GraphQL @oneOf: Exactly One Input, Enforced by the Schema

Every GraphQL schema I've worked on eventually grows a field that can be looked up more than one way. You want a user by ID, or by email, or by username. The type system has no way to say "exactly one of these," so you pick one of two bad options: three root fields that do the same thing, or one field with three nullable arguments and a pile of...