Error using 'for await' on a union.

[rbuckton]

TypeScript Version: nightly (2.4.0-dev.20170530)

Code

async function f<T>(source: Iterable<T> | AsyncIterable<T>) {
  for await (const x of source) { 
  }
}

Expected behavior:

x has type T

Actual behavior:

error TS2504: Type must have a '[Symbol.asyncIterator]()' method that returns an async iterator.