Bug report

Negative indices on strings should be allowed. When setting a string explicitly that has length greater than 0, PHPstan wrongfully concludes that "Offset -1 does not exist on 'mystring'". This fails when wrapped in a strlen check as well.

This issue only exists when setting the string (e.g. $mystring = 'mystring';), and does not happen when using a variable string from a function (e.g. function foo(string $mystring) {...} ).

Code snippet that reproduces the problem

https://phpstan.org/r/f482db20-093c-442e-874b-13237efd1e4d

Expected output

PHPStan should not throw an error for negative indices on static strings.

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