RFC: Enforce Interfaces having at least one implementing Object

It is unclear whether the spec validation from graphql/graphql-spec#424 as implemented by #1277 is what we want to have for the GraphQL Schema long term. This RFC lets us explore that option more, to see whether the breaking schema change is the right path forward.

Author: mjmahone

src/type/__tests__/validation-test.js

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

src/type/validate.js

@@ -257,6 +257,9 @@ 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);
@@ -364,6 +367,21 @@ 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,