When there are several dependencies declare in Package.swift. And one of them, let us call PackageA, only has one tag version 1.0.9 in git. And then in Package.swift, user uses a wrong majorversion 2. Then run "swift build". Observe the result.
Only return an error message "unsatisfiable".
It should return an error message with which dependency does not work out, like the version of dependency Package A could not be found. In some certain case, like package has only one version, it would be better if package manager could give some suggestions.