For projects that have tests, PHPStan will report a bunch of false positives, where the base classes of the test infrastructure are missing, if Drupal's developer dependencies are not installed.
For projects that do not have tests, Drupal's developer dependencies are not required, and installing them is an extra, unnecessary burden on novice users, who want to upgrade their site from Drupal 9 to Drupal 10.
Want to use Upgrade Status on a site without tests, and be told to install non-essential Drupal developer dependencies.
It could be better to catch this kind of error, and add a tip to install Drupal's developer dependencies, directly within the PHPStan error.
We would need some kind of reliable list of errors that show up for these cases first.
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The errors are already shown but they are not useful I think to help fix them. Modified the title.
We would need to have some reliable patterns to at least identify the common errors in this category to add instructions at the end or start.
Thanks for updating the title, I am not sure how all these parts interconnect, so I just threw everything in the mix :)