Bug report

I stumbled across this while working on phpstan/phpstan-src#311.

In the test sample below, the given array may have legal keys, but PHPStan doesn't think so. Without phpstan/phpstan-src#311, this results in erroneous coercion into array; with it, it results in the string remaining unchanged.

Code snippet that reproduces the problem

https://phpstan.org/r/ed1af86c-a6b8-4b4c-9824-11c720e01b90
https://3v4l.org/fsv1d

Expected output

No error should be reported on line 9.

Possibly, an error could be reported on line 7, because $k is not guaranteed to be an int; however, I think this might go against the idea of BenevolentUnionType so I'm not sure.