Add the missing @flow annotation

[ryerh]

Add @flow annotation to packages/react-reconciler/src/ReactFiber.js.

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[aweary]

😮 I'm surprised we didn't catch this earlier. We'll need to fix all the new Flow errors before merging. Do you want to give that a try @ryerh? Happy to help if so.

cc @acdlite

[ryerh]

@aweary I'm willing to contribute😊 I'll try to fix all the new Flow errors and let CI pass, in my free time this week.

But if you do think those errors are urgent, you can just call someone familiar with the code to fix it then close this PR instead.

[ryerh]

Hi @aweary , I have fixed most of the flow errors, but there are three errors remain to be discussed.

They are:

1. When the code runs to this line ReactFiber.js#L418, the fiberTag variable should have been assigned a proper value rather than null, otherwise an error will be thrown. But flow doesn't known anything about it.
2. When the code runs to this line ReactFiberScheduler.js#L1081, the nextUnitOfWork should have been assigned a proper value rather than null, otherwise the loop will be breaked. But flow doesn't known anything about it, neither.
3. ReactFiberScheduler.js#L1085, same as above.

How do we solve them?

[aweary]

Since this requires some complex decisions to be made about internal types, I'm going to close this in favor of @gaearon's fix in #12842.

Thanks for putting in the work here @ryerh! We appreciate it.

[gaearon]

Sorry, I completely missed this PR and had no idea about this 😞
I independently found it today and am working on #12842.

@aweary If you see something like this next time please ping me directly :-). This is an extremely bad issue and we shouldn’t have missed it.

[gaearon]

@ryerh I'm sorry to duplicate your work. I should've read this PR when I saw it on notifications. I initially thought the PR is just a one-line change (from its title) and didn't look at it, and then forgot about it.

[aweary]

I will! I had pinged @acdlite but he probably didn't see it, I'll be sure to sound the alarm more urgently next time 👍

[gaearon]

I mean on FB workplace :-) I don't think either of us check GH mentions, there's way too much noise and we can't keep up.

[ryerh]

@aweary @gaearon I am feeling happy to be helpful.

Some other words: I think writing flow-oriented code in order to let flow-check pass makes me feel tricky. The typing system should work for safety, not for programming burden.

[gaearon]

It's a tradeoff. But writing this kind of code isn't just appeasing the type checker, it legitimately forces you to deal with potentially buggy code paths by making them safer or more explicit.