[pull] main from facebook:main #276
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…et value is derived from a ref (#34462) @stipsan found this issue where the compiler would bailout on the `useLayoutEffect` examples in the React docs. While setState in an effect is typically an anti-pattern due to the fact that it hurts performance through cascading renders, the one scenario where it _is_ allowed is if the value being set flows from a ref.
Two small QoL improvements inspired by feedback:
* `if (ref.current === undefined) { ref.current = ... }` is now allowed.
* `if (!ref.current) { ref.current = ... }` is still disallowed, but we
emit an extra hint suggesting the `if (!ref.current == null)` pattern.
I was on the fence about the latter. We got feedback asking to allow `if
(!ref.current)` but if your ref stores a boolean value then this would
allow reading the ref in render. The unary form is also less precise in
general due to sketchy truthiness conversions. I figured a hint is a
good compromise.
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[//]: # (BEGIN SAPLING FOOTER)
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* __->__ #34449
* #34424
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