introduce new Intersection type

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 graphql#3527

docs-old/APIReference-GraphQL.md

@@ -66,6 +66,12 @@ _Type Definitions_
       A union type within GraphQL that defines a list of implementations.
     </a>
   </li>
+  <li>
+    <a href="../type/#graphqlintersectiontype">
+      <pre>class GraphQLIntersectionType</pre>
+      An intersection type within GraphQL that defines a list of constraining types.
+    </a>
+  </li>
   <li>
     <a href="../type/#graphqlenumtype">
       <pre>class GraphQLEnumType</pre>

docs-old/APIReference-TypeSystem.md

@@ -54,6 +54,12 @@ _Definitions_
       A union type within GraphQL that defines a list of implementations.
     </a>
   </li>
+  <li>
+    <a href="#graphqlintersectiontype">
+      <pre>class GraphQLIntersectionType</pre>
+      An intersection type within GraphQL that defines a list of constraining types.
+    </a>
+  </li>
   <li>
     <a href="#graphqlenumtype">
       <pre>class GraphQLEnumType</pre>

src/__testUtils__/kitchenSinkSDL.ts

@@ -79,6 +79,18 @@ extend union Feed = Photo | Video
 
 extend union Feed @onUnion
 
+intersection Resource = Feed & Node
+
+intersection AnnotatedIntersection @onIntersection = Feed & Node
+
+intersection AnnotatedIntersectionTwo @onIntersection = Feed & Node
+
+intersection UndefinedIntersection
+
+extend intersection Resource = Media & Accessible
+
+extend intersection Resource @onIntersection
+
 scalar CustomScalar
 
 scalar AnnotatedScalar @onScalar

src/execution/__tests__/abstract-test.ts

@@ -8,6 +8,7 @@ import { parse } from '../../language/parser';
 import {
   assertInterfaceType,
   GraphQLInterfaceType,
+  GraphQLIntersectionType,
   GraphQLList,
   GraphQLObjectType,
   GraphQLUnionType,
@@ -352,6 +353,94 @@ describe('Execute: Handles execution of abstract types', () => {
     });
   });
 
+  it('isTypeOf used to resolve runtime type for Intersection', async () => {
+    const DogType = new GraphQLObjectType({
+      name: 'Dog',
+      isTypeOf(obj, context) {
+        const isDog = obj instanceof Dog;
+        return context.async ? Promise.resolve(isDog) : isDog;
+      },
+      interfaces: () => [PetType],
+      fields: {
+        name: { type: GraphQLString },
+        woofs: { type: GraphQLBoolean },
+      },
+    });
+
+    const CatType = new GraphQLObjectType({
+      name: 'Cat',
+      isTypeOf(obj, context) {
+        const isCat = obj instanceof Cat;
+        return context.async ? Promise.resolve(isCat) : isCat;
+      },
+      interfaces: () => [PetType],
+      fields: {
+        name: { type: GraphQLString },
+        meows: { type: GraphQLBoolean },
+      },
+    });
+
+    const PetType = new GraphQLInterfaceType({
+      name: 'Pet',
+      fields: {
+        name: { type: GraphQLString },
+      },
+    });
+
+    const CatOrDogType = new GraphQLUnionType({
+      name: 'CatOrDog',
+      types: [DogType, CatType],
+    });
+
+    const CatOrDogPetType = new GraphQLIntersectionType({
+      name: 'CatOrDogPet',
+      types: [CatOrDogType, PetType],
+    });
+
+    const schema = new GraphQLSchema({
+      query: new GraphQLObjectType({
+        name: 'Query',
+        fields: {
+          catOrDogPets: {
+            type: new GraphQLList(CatOrDogPetType),
+            resolve() {
+              return [new Dog('Odie', true), new Cat('Garfield', false)];
+            },
+          },
+        },
+      }),
+    });
+
+    const query = `{
+      catOrDogPets {
+        ... on Pet {
+          name
+        }
+        ... on Dog {
+          woofs
+        }
+        ... on Cat {
+          meows
+        }
+      }
+    }`;
+
+    expect(await executeQuery({ schema, query })).to.deep.equal({
+      data: {
+        catOrDogPets: [
+          {
+            name: 'Odie',
+            woofs: true,
+          },
+          {
+            name: 'Garfield',
+            meows: false,
+          },
+        ],
+      },
+    });
+  });
+
   it('resolveType can throw', async () => {
     const PetType = new GraphQLInterfaceType({
       name: 'Pet',
@@ -497,6 +586,78 @@ describe('Execute: Handles execution of abstract types', () => {
     });
   });
 
+  it('resolve Intersection type using __typename on source object', async () => {
+    const schema = buildSchema(`
+      type Query {
+        catOrDogPets: [CatOrDogPet]
+      }
+
+      union CatOrDog = Cat | Dog
+
+      interface Pet {
+        name: String
+      }
+
+      intersection CatOrDogPet = CatOrDog & Pet
+
+      type Cat implements Pet {
+        name: String
+        meows: Boolean
+      }
+
+      type Dog implements Pet {
+        name: String
+        woofs: Boolean
+      }
+    `);
+
+    const query = `
+      {
+        catOrDogPets {
+          ... on Pet {
+            name
+          }
+          ... on Dog {
+            woofs
+          }
+          ... on Cat {
+            meows
+          }
+        }
+      }
+    `;
+
+    const rootValue = {
+      catOrDogPets: [
+        {
+          __typename: 'Dog',
+          name: 'Odie',
+          woofs: true,
+        },
+        {
+          __typename: 'Cat',
+          name: 'Garfield',
+          meows: false,
+        },
+      ],
+    };
+
+    expect(await executeQuery({ schema, query, rootValue })).to.deep.equal({
+      data: {
+        catOrDogPets: [
+          {
+            name: 'Odie',
+            woofs: true,
+          },
+          {
+            name: 'Garfield',
+            meows: false,
+          },
+        ],
+      },
+    });
+  });
+
   it('resolve Interface type using __typename on source object', async () => {
     const schema = buildSchema(`
       type Query {