So I’m aware this is not a Swift issue per-se, but it is closely related.
Reproducing the issue is easy: clone Frizlab’s Demo Projects / Xcode Tests Compilation Failure · GitLab and build it for testing (for iOS; for macOS it works).
The incorrect SwiftCompilerPlugin is used, and compilation does not work.
Disabling prebuilds works (defaults write com.apple.dt.Xcode IDEPackageEnablePrebuilts NO) as a workaround (the derived data for the project has to be removed too before retrying).
FB21914833