Variables, declared in the right part of ??= expression are considered always existing and not null

[ethernidee]

Bug report

It seems, that ??= is treated as "=" if new variables are initialized in the right part. Example:

$result = $this->cache[$keyName] ??= ($newIndex = count($this->cache) + 1);
// WRONG ERROR: Variable $newIndex in isset() always exists and is not nullable.
if (isset($newIndex)) {
	$this->recordNewCacheItem($keyName);
}

This code should be treated as

if (isset($this->cache[$keyName]) {
	$result = $this->cache[$keyName];
} else {
	$newIndex = count($this->cache) + 1;
	$this->cache[$keyName] = $newIndex;
	$result = $newIndex;
}

Code snippet that reproduces the problem

https://phpstan.org/r/44f585d5-f09f-42f7-abf8-bd612169930f

Expected output

Should be no error.

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[ondrejmirtes]

Fixed: phpstan/phpstan-src@846f44e

[github-actions]

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