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Error Identifier: generics.variance

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-covariant T
 */
interface Collection
{
	/**
	 * @param T $item
	 */
	public function add($item): void;
}

Why is it reported? #

The template type T is declared as covariant (@template-covariant), but it appears in a contravariant position (as a method parameter type). Covariant types can only appear in “output” positions (return types), while contravariant types can only appear in “input” positions (parameter types). Using a template type in the wrong position breaks type safety.

In this example, T is covariant but is used as a parameter type in add(), which is a contravariant position.

How to fix it #

If the type should be covariant (read-only collection), remove the method that uses it in a contravariant position:

 /**
  * @template-covariant T
  */
 interface Collection
 {
-	/**
-	 * @param T $item
-	 */
-	public function add($item): void;
+	/**
+	 * @return T
+	 */
+	public function first();
 }

If the type needs to appear in both parameter and return positions, make it invariant:

 /**
- * @template-covariant T
+ * @template T
  */
 interface Collection
 {
 	/**
 	 * @param T $item
 	 */
 	public function add($item): void;
 }

How to ignore this error #

You can use the identifier generics.variance to ignore this error using a comment:

// @phpstan-ignore generics.variance
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: generics.variance

Rules that report this error #

• PHPStan\Rules\Generics\ClassAncestorsRule [1]
• PHPStan\Rules\Generics\EnumAncestorsRule [1]
• PHPStan\Rules\Generics\FunctionSignatureVarianceRule [1]
• PHPStan\Rules\Generics\InterfaceAncestorsRule [1]
• PHPStan\Rules\Generics\MethodSignatureVarianceRule [1]
• PHPStan\Rules\Generics\PropertyVarianceRule [1]
• PHPStan\Rules\Generics\UsedTraitsRule [1]

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