//commonjs-export-function.js
module.exports = function() {
    return 'func';
};

//commonjs-export-function.d.ts
declare function func(): string;
export = func;

//main.ts
import { func } from './commonjs-function';

console.log(func());

tsc main.ts && node main.js
main.ts(1,22): error TS2497: Module '"/Users/aleksandar/projects/typescript-playground/commonjs-function"' resolves to a non-module entity and cannot be imported using this construct.

*~ Note that ES6 modules cannot directly export callable functions.
*~ This file should be imported using the CommonJS-style:
*~   import x = require('someLibrary');
...
export = MyFunction;
declare function MyFunction(): string;

//commonjs-export-function.d.ts
declare function func(): string;
export = func;

//main.ts
import func = require('./commonjs-export-function');

declare module 'promise-map-limit' {

  type IIteratee<T, R> = (value: T) => Promise<R> | R;

  function mapLimit<T, R>(
    iterable: Iterable<T>,
    concurrency: number,
    iteratee: IIteratee<T, R>
  ): Promise<R[]>;

  export = mapLimit;

}