Throws descriptive error when non-type used instead of interface

src/type/__tests__/validation-test.js

@@ -824,6 +824,22 @@ describe('Type System: Object fields must have output types', () => {
 });
 
 describe('Type System: Objects can only implement unique interfaces', () => {
+  it('rejects an Object implementing a non-type values', () => {
+    const schema = new GraphQLSchema({
+      query: new GraphQLObjectType({
+        name: 'BadObject',
+        interfaces: [undefined],
+      }),
+    });
+
+    expect(validateSchema(schema)).to.containSubset([
+      {
+        message:
+          'Type BadObject must only implement Interface types, it cannot implement undefined.',
+      },
+    ]);
+  });
+
   it('rejects an Object implementing a non-Interface type', () => {
     const schema = buildSchema(`
       type Query {

src/type/schema.js

@@ -152,11 +152,13 @@ export class GraphQLSchema {
       const type = this._typeMap[typeName];
       if (isObjectType(type)) {
         type.getInterfaces().forEach(iface => {
-          const impls = this._implementations[iface.name];
-          if (impls) {
-            impls.push(type);
-          } else {
-            this._implementations[iface.name] = [type];
+          if (isInterfaceType(iface)) {
+            const impls = this._implementations[iface.name];
+            if (impls) {
+              impls.push(type);
+            } else {
+              this._implementations[iface.name] = [type];
+            }
           }
         });
       }

src/type/validate.js

@@ -343,6 +343,15 @@ function validateObjectInterfaces(
 ): void {
   const implementedTypeNames = Object.create(null);
   object.getInterfaces().forEach(iface => {
+    if (!isInterfaceType(iface)) {
+      context.reportError(
+        `Type ${String(object)} must only implement Interface types, ` +
+          `it cannot implement ${String(iface)}.`,
+        getImplementsInterfaceNode(object, iface),
+      );
+      return;
+    }
+
     if (implementedTypeNames[iface.name]) {
       context.reportError(
         `Type ${object.name} can only implement ${iface.name} once.`,
@@ -360,15 +369,6 @@ function validateObjectImplementsInterface(
   object: GraphQLObjectType,
   iface: GraphQLInterfaceType,
 ): void {
-  if (!isInterfaceType(iface)) {
-    context.reportError(
-      `Type ${String(object)} must only implement Interface types, ` +
-        `it cannot implement ${String(iface)}.`,
-      getImplementsInterfaceNode(object, iface),
-    );
-    return;
-  }
-
   const objectFieldMap = object.getFields();
   const ifaceFieldMap = iface.getFields();