allow unions to declare implementation of interfaces

At a glance

• Identifier: #939
• Stage: RFC 1 / Proposal
• Champion: @yaacovCR
• Latest activity: Spec PR closed on 2026-03-05
• Spec PR: https://github.com/graphql/graphql-spec/pull/939
• Related:
  • #373

Spec PR description

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
}