infinite generator transform is no longer infinite

[mfulton26]

🔎 Search Terms

A function returning 'never' cannot have a reachable end point.

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about never and generator.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mAVAAggTwgGwKYB4AqAfABQxTYBOAhgEY4BcyAkudXXkQJSMDi2YlKlDgUCAGmQCAbpQngAJtmAwB8wsgDeAKGTIA7gAsYOZMSgUQ2TsjQxsmeSjKD2Abi0BfLVtCRYCFGwADyoAWwAHHDNgqEYAZ3MVAHNuZD4BamFRAFoARmQAH2RciWlKdW1dYBFTCAQE1AMqCmQ4YFQMKIBBCmo0ADpgCjhQ6KCoTkmbOwdG5uQAXiXkACJcFeQAfmQ85EZc9w8gA

💻 Code

function* cycle<T>(iterable: Iterable<T>): Generator<T, never, undefined> {
  while (true) yield* iterable;
}

function* example(text: string): Generator<-1 | 1, never> {
  for (const char of cycle(Array.from(text))) yield char === "<" ? -1 : 1;
}

🙁 Actual behavior

The second generator function build on top of the first where consumers of cycle do not need to check IteratorResult#done as the never return type is interpreted as never done. This is great for direct callers of cycle() but other generator functions building on top of cycle() are not inferred too as having a return type of never.

🙂 Expected behavior

A loop over an infinite generator (i.e. one with a return type of never) is also inferred as an infinite loop.

Additional information about the issue

A throw statement can be added after the loop as a workaround but I think this shouldn't be necessary:

function* example(text: string): Generator<-1 | 1, never> {
  for (const char of cycle(Array.from(text))) yield char === "<" ? -1 : 1;
  throw new Error("unreachable");
}

[justmoon]

Here is a slightly more minimal example (Playground):

function* zeros(): Generator<number, never> {
  for (;;) yield 0
}

function* ones(): Generator<number, never> {
  for (const num of zeros()) yield num+1
}

Gives error "A function returning 'never' cannot have a reachable end point. (2534)"

I expected TypeScript to be able to tell that a for..of on a Generator<..., never> could never finish similar to how it's able to tell that the for (;;) in the first function would never finish.