introduce new intersection type

see graphql/graphql-js#3550

spec/Appendix B -- Grammar Summary.md

@@ -343,6 +343,19 @@ UnionTypeExtension :
 - extend union Name Directives[Const]? UnionMemberTypes
 - extend union Name Directives[Const]
 
+IntersectionTypeDefinition : Description? intersection Name Directives[Const]?
+IntersectionMemberTypes?
+
+UnionMemberTypes :
+
+- IntersectionMemberTypes | NamedType
+- = `|`? NamedType
+
+IntersectionTypeExtension :
+
+- extend intersection Name Directives[Const]? IntersectionMemberTypes
+- extend intersection Name Directives[Const]
+
 EnumTypeDefinition :
 
 - Description? enum Name Directives[Const]? EnumValuesDefinition
@@ -402,6 +415,7 @@ TypeSystemDirectiveLocation : one of
 - `ARGUMENT_DEFINITION`
 - `INTERFACE`
 - `UNION`
+- `INTERSECTION`
 - `ENUM`
 - `ENUM_VALUE`
 - `INPUT_OBJECT`

spec/Section 2 -- Language.md

@@ -528,8 +528,8 @@ Fragments are the primary unit of composition in GraphQL.
 
 Fragments allow for the reuse of common repeated selections of fields, reducing
 duplicated text in the document. Inline Fragments can be used directly within a
-selection to condition upon a type condition when querying against an interface
-or union.
+selection to condition upon a type condition when querying against an interface,
+union, or intersection.
 
 For example, if we wanted to fetch some common information about mutual friends
 as well as friends of some user:

spec/Section 3 -- Type System.md

@@ -261,6 +261,7 @@ TypeDefinition :
 - ObjectTypeDefinition
 - InterfaceTypeDefinition
 - UnionTypeDefinition
+- IntersectionTypeDefinition
 - EnumTypeDefinition
 - InputObjectTypeDefinition
 
@@ -276,7 +277,7 @@ Scalars and Enums form the leaves in response trees; the intermediate levels are
 `Object` types, which define a set of fields, where each field is another type
 in the system, allowing the definition of arbitrary type hierarchies.
 
-GraphQL supports two abstract types: interfaces and unions.
+GraphQL supports three abstract types: interfaces, unions and intersections.
 
 An `Interface` defines a list of fields; `Object` types and other Interface
 types which implement this Interface are guaranteed to implement those fields.
@@ -287,6 +288,11 @@ A `Union` defines a list of possible types; similar to interfaces, whenever the
 type system claims a union will be returned, one of the possible types will be
 returned.
 
+An `Intersection` defines a list of constraining abstract types. If a field
+claims it returns an Intersection type, it will return only types that are
+contained within all of the Intersections's Unions and types that implement all
+of the Intersection's Interfaces.
+
 Finally, oftentimes it is useful to provide complex structs as inputs to GraphQL
 field arguments or variables; the `Input Object` type allows the schema to
 define exactly what data is expected.
@@ -314,9 +320,9 @@ to arguments and variables as well as the values output by fields. These two
 uses categorize types as _input types_ and _output types_. Some kinds of types,
 like Scalar and Enum types, can be used as both input types and output types;
 other kinds of types can only be used in one or the other. Input Object types
-can only be used as input types. Object, Interface, and Union types can only be
-used as output types. Lists and Non-Null types may be used as input types or
-output types depending on how the wrapped type may be used.
+can only be used as input types. Object, Interface, Union, and Intersection
+types can only be used as output types. Lists and Non-Null types may be used as
+input types or output types depending on how the wrapped type may be used.
 
 IsInputType(type) :
 
@@ -332,7 +338,7 @@ IsOutputType(type) :
 - If {type} is a List type or Non-Null type:
   - Let {unwrappedType} be the unwrapped type of {type}.
   - Return IsOutputType({unwrappedType})
-- If {type} is a Scalar, Object, Interface, Union, or Enum type:
+- If {type} is a Scalar, Object, Interface, Union, Intersection, or Enum type:
   - Return {true}
 - Return {false}
 
@@ -706,8 +712,8 @@ Must only yield exactly that subset:
 ```
 
 A field of an Object type may be a Scalar, Enum, another Object type, an
-Interface, or a Union. Additionally, it may be any wrapping type whose
-underlying base type is one of those five.
+Interface, a Union, or an Intersection. Additionally, it may be any wrapping
+type whose underlying base type is one of those five.
 
 For example, the `Person` type might include a `relationship`:
 
@@ -928,7 +934,13 @@ IsValidImplementationFieldType(fieldType, implementedFieldType):
 5. If {fieldType} is an Object or Interface type and {implementedFieldType} is
    an Interface type and {fieldType} declares it implements
    {implementedFieldType} then return {true}.
-6. Otherwise return {false}.
+6. If {fieldType} is an IntersectionType and {implementedFieldType} is an Union
+   type and {fieldType} is a member type of {implementedFieldType} then return
+   {true}.
+7. If {fieldType} is an IntersectionType and {implementedFieldType} is an
+   Interface type and at least one of the members of {fieldType} declares it
+   implements {implementedFieldType} then return {true}.
+8. Otherwise return {false}.
 
 ### Field Arguments
 
@@ -977,7 +989,7 @@ May return the result:
 ```
 
 The type of an object field argument must be an input type (any type except an
-Object, Interface, or Union type).
+Object, Interface, Union, or Intersection type).
 
 ### Field Deprecation
 
@@ -1054,8 +1066,8 @@ objects and interfaces can then implement these interfaces which requires that
 the implementing type will define all fields defined by those interfaces.
 
 Fields on a GraphQL interface have the same rules as fields on a GraphQL object;
-their type can be Scalar, Object, Enum, Interface, or Union, or any wrapping
-type whose base type is one of those five.
+their type can be Scalar, Object, Enum, Interface, Union, or Intersection, or
+any wrapping type whose base type is one of those five.
 
 For example, an interface `NamedEntity` may describe a required field and types
 such as `Person` or `Business` may then implement this interface to guarantee
@@ -1415,6 +1427,154 @@ Union type extensions have the potential to be invalid if incorrectly defined.
 5. Any non-repeatable directives provided must not already apply to the original
    Union type.
 
+## Intersections
+
+IntersectionTypeDefinition : Description? intersection Name Directives[Const]?
+IntersectionMemberTypes?
+
+IntersectionMemberTypes :
+
+- IntersectionMemberTypes | NamedType
+- = `|`? NamedType
+
+GraphQL Intersections are higher order abstract types that represent objects
+satisfying the requirements of all of the Intersection's abstract member types.
+Intersections combine the features of interfaces and unions; the objects
+represented by an Intersection must implement all of the Intersection's
+interfaces and must also be included within all of its unions.
+
+Just as with unions, intersections do not directly define any fields, so **no**
+fields may be queried on this type without the use of type refining fragments or
+inline fragments (with the exception of the meta-field {\_\_typename}).
+
+For example, we might define the following types:
+
+```graphql example
+interface Link {
+  link: Downloadable
+}
+
+type SearchResultLink implements Link {
+  link: DownloadableSearchResult
+}
+
+interface Downloadable {
+  url: string
+}
+
+union SearchResult = Photo | Person
+
+intersection DownloadableSearchResult = SearchResult & Downloadable
+
+type Person implements Downloadable {
+  url: String
+  name: String
+  age: Int
+}
+
+type Photo implements Downloadable {
+  url: String
+  height: Int
+  width: Int
+}
+
+type SearchQuery {
+  firstDownloadableSearchResult: DownloadableSearchResult
+}
+```
+
+Because intersection `DownloadableSearchResult` includes interface
+`Downloadable` as a constraining member type, the possible types of the
+intersection implement the `Downloadable` interface. The `link` field within
+`SearchResultLink` therefore implements the `link` field of interface `Link`.
+
+Just as with unions, the below could be ambiguous and is invalid.
+
+```graphql counter-example
+{
+  firstDownloadableSearchResult {
+    url
+    name
+    height
+  }
+}
+```
+
+A valid operation includes typed fragments (in this example, inline fragments):
+
+```graphql example
+{
+  firstDownloadableSearchResult {
+    ... on Downloadable {
+      url
+    }
+    ... on Person {
+      name
+    }
+    ... on Photo {
+      height
+    }
+  }
+}
+```
+
+Intersection members may be defined with an optional leading `&` character to
+aid formatting when representing a longer list of constraining types:
+
+```raw graphql example
+intersection DownloadableSearchResult =
+  & Downloadable
+  & SearchResult
+```
+
+**Result Coercion**
+
+The intersection type should have some way of determining which object a given
+result corresponds to. Once it has done so, the result coercion of the
+intersection is the same as the result coercion of the object.
+
+**Input Coercion**
+
+Intersections are never valid inputs.
+
+**Type Validation**
+
+Intersection types have the potential to be invalid if incorrectly defined.
+
+1. An Intersection type must include one or more unique member types.
+2. The member types of a Intersection type must all be Interface or Union base
+   types; Scalar, Enum, Object and other Intersection types must not be member
+   types of an Intersection. Similarly, wrapping types must not be member types
+   of an Intersection.
+
+### Intersection Extensions
+
+IntersectionTypeExtension :
+
+- extend intersection Name Directives[Const]? IntersectionMemberTypes
+- extend intersection Name Directives[Const]
+
+Intersection type extensions are used to represent an intersection type which
+has been extended from some original intersection type. Similar to unions, this
+might by utilized by a GraphQL service which is itself an extension of another
+GraphQL service.
+
+**Type Validation**
+
+Intersection type extensions have the potential to be invalid if incorrectly
+defined.
+
+1. The named type must already be defined and must be a Intersection type.
+2. The member types of a Intersection type must all be Interface or Union base
+   types; Scalar, Enum, Object and other Intersection types must not be member
+   types of an Intersection. Similarly, wrapping types must not be member types
+   of an Intersection.
+3. All member types of an Intersection type extension must be unique.
+4. All member types of an Intersection type extension must not already be a
+   member of the original Intersection type.
+5. Any non-repeatable directives provided must not already apply to the original
+   Intersection type.
+
 ## Enums
 
 EnumTypeDefinition :
@@ -1876,6 +2036,7 @@ TypeSystemDirectiveLocation : one of
 - `ARGUMENT_DEFINITION`
 - `INTERFACE`
 - `UNION`
+- `INTERSECTION`
 - `ENUM`
 - `ENUM_VALUE`
 - `INPUT_OBJECT`

spec/Section 4 -- Introspection.md

@@ -71,11 +71,12 @@ underscores {"\_\_"}.
 GraphQL supports type name introspection within any selection set in an
 operation, with the single exception of selections at the root of a subscription
 operation. Type name introspection is accomplished via the meta-field
-`__typename: String!` on any Object, Interface, or Union. It returns the name of
-the concrete Object type at that point during execution.
+`__typename: String!` on any Object, Interface, Union, Intersection. It returns
+the name of the concrete Object type at that point during execution.
 
-This is most often used when querying against Interface or Union types to
-identify which actual Object type of the possible types has been returned.
+This is most often used when querying against Interface, Union, or Intersection
+types to identify which actual Object type of the possible types has been
+returned.
 
 As a meta-field, `__typename` is implicit and does not appear in the fields list
 in any defined type.
@@ -140,7 +141,9 @@ type __Type {
   fields(includeDeprecated: Boolean = false): [__Field!]
   # must be non-null for OBJECT and INTERFACE, otherwise null.
   interfaces: [__Type!]
-  # must be non-null for INTERFACE and UNION, otherwise null.
+  # must be non-null for INTERSECTION, otherwise null.
+  memberTypes: [__Type!]
+  # must be non-null for INTERFACE, UNION, and INTERSECTION, otherwise null.
   possibleTypes: [__Type!]
   # must be non-null for ENUM, otherwise null.
   enumValues(includeDeprecated: Boolean = false): [__EnumValue!]
@@ -157,6 +160,7 @@ enum __TypeKind {
   OBJECT
   INTERFACE
   UNION
+  INTERSECTION
   ENUM
   INPUT_OBJECT
   LIST
@@ -210,6 +214,7 @@ enum __DirectiveLocation {
   ARGUMENT_DEFINITION
   INTERFACE
   UNION
+  INTERSECTION
   ENUM
   ENUM_VALUE
   INPUT_OBJECT
@@ -256,6 +261,7 @@ possible value of the `__TypeKind` enum:
 - {"OBJECT"}
 - {"INTERFACE"}
 - {"UNION"}
+- {"INTERSECTION"}
 - {"ENUM"}
 - {"INPUT_OBJECT"}
 - {"LIST"}
@@ -310,6 +316,25 @@ Fields\:
   union. They must be object types.
 - All other fields must return {null}.
 
+**Intersection**
+
+Intersections are a higher-order abstract type defined by a list of constraining
+abstract types. The list of constraining abstract member types is accessible in
+`memberTypes`. The possible types of an intersection are explicitly listed out
+in `possibleTypes`. Types can be made parts of intersections without
+modification of that type.
+
+Fields\:
+
+- `kind` must return `__TypeKind.INTERSECTION`.
+- `name` must return a String.
+- `description` may return a String or {null}.
+- `memberTypes` returns the list of constraining abstract types defined by the
+  intersection. They must be interface or union types.
+- `possibleTypes` returns the list of types that can be represented within this
+  intersection. They must be object types.
+- All other fields must return {null}.
+
 **Interface**
 
 Interfaces are an abstract type where there are common fields declared. Any type
@@ -470,6 +495,7 @@ supported. All possible locations are listed in the `__DirectiveLocation` enum:
 - {"ARGUMENT_DEFINITION"}
 - {"INTERFACE"}
 - {"UNION"}
+- {"INTERSECTION"}
 - {"ENUM"}
 - {"ENUM_VALUE"}
 - {"INPUT_OBJECT"}