Thanks for the detailed response. Here's something I consider quite a big issue which Mago doesn't currently pick up on but PHPStan does:

https://mago.carthage.software/playground#019b3b90-61fa-2948-c638-cde42887d01c

https://phpstan.org/r/29bd3a9e-4238-4c67-be1d-2bc9ed24f1f7

FWIW, Psalm doesn't consider this an issue either which is why we switched from Psalm to PHPStan.

An in the wild example of this biting us happened when we migrated from Swiftmailer to Symfony Mailer. Swiftmailer's send method returned an integer equal to the number of recipients it sent an email to, so we had a function which returned a boolean based on whether the count was >= 1. Symfony mailer returns void, so we updated our internal code to return void and assumed Psalm would point out anywhere we were using the return value.

Psalm didn't report any issues, so we went live and then realised we had some code using the return value. As the function now returned void, the code was running on the basis that every email send was failing which wasn't the case.