Union of Generator and AsyncGenerator introduces spurious undefined next() value path

[cefn]

Bug Report

I was creating a coroutine-based interactive prompt library. It can consume either synchronous or asynchronous co-routines.

There doesn't seem to be any runtime issue associated with this approach, (in some configurations the library harmlessly awaits on a synchronous generator.next()).

However, any function which is declared as fulfilling a union of async+sync generators faces a compile error when a spurious undefined value is indicated as possible from a yield even though the next() call only accepts strings.

🔎 Search Terms

yield, undefined, typescript, asyncgenerator, generator

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about AsyncGenerator

⏯ Playground Link

Playground link with relevant code

💻 Code

type SyncSequenceFactory = () => Generator<string, string, string>;

type AsyncSequenceFactory = () => AsyncGenerator<string, string, string>;

type SequenceFactory = SyncSequenceFactory | AsyncSequenceFactory

const syncFactory: SyncSequenceFactory = function* (){
  let name = "";
  while(!name){
    name = yield "What is your name?"
  }
  return `That's the end of the game, ${name}`
} 

const asyncFactory: AsyncSequenceFactory = async function* (){
  let name = "";
  while(!name){
    name = yield "What is your name?"
  }
  return `That's the end of the game, ${name}`
} 

const looserSyncFactory: SequenceFactory = function* (){
  let name = "";
  while(!name){
    name = yield "What is your name?"
  }
  return `That's the end of the game, ${name}`
} 

const looserAsyncFactory: SequenceFactory = async function* (){
  let name = "";
  while(!name){
    name = yield "What is your name?"
  }
  return `That's the end of the game, ${name}`
} 

🙁 Actual behavior

Compiler reports Type 'string | undefined' is not assignable to type 'string'.

🙂 Expected behavior

I don't believe this possible execution path is actually made possible by the union, since it is not possible in either of the unioned cases.