introduce new Intersection type

[yaacovCR]

Intersections of unions and interfaces can be considered to "implement" their unions and interface members.

A type with a field of type Intersection will satisfy an interface where the field defined in the interface is one of the member types of the intersection.

Alternative to #3527

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[yaacovCR]

graphql/graphql-spec#939 (comment)

[yaacovCR]

intersection "implementing" interface

# 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 = Cat | Dog

intersection HousePetNode = HousePet & Pet & Node

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

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

# query
type Query {
  housePets: HousePetConnection
}

intersection "implementing" interface AND narrowing unions

# 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
}

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

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

union HousePet = Cat | Dog | Fish
union PetWithFur = Cat | Dog | Lion

intersection HousePetWithFurNode = HousePet & PetWithFur & Pet & Node

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

type HousePetWithFurNodeConnection implements Connection {
  pageInfo: PageInfo!
  edges: [HousePetWithFurEdge]
}

# query
type Query {
  housePetsWithFur: HousePetWithFurNodeConnection
}

[yaacovCR]

spec PR: graphql/graphql-spec#941

[yaacovCR]

discussion @ graphql/graphql-wg#944

[yaacovCR]

Summary of discussion from WG https://github.com/graphql/graphql-wg/blob/main/agendas/2022/2022-05-05.md :

union HousePet implements Node was overwhelmingly felt to be best because it is (1) expressive (2) concise (3) less complex (4) will lead to early validation error, which overall would be most valuable. Intersections were felt to be powerful tools that would then propagate a small number of breaking changes to a large number of dangerous and --more importantly-- possibly silent errors!

The action item will then be to move forward as possible with graphql/graphql-spec#939 . Immediate next steps is to refine the implementation at #3527 with further checking for all the necessary validation and test changes.