Invalid code typechecks in generators with strictNullChecks #22290

TypeScript Version: 2.8.0-dev.20180302

Search Terms: generator, yield, strictNullChecks, typechecking

Code

function* myGenerator(): IterableIterator<number> {
  yield;
  yield 2;
}

for (let n of myGenerator()) {
  let number: number = n;
  console.log(number);
}

tsconfig.json:

{
  "compilerOptions": {
    "target":"es6",
    "strictNullChecks": true,
  }
}

Expected behavior:
I would expect yield; (without expression) to be forbidden if the generator return type (IterableIterator<number>) doesn't allow undefined.

Actual behavior:
This compiles fine, prints 'undefined', '2' (we're essentially allowing undefined to be assigned to non-nullable number).

Playground Link: link