Likely to cause new Flow errors:

• When component syntax support is enabled, upper case functions with component-like name but doesn't return React.Node will no longer be treated like components. Thus, all the hooks call in such functions will have react-rule-hook-definitely-not-in-component-or-hook errors.
• Usage of Object.assign will now trigger unsafe-object-assign lint error that's on by default. The type checking behavior for Object.assign will otherwise stay the same for now.
• When Flow decides that the hook call definitely doesn't happen within component or hook body, it will emit errors with code react-rule-hook-definitely-not-in-component-or-hook.

Parser:

• Parse the TS nonnull assertion operator.