Preserve ordered yield types when no logic is involved

Suggestion

🔍 Search Terms

yield ordered return types

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Make types more definite for each yielded return value in a generator function

See also https://stackoverflow.com/questions/66538431/function-chain-factory-in-typescript-preserving-input-output-types

📃 Motivating Example

Right now, if I do this:

function* work() {
  yield 1;
  yield 'test';
  return [123];
}

Right now this gives:

const g = work();
g.next().value  // string | number | number[]
g.next().value  // string | number | number[]
g.next().value  // string | number | number[]

But since there's no logic/branching/etc, it would be more helpful to me to have:

const g = work();
g.next().value  // number
g.next().value  // string
g.next().value  // number[]

💻 Use Cases

Sometimes I want to use a function generator to represent a series of steps that I want to examine (whether at runtime or during testing, etc) before deciding whether to continue. In other words, I want to break up a function into a series of "pauseable" steps.

The issue names no files, tests, or entry points. Start by validating the generator examples and reviewing the linked Stack Overflow use case. Done means successive generator results expose the ordered yield and return types without changing JavaScript runtime behavior.