Error Identifier: method.alreadyNarrowedType

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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);

class TypeChecker
{
	/** @phpstan-assert-if-true string $value */
	public function isString(mixed $value): bool
	{
		return is_string($value);
	}
}

function doFoo(TypeChecker $checker, string $value): void
{
	if ($checker->isString($value)) {
		echo 'already known to be string';
	}
}

Why is it reported? #

A method call that acts as a type-checking or type-narrowing operation always evaluates to true. PHPStan has already inferred enough type information to determine that the result of this method call is always true in this context.

In the example above, $value is already typed as string, and TypeChecker::isString() uses @phpstan-assert-if-true string $value to narrow the type. Since the type is already string, the method will always return true.

This typically occurs when the same type check is performed redundantly, or when the parameter types already guarantee the result.

How to fix it #

Remove the redundant check if the type is already known:

 function doFoo(TypeChecker $checker, string $value): void
 {
-	if ($checker->isString($value)) {
-		echo 'already known to be string';
-	}
+	echo 'already known to be string';
 }

Or fix the parameter type if the check should be meaningful:

-function doFoo(TypeChecker $checker, string $value): void
+function doFoo(TypeChecker $checker, mixed $value): void
 {
 	if ($checker->isString($value)) {
-		echo 'already known to be string';
+		echo 'confirmed string';
 	}
 }

How to ignore this error #

You can use the identifier method.alreadyNarrowedType to ignore this error using a comment:

// @phpstan-ignore method.alreadyNarrowedType
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: method.alreadyNarrowedType

Rules that report this error #

• PHPStan\Rules\Comparison\ImpossibleCheckTypeMethodCallRule [1]