Intersection Type Error

Why This Happens

PHP 8.1 intersection types require a value to satisfy all specified types simultaneously. If a class implements Countable but not Iterator, it fails the Countable&Iterator type check. The class must implement all interfaces in the intersection.

The Problem

function process(Countable&Iterator $handler): void {
    // ...
}

class SimpleList implements Countable {
    public function count(): int { return 0; }
}
process(new SimpleList()); // Missing Iterator

The Fix

function process(Countable&Iterator $handler): void {
    // ...
}

class SimpleList implements Countable, Iterator {
    private array $items = [];
    public function count(): int { return count($this->items); }
    public function current(): mixed { return current($this->items); }
    public function key(): mixed { return key($this->items); }
    public function next(): void { next($this->items); }
    public function rewind(): void { reset($this->items); }
    public function valid(): bool { return key($this->items) !== null; }
}

Step-by-Step Fix

1. 1

   Read the type requirements

   The error message lists all interfaces required by the intersection type using the & separator.

2. 2

   Check the class implements list

   Look at the class definition to see which interfaces it implements and which are missing.

3. 3

   Implement missing interfaces

   Add the missing interface to the class implements list and implement all required methods.