Is the rule graph subclass aware? I'm thinking about this class hierarchy: • PackageFieldSet • NfpmPackageFieldSet • NfpmRpmPackageFieldSet With rules: • UnionRule(PackageFieldSet, NfpmRpmPackageFieldSet) • nfpm_package(NfpmPackageFieldSet) -> BuiltArtifact I can't tell what's missing from the role graph errors I'm seeing, but I suspect the rule graph does not treat NfpmRpmPackageFieldSet as compatible with NfpmPackageFieldSet, so it can't find the last rule given the subclass. Is that right?

> Is the rule graph subclass aware? It is not.

I'm not too sure of it. But I think it isn't, and that `UnionRule`s act as a rule that allows you to move between the parent class and the child class. To illustrate,

Get(InferredDependencies, InferDependenciesRequest)

might go

InferDependenciesRequest--[UnionRule]-->InferFortranDependencies--[infer_fortran_dependencies]-->InferredDependencies

The rule engine treats all types as distinct regardless of subclass relationship. You can use subclassing for code sharing but the engine won't consider the types as compatible for purposes of finding rules to apply.

Thank you! That clears up why the rule wasn't working.

But shouldn't you be using call-by-name anyway for new rules? 🙂

@happy-kitchen-89482 well, now I am using call-by-name. I ran migrate-call-by-name, fmt, and then fixed the issues mypy found. https://github.com/pantsbuild/pants/pull/19308/files/2bee818cbe5b81eb44d008530fb34f4d5e3aeb2f..3f9b63826d1fcc3060cafd876ba8989f359b5789