Support iterating generators in browsers without Symbol

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #13128
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

In browsers without Symbol support and without a Symbol polyfill, regenerator uses @@iterator/@@asyncIterator to mark (async) generators as iterable. (ref)

Since from a user point of view regenerator is effectively part of Babel (because we use it by default in @babel/preset-env), we should make sure that it works well with the other plugins.

This PR adds checks for @@iterator/@@asyncIterator in the spread and for-of helpers, so that they work with compiled generators.

[babel-bot]

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[matthieusieben]

That was fast 🤩
You rock

[zloirock]

It's a crutch and solves only a part of the problem. As you could see, #13128 use polyfilling with corejs: { version: 3 }, useBuiltIns: 'usage'. The problem is that polyfills are injected only to the userland code, not regenerator/runtime. But regenerator uses other features that should be polyfilled too.

The actual core-js features work with "@@iterator" string like with Symbol.iterator, but not with "@@asyncIterator" - in case of #13128, Babel async generators will not be able to work with core-js features like iterators helpers. More other, I planned to drop "@@iterator" fallback from core-js@4.

The proper solution here - if you load a polyfill with useBuiltIns: 'usage' and required to inject regenerator/runtime, inject all polyfills required for regenerator/runtime.

[nicolo-ribaudo]

The goal of this PR is to allow using generators without a polyfill, similarly to how array spread or array for-of works without polyfills.

More other, I planned to drop "@@iterator" fallback from core-js@4.

Wouldn't that make it incompatible with some Firefox versions, that implemented the iterator protocol before implementing symbol support?

[zloirock]

The goal of this PR is to allow using generators without a polyfill, similarly to how array spread or array for-of works without polyfills.

However, #13128 uses polyfills.

Wouldn't that make it incompatible with some Firefox versions, that implemented the iterator protocol before implementing symbol support?

Nobody bothers to implement iterators from scratch. It will not work with for-of syntax in critically old FF versions, but anyway it should be transpiled. About collections iterators - for them, it's just a key that can be redefined.

[nicolo-ribaudo]

I'm merging this since:

• It solves the case without polyfills (and Babel helpers should be compatible with each other even without loading a polyfill)
• It currently solves the case with a polyfill in most cases

As @zloirock noted the fix isn't complete, and we'll need to polyfill symbols when using generators.