Rule defined-types-are-used should detect reference loops

[Oliver Wright]

The following schema:

type Query {
    field: Int!
}

type A {
    b: B!
}

type B {
    a: A!
}

Will currently not show any errors, despite types A and B being inaccessible types (Due to the cyclic dependencies they count as referenced)

The same is true for self-referential types:

type Query {
    field: Int!
}

type A {
    a: A!
}

[Christian Joudrey]

Thanks for opening the issue @meiamsome. That's an interesting observation.

Today's defined-types-are-used is indeed a bit naive in this respect.

If you have suggestions on how to improve this feel free to share them.

I get the feeling we'd have to walk the graph starting at Query and Mutation to determine if something is truly accessible or not. We'd have to consider interfaces in this too, i.e. if Node is accessible then anything that implements Node should be considered accessible.

Not sure how complex that'd be though. 🤔

[Oliver Wright]

Yes, I imagine it's something like a memoized BFS or DFS. I think also this rule does not currently obey inline lint-disables either, so that's something to take in to account I guess.

I would be interested in writing a PR to add this functionality in the near future, but I don't have any code yet - thought I'd at least document the issue to begin with.