Bug report

Call to function is_string() with non-empty-string|false will always evaluate to true.
🪪 function.alreadyNarrowedType

<?php declare(strict_types = 1);

class HelloWorld
{
    public function dumpToLog(mixed $dumpToLog): ?string
    {
        if (is_string($dumpToLog)) {
            if (file_exists($dumpToLog) || is_string($dumpToLog = realpath($dumpToLog))) {
                throw new \RuntimeException('Refusing to overwrite existing file: ' . $dumpToLog);
            }
        }
        return null;
    }
}

The exact cause of the error is because $dumpToLog gets reassigned inside the is_string() function. Removing the assignment (or assigning to a differently named variable) makes the error go away.

Code snippet that reproduces the problem

https://phpstan.org/r/7856e3b2-805f-4edb-996e-e3042642315e

Expected output

No error, the type is not sufficiently narrowed (is_string(non-empty-string|false) should return bool not true).

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