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allow unions to declare implementation of interfaces

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Cf. https://github.com/graphql/graphql-spec/pull/373#issuecomment-375489730

# generic types
interface Node {
  id: ID!
}

interface Connection {
  pageInfo: PageInfo!
  edges: [Edge]
}

interface Edge {
  cursor: String
  node: Node
}

type PageInfo {
  hasPreviousPage: Boolean
  hasNextPage: Boolean
  startCursor: String
  endCursor: String
}

# schema-specific types
interface Pet {
  id: ID!
  name: String
}

type Cat implements Pet & Node {
  id: ID!
  name: String
}

type Dog implements Pet & Node {
  id: ID!
  name: String
}

union HousePet implements Pet & Node = Cat | Dog

# house-pet-specific types
type HousePetEdge implements Edge {
  cursor: String
  node: HousePet  # <<< This is what is unlocked by this PR
}

type HousePetConnection implements Connection {
  pageInfo: PageInfo!
  edges: [HousePetEdge]
}

# query
type Query {
  housePets: HousePetConnection
}