<?php
class A { public int $a = 0; }
class B { public int $b = 0; }

/** @template T */
interface Generic
{
    /** @psalm-return iterable<T> */
    public function fetchAll();
}

/** @param Generic<A> $ag */
function (Generic $ag): void {
    // psalm itself can infer the type well enough
    $collection = $ag->fetchAll();
    foreach ($collection as $_) {
        /** @psalm-trace $_ */
    }

    // need this @var to get the power of auto-completion in phpstorm
    /** @var A[] */
    $collection = $ag->fetchAll();
    foreach ($collection as $item) {
        echo $item->a; // auto-completion can be used here
        /** @psalm-trace $item */
    }

    // but explicitly declared @var hides the programmer's mistake
    /** @var B[] */
    $collection = $ag->fetchAll();
    foreach ($collection as $item) {
        echo $item->b; // the item is actually A, it has no member named b
        /** @psalm-trace $item */
    }

    // if psalm ignores @var which co-located with a specific annotation,
    // programmers can annotate it for auto-completion,
    // while psalm still catch type errors.
    /**
     * @var B[]
     * @psalm-ignore-var
     */
    $collection = $ag->fetchAll();
    foreach ($collection as $item) {
        echo $item->b; // if @var is ignored, this error can be caught in psalm
        /** @psalm-trace $item */
    }

};