This may be an Xcode problem not with SPM, but when I add a package into my App project, as a local package, whenever I change the dependencies of the package, even if the change to the version conflicts with the app's version constraints, the Xcode build the app just fine. I have to close and reopen the project again before the Xcode detects the change and raise an error during the build process. Why can't SPM/Xcode just detect the change while the app is opened, and raise an error instead of using whatever cache it is using?
Currently, the workaround is to never edit the package in the app project (even though I should be able to), but to open the package as separate project to make edits, then reopen the app project to build the app.
Another workaround I'm trying out is to add the xcodebuild -resolvePackageDependencies as a step during the build process, to force the build to cancel if theres package dependency resolution errors. But why is Xcode just ignoring resolution errors ??