Asserting false on result of is_null should remove null from type

[mr-feek]

https://psalm.dev/r/4de47158f1

Per https://github.com/facade/ignition/pull/292/files#r455046472, I believe psalm should be able to infer this.

The expected functionality seems to work for the inverse:

https://psalm.dev/r/afc769579e

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/4de47158f1

<?php

/**
* @param mixed $foo
* @psalm-assert false $foo
*/
function abort_if($foo): void
{
    if ($foo) {
        throw new \RuntimeException();
    }
}

/**
* @param string|null $foo
*/
function removeNullable($foo): string
{
    abort_if(is_null($foo));
    return $foo;
}

Psalm output (using commit 56047dd):

ERROR: NullableReturnStatement - 20:12 - The declared return type 'string' for removeNullable is not nullable, but the function returns 'null|string'

ERROR: InvalidNullableReturnType - 17:32 - The declared return type 'string' for removeNullable is not nullable, but 'null|string' contains null

https://psalm.dev/r/afc769579e

<?php

/**
* @param mixed $foo
* @psalm-assert true $foo
*/
function abort_if_false($foo): void
{
    if (!$foo) {
        throw new \RuntimeException();
    }
}

/**
* @param string|null $foo
* @return null
*/
function onlyNullable($foo)
{
    abort_if_false(is_null($foo));
    return $foo;
}

Psalm output (using commit 56047dd):

No issues!

[mr-feek]

Thanks @muglug !