Component functiosn aren’t executed recusively, they’re pushed into a stack to be processed later on. When you return the <Ctx.Provider value=…><Foo /></Ctx.Provider> VDOM, that Foo component won’t be executed within the sync execution of Ctx.Provider. If Foo were to useContext(Ctx), that would be tracked as part of the component’s internal state (which itself is stored in a AC), it woulnd’t store the context on its own AC variable.

I think Stephen was arguing for a more narrow scope

it's not that it wouldn't work, but that it wouldn't be idiomatic, or it'd take a lot more boilerplate than the alternative

No, there’s a lot of cases where taking a callback just doesn’t work. The example of storing a database connection in the top-level of one file and then trying to use it in another, for example. A common bootstrapping practice with top-level await.

There are also quite a few cases where APMs need to use enterWith because we just don’t have the ability to wrap a desired scope in any sort of callback—we can’t change how user code behaves yet need to be able to flow context around it.

this is about whether the proposal should support an api like AsyncLocalStorage#enterWith? What's the reason to not support it?

The reason is that it breaks encapsulation. The current proposal means that you have strong guarantees that a variable won't change during the course of a function, but if a child task can enterWith a new value, then it can affect the parent task in unexpected ways. It also runs afoul of the requirements imposed by SES, and is unlikely to be accepted by the committee.

That said, if you want to make your own enterWithable variable, it's easy enough:

class EnterableVar<T> {
  private readonly internal: AsyncContext.Variable<[T]>;
  constructor(opts = {}) {
    this.internal = new AsyncContext.Variable({...opts, defaultValue: [opts.defaultValue]});
  }
  run(val, fn) {
    return this.internal.run([val], fn);
  }
  get() {
    return this.internal.get()[0];
  }
  enterWith(val) {
    this.internal.get()[0] = val;
  }
}