Add support for iterating over enums

[Timmmm]

I'm fairly confident about the codegen stuff, but I have no idea about the type checker.

I also couldn't make it work with type aliases - it complains that the alias is only a type but you're trying to use it as a value. Edit: Actually I think that makes sense.

Fixes #42457

[awerlogus]

Does it work with https://www.typescriptlang.org/tsconfig#preserveConstEnums disabled?

[Timmmm]

Does it work with https://www.typescriptlang.org/tsconfig#preserveConstEnums disabled?

You mean enabled? In any case it doesn't work with const enums at all. I think that's what you'd expect given what const enums are.

I also couldn't make it work with type aliases

I realised why this is - you can only iterate over objects. It works with enums because enums are objects (I think they're objects and types). It doesn't work with the type alias because those aren't objects. So it turns out you can do a sort of enum alias by copying the object like this:

enum Test {
  Foo,
  Bar,
}

const AlsoTest = Test;

for (const m of Alsotest) { ...

Kind of confusing but I guess it makes sense.

[awerlogus]

You mean enabled?

No. I asked this because if you disable that option, compiler won't generate the object you can iterate on. So, when somebody will try to iterate some enum, it will produce a runtime error. You must ensure that preserveConstEnums is enabled or show an error otherwise.

[Timmmm]

Ah right. If you check the code (it's really short!) you can see where it only enables iteration over RegularEnums precisely for that reason.

[Timmmm]

Ok I think this is actually ready. I improved the code a bit and added a simple test.

I have not updated the test baselines so that review is easier, because a million of them change (every one with an enum).

[typescript-bot]

The TypeScript team hasn't accepted the linked issue #42457. If you can get it accepted, this PR will have a better chance of being reviewed.

[sandersn]

Given that we want to wait to make runtime changes to enum until they reach stage 3 in TC39, I'm going to close this PR. We might be able to re-use parts of it for the standard enums, but the whole PR is likely to be much more complex.