Missing error when awaiting dynamic import of module with 'then' export

TypeScript Version: 3.0.0-dev.20180605

Search Terms:

Code

// a.ts
export function then() { return true; }

// b.ts
async function test() {
  await import('./a');
}

// c.ts
// this is basically the same as b.ts
import * as ns from './a';
function myImport() {
  return Promise.resolve(ns);
}
async function test() {
  await myImport();
}

Expected behavior:
in b.ts: [ts] Type of 'await' operand must either be a valid promise or must not contain a callable 'then' member.
This would catch a lot of bugs with dynamic imports of modules that contain a then function. The resulting Promise will never resolve: https://github.com/tc39/proposal-dynamic-import/issues/47

Actual behavior:
No error in b.ts. The resulting type of the await is {}.

Playground Link:

Related Issues:

Start by reproducing the a.ts, b.ts, and c.ts examples with the reported TypeScript version, then compare dynamic import handling with the Promise.resolve namespace case. Trace the await operand checking and dynamic-import type inference. Done means b.ts reports the callable then diagnostic and a regression test covers the module export case.