Allow interfaces to have no implementors (graphql#1376)

* Allow interfaces to have no implementors Reverts graphql#1277, and implements spec removal from graphql#459. While it's an open question whether this validation is good, we need to provide an upgrade path for schemas that currently do not satisfy this validation rule. * Update test to accept unimplemented interface

Author: mjmahone

src/type/__tests__/validation-test.js

@@ -1205,7 +1205,7 @@ describe('Type System: Interface fields must have output types', () => {
  ]);
  });
 
- it('rejects an interface not implemented by at least one object', () => {
+ it('accepts an interface not implemented by at least one object', () => {
  const schema = buildSchema(`
  type Query {
  test: SomeInterface
@@ -1215,13 +1215,7 @@ describe('Type System: Interface fields must have output types', () => {
  foo: String
  }
  `);
- expect(validateSchema(schema)).to.deep.equal([
- {
- message:
- 'Interface SomeInterface must be implemented by at least one Object type.',
- locations: [{ line: 6, column: 7 }],
- },
- ]);
+ expect(validateSchema(schema)).to.deep.equal([]);
  });
 });
 

src/type/validate.js

@@ -257,9 +257,6 @@ function validateTypes(context: SchemaValidationContext): void {
  } else if (isInterfaceType(type)) {
  // Ensure fields are valid.
  validateFields(context, type);
-
- // Ensure Interfaces include at least 1 Object type.
- validateInterfaces(context, type);
  } else if (isUnionType(type)) {
  // Ensure Unions include valid member types.
  validateUnionMembers(context, type);
@@ -367,21 +364,6 @@ function validateObjectInterfaces(
  });
 }
 
-function validateInterfaces(
- context: SchemaValidationContext,
- iface: GraphQLInterfaceType,
-): void {
- const possibleTypes = context.schema.getPossibleTypes(iface);
-
- if (possibleTypes.length === 0) {
- context.reportError(
- `Interface ${iface.name} must be implemented ` +
- `by at least one Object type.`,
- iface.astNode,
- );
- }
-}
-
 function validateObjectImplementsInterface(
  context: SchemaValidationContext,
  object: GraphQLObjectType,