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@derek-miller It looks like GraphQL specification doesn't handle this case:
http://facebook.github.io/graphql/June2018/#sec-Objects

An object field type is a valid sub‐type if it is an Object type and the interface field type is either an Interface type or a Union type and the object field type is a possible type of the interface field type.

Spec doesn't define possible types here but according to other places it just an object types inherited from interface:
http://facebook.github.io/graphql/June2018/#sec-Interface

possibleTypes returns the list of types that implement this interface. They must be object types.

Proposed behaviour requires a non trivial change to the spec and should be accepted to the spec first: https://github.com/facebook/graphql
I would suggest to open an issue/PR there.

Actually even this part is enough to reject schema:

An object field type is a valid sub‐type if it is an Object type

I'm going to close this: if you want to propose allowing Unions to be sub-types of Interfaces, that would be a different proposal, which would need to include a PR on the GraphQL Spec as well as updates here to showcase the implementation (as @IvanGoncharov mentioned).

@derek-miller thank you, though, for asking a detailed question with actual test cases! The presentation of your question as a PR made it easier to understand what was desired.