Type annotation needed only when component is root of exported component?

[FezVrasta]

Flow version: 0.110.1

Expected behavior

The un-annotated component should not need any type annotations.

Actual behavior

Flow requires me to annotate a component if I'm gonna use it inside an exported component, but this happens only if the un-annotated component is rendered as root of the exported component.

If I wrap the un-annotated component with something else (a fragment, a div, etc), then Flow doesn't require me to annotate it.

I know Flow wants us to annotate the exported functions, components, etc... but I'm not exporting that component, I'm using it inside a component that's getting exported.

Why is it behaving this way?

• Link to Try-Flow or Github repo: Try-Flow repro here

[nmote]

This is because Flow infers the return type of Ko to be something like React$Element<typeof ContainerB>, which causes the type of ContainerB to end up in the externally-visible interface. This means that all input positions must be annotated. If you don't want that type to end up in the externally-visible interface, you can use a more generic return type annotation for Ko, like React$Node. See this playground example

[FezVrasta]

Thank you for the information!

What's the need to use React$Element<typeof ContainerB> as return type? Shouldn't be enough to return one of the types that compose React$Node (for example, if my component returns an array?

[nmote]

In most cases React$Node should work just fine. Flow just infers the more specific type by default.