This just floated up on my screen, while working on feedback for the try/catch support for `@rule`s.

native_engine.IntrinsicError: Invalid Get. Because an input type for `Get(TargetFilesGeneratorSettings, TargetFilesGeneratorSettingsRequest, <pants.backend.python.target_types.PythonFilesGeneratorSettingsRequest object at 0x1073c2f40>)` was annotated with `@union`, the value for that type should be a member of that union. Did you intend to register a `UnionRule`? If not, you may be using the incorrect explicitly declared type.