@psalm-mutation-free only works *once* inside if

[ThomasLandauer]

https://psalm.dev/r/fb594bf0e4

This line is not raising an error, due to @psalm-mutation-free and if (is_string()):

$cat->setBar($foo->getBar());

However, the next line does give an error - so it looks like @psalm-mutation-free isn't "working" here anymore:

echo $foo->getBar() . $foo->getBar();

If I delete the first line, the second line does not raise the error. This makes me think that @psalm-mutation-free is only working at the first call.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/fb594bf0e4

<?php

class Foo
{
    private ?string $bar = null;
    
    /**
     * @psalm-mutation-free
     */
    public function getBar(): ?string
    {
        return $this->bar;
    }
}

class Cat
{
    private ?string $bar = null;

    public function setBar(string $bar): void
    {
        $this->bar = $bar;
    }

}

$foo = new Foo();
$cat = new Cat();
if (is_string($foo->getBar())) {
    $cat->setBar($foo->getBar());
    echo $foo->getBar() . $foo->getBar();
}

Psalm output (using commit 10ea05a):

INFO: PossiblyNullOperand - 31:10 - Cannot concatenate with a possibly null null|string

INFO: PossiblyNullOperand - 31:27 - Cannot concatenate with a possibly null null|string

[someniatko]

Psalm cannot know what happens inside the setBar() when it evaluates usages of this method. You could for example change $this->bar to null inside of it.

[ThomasLandauer]

Hm. But it's $cat->setBar(). How could this modify the value of $foo->getBar()?

[AndrolGenhald]

Psalm isn't smart enough to tell that $cat can't contain a reference to $foo.

[orklah]

and most importantly, Psalm doesn't check method name like setXXX or getXXX, so the method name could as well be called $cat->changeTheValueOfEveryFooInstanceInMemory($foo->getBar()); 😄

[ThomasLandauer]

I see, indeed, it could even be $cat->setBar('123') (i.e. no need for $foo->getBar() as argument).

So the exact rule is that there must not be a call to any function (more precisely: to any function without @psalm-mutation-free) between the if (is_string()) and the actual call to $foo->getBar()?

This is somewhat surprising (at least to me) - what about adding more explanation to https://psalm.dev/docs/running_psalm/issues/PossiblyNullArgument/ ?
Are those error docs pages so minimalist on purpose, or just cause nobody took the time yet to add some hints about how the issue could be resolved?

[orklah]

Mainly the latter, contributions are very welcome :)