False positives when using isset($array[$key]) === false instead of !isset($array[$key])

[aboks]

We have long ago adopted a code style that enforces an explicit === false instead of a ! negation within if conditions. I noticed that this sometimes leads to false positives when using an isset() check for a key within an array, while Psalm does not find issues on the equivalent version with a ! instead of === false.

One that I've been able to reproduce in isolation:

• https://psalm.dev/r/6ae508f44e (with isset(...) === false, gives issues)
• https://psalm.dev/r/5681b1b89c (with !isset(...), no issues)

I also see some RedundantPropertyInitializationCheck issues on if (isset($this->array[$key]) === false) patterns on our code base (like Psalm does not understand that we're checking if a specific element is set rather than if the whole array is initialized), which disappear when I switch === false to !. I haven't been able to reproduce these in an isolated case though, and hope these maybe also disappear if the above false positive is solved.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/6ae508f44e

<?php

interface UuidInterface {
    public function toString(): string;
}

class Foo {}

class FooCollection {
    /** @var Foo[] */
    private array $foos = [];
    
    public function addFoo(UuidInterface $uuid, Foo $foo): void {
        $this->foos[$uuid->toString()] = $foo;
    }
    
    public function getFoo(UuidInterface $uuid): Foo {
        if (isset($this->foos[$uuid->toString()]) === false) {
            throw new \LogicException("No such foo");
        }
        return $this->foos[$uuid->toString()];
    }
}

Psalm output (using commit a9775c6):

INFO: MixedArrayAccess - 21:16 - Cannot access array value on mixed variable $this->foos

INFO: MixedReturnStatement - 21:16 - Could not infer a return type

INFO: MixedInferredReturnType - 17:50 - Could not verify return type 'Foo' for FooCollection::getFoo

https://psalm.dev/r/5681b1b89c

<?php

interface UuidInterface {
    public function toString(): string;
}

class Foo {}

class FooCollection {
    /** @var Foo[] */
    private array $foos = [];
    
    public function addFoo(UuidInterface $uuid, Foo $foo): void {
        $this->foos[$uuid->toString()] = $foo;
    }
    
    public function getFoo(UuidInterface $uuid): Foo {
        if (!isset($this->foos[$uuid->toString()])) {
            throw new \LogicException("No such foo");
        }
        return $this->foos[$uuid->toString()];
    }
}

Psalm output (using commit a9775c6):

No issues!