What are common mistakes developers make with variance and invariance?

Variance and invariance are important concepts in programming, particularly in type systems. They determine how type parameters behave in relation to subtyping. However, developers often make some common mistakes while working with them. Here are a few:

Ignoring type relationships: Some developers assume that type parameters are interchangeable without considering their relationships, leading to type safety issues.
Incorrectly applying covariance and contravariance: Misunderstanding when to use covariant and contravariant types can result in runtime errors, especially in collections.
Not using wildcards effectively: Wildcards can enhance the flexibility of type parameters, but failing to use them appropriately can complicate the code unnecessarily.
Confusing generics with subtyping: Developers sometimes confuse generics with classic subtyping rules, leading to improper implementations and type constraints.

Example


        // Example in PHP demonstrating covariance and contravariance
        class Animal {}
        class Dog extends Animal {}
        class Cat extends Animal {}
        
        class Box<T> {
            private $item;
            
            public function addItem(T $item) {
                $this->item = $item;
            }
            
            public function getItem(): T {
                return $this->item;
            }
        }
        
        $dogBox = new Box<Dog>();
        $dogBox->addItem(new Dog());
        
        // This would be a type error 
        // $animalBox = new Box<Animal>(); 
        // $animalBox->addItem(new Dog());

What are common mistakes developers make with variance and invariance?

Example

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What are common mistakes developers make with variance and invariance?

Example

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