Generator does not work with federated schema

Describe the bug The code generator throws an error when a federated schema is passed in.

To Reproduce Steps to reproduce the behavior: Hello, I am trying to break down my graphql schema into different files and modules. I am working with a federated schema. I have imported the different files and generated a GraphQLSchema object in an index.ts file like so:

import { gql } from "apollo-server";
import fs from "fs";
import path from "path";
import { resolvers } from "../server/resolvers";
import { buildFederatedSchema } from "@apollo/federation";

const { buildSchema } = require("graphql");

const mainSchema = gql(
  fs.readFileSync(path.join(__dirname, "./schema.graphql"), "utf-8")
);

const expiryDate = gql(
  fs.readFileSync(path.join(__dirname, "./expiryDate.graphql"), "utf-8")
);

export const schema = buildFederatedSchema({
  resolvers,
  typeDefs: [mainSchema, expiryDate],
});

To generate this schema for typescript, I use the code generator with the following codegen.yaml :

overwrite: true
schema:
  - "./src/typeDefs/index.ts"
documents: null
generates:
  ./generated/graphql.ts:
    plugins:
      - "typescript"
      - "typescript-resolvers"
    config:
      contextType: "../src/interfaces/IContext#IContext"
      federation: true
      useIndexSignature: true

When I run the command to generate the code, I get the following error:

Failed to load schema from ./src/typeDefs/index.ts:

    Cannot read property 'concat' of undefined
    TypeError: Cannot read property 'concat' of undefined
at extendDefinition (/Users/talaltoukan/Developer/Ziina/ziina/node_modules/@graphql-toolkit/common/index.cjs.js:362:45)
at Object.printSchemaWithDirectives (/Users/talaltoukan/Developer/Ziina/ziina/node_modules/@graphql-toolkit/common/index.cjs.js:342:64)
at /Users/talaltoukan/Developer/Ziina/ziina/node_modules/@graphql-toolkit/core/index.cjs.js:629:46
at Array.map (<anonymous>)

dotansimha / graphql-code-generator

Generator does not work with federated schema #6099