add a PHPDoc type to the variable to help PHPStan understand the resolved type:

+/** @var \Closure(string): string $fn */
 $fn = identity(...);
 $result = $fn('hello');

How to ignore this error #

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

// @phpstan-ignore callable.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: callable.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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