PossiblyInvalidArrayOffset - Psalm wrongly expects string array keys when only int is possible

[BenMorel]

A snippet is worth a thousand words, so here we go:

https://psalm.dev/r/62dbf6620f

Bug 🐛

As you can see, the OWNERSHIP_STATUS_TRANSLATION_KEYS constant can only have OwnershipStatus::* keys, so int only.

But Psalm complains:

ERROR: PossiblyInvalidArrayOffset - 34:16 - Cannot access value on variable Translator::OWNERSHIP_STATUS_TRANSLATION_KEYS using offset value of 2, expecting '1' or '2'

Not sure where the string type '1' or '2' comes from?

Feature request ✨

I'll happily create another issue for this if you wish, but as you can see, Psalm cannot infer the return type of getTranslationKey(), even though it looks pretty obvious.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/62dbf6620f

<?php

class OwnershipStatus
{
    public const NOT_OWNED = 0;
    public const OWNED     = 1;
    public const WISHED    = 2;
}

class BookActivity {
    /**
     * @psalm-var OwnershipStatus::*
     */
    private int $ownershipStatus = OwnershipStatus::NOT_OWNED;
    
    /**
     * @psalm-return OwnershipStatus::*
     */
    public function getOwnershipStatus(): int
    {
        return $this->ownershipStatus;
    }
}

class Translator
{
    private const OWNERSHIP_STATUS_TRANSLATION_KEYS = [
        OwnershipStatus::OWNED  => 'Activity:OwnBook:OWNED',
        OwnershipStatus::WISHED => 'Activity:OwnBook:WISHED'
    ];

    public function getTranslationKey(BookActivity $activity): string
    {
        return self::OWNERSHIP_STATUS_TRANSLATION_KEYS[$activity->getOwnershipStatus()];
    }
}

Psalm output (using commit b5a3f45):

ERROR: PossiblyInvalidArrayOffset - 34:16 - Cannot access value on variable Translator::OWNERSHIP_STATUS_TRANSLATION_KEYS using offset value of 2, expecting '1' or '2'

INFO: MixedReturnStatement - 34:16 - Could not infer a return type

INFO: MixedInferredReturnType - 32:64 - Could not verify return type 'string' for Translator::getTranslationKey

[weirdan]

The message is obviously misleading, but the code is not safe either. You're not handling OwnershipStatus::NOT_OWNED, and that's why Psalm complains. See https://psalm.dev/r/ff55667bb8 (if you comment out the assert, you get the original issue reported).

The message should instead read: Cannot access value on variable Translator::OWNERSHIP_STATUS_TRANSLATION_KEYS using offset value of 0, expecting 1 or 2

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/ff55667bb8

<?php

class OwnershipStatus
{
    public const NOT_OWNED = 0;
    public const OWNED     = 1;
    public const WISHED    = 2;
}

class BookActivity {
    /**
     * @psalm-var OwnershipStatus::*
     */
    private int $ownershipStatus = OwnershipStatus::NOT_OWNED;
    
    /**
     * @psalm-return OwnershipStatus::*
     */
    public function getOwnershipStatus(): int
    {
        return $this->ownershipStatus;
    }
}

class Translator
{
    private const OWNERSHIP_STATUS_TRANSLATION_KEYS = [
        OwnershipStatus::OWNED  => 'Activity:OwnBook:OWNED',
        OwnershipStatus::WISHED => 'Activity:OwnBook:WISHED'
    ];

    public function getTranslationKey(BookActivity $activity): string
    {
        /** @psalm-trace $status */
        $status = $activity->getOwnershipStatus();
        assert($status !== 0);
        /** @psalm-trace $status */;
        return self::OWNERSHIP_STATUS_TRANSLATION_KEYS[$status];
    }
}

Psalm output (using commit 3e9c5d3):

INFO: Trace - 35:9 - $status: int(0)|int(1)|int(2)

INFO: Trace - 37:36 - $status: int(1)|int(2)

[BenMorel]

@weirdan Indeed, good point! This is definitely a cosmetic issue then. The exception message is very misleading. It should indeed be "using offset value of 0, expecting 1 or 2".

Leaving this open as a request to fix the exception message then.

By the way, is there a way to declare that a method can accept/return any of these constants, but NOT_OWNED? Or do I have to list all possible values?

[BenMorel]

Thanks a lot, @muglug !