is_* with null coalesce on array value does not reduce the type

[BackEndTea]

Bug report

When doing an is_string check on an array access, within an if with a null coalesce (in case the key is not present), does not make PHPStan understand that they key must be there.

Code snippet that reproduces the problem

https://phpstan.org/r/d3ff8430-a6ae-498a-b09e-fed774f9099d

Expected output

I expected both outputs to be Dumped type: array('foo' => string)

I know this isn't the best code, and an isset would've been better here, but i noticed this in our code base.

[phpstan-bot]

@BackEndTea After the latest commit in dev-master, PHPStan now reports different result with your code snippet:

@@ @@
- 9: Dumped type: array(?'foo' => string)
-13: Dumped type: array('foo' => string)
+No errors

[phpstan-bot]

@BackEndTea After the latest commit in dev-master, PHPStan now reports different result with your code snippet:

@@ @@
- 9: Dumped type: array(?'foo' => string)
-13: Dumped type: array('foo' => string)
+ 9: Dumped type: array{foo?: string}
+13: Dumped type: array{foo: string}

Full report

Line	Error
9	Dumped type: array{foo?: string}
13	Dumped type: array{foo: string}

[github-actions]

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