psalm/docs/running_psalm/issues/PossiblyNullArgument.md

PossiblyNullArgument

Emitted when calling a function with a value that’s possibly null when the function does not expect it

<?php

function foo(string $s): void {}
foo(rand(0, 1) ? "hello" : null);

Common Problem Cases

Using a Function Call inside if

<?php

if (is_string($cat->getName()) {
    foo($cat->getName());
}

This fails since it's not guaranteed that subsequent calls to $cat->getName() always give the same result.

Possible Solutions

• Use a variable:

  <?php
  
  $catName = $cat->getName();
  if (is_string($catName) {
      foo($catName);
  }
  unset($catName);
  

• Add @psalm-mutation-free to the declaration of the function

Calling Another Function After if

<?php

if (is_string($cat->getName()) {
    changeCat();
    foo($cat->getName());
}

This fails since psalm cannot know if changeCat() does actually modify $cat.

Possible Solutions

• Add @psalm-mutation-free to the declaration of the other function (here: changeCat()) too
• Use a variable: See above