[Compiler]: Detection for calling components as functions (instead of JSX)

[niklas-io]

What kind of issue is this?

• React Compiler core (the JS output is incorrect, or your app works incorrectly after optimization)
• babel-plugin-react-compiler (build issue installing or using the Babel plugin)
• eslint-plugin-react-compiler (build issue installing or using the eslint plugin)
• react-compiler-healthcheck (build issue installing or using the healthcheck script)

Link to repro

https://playground.react.dev/#N4Igzg9grgTgxgUxALhAMygOzgFwJYSYAEAsgJ4CCADlQBQCURwAOsUXIWDkQNpcCGOKGAA0RMAhwBlHIOEBdIgF4iwhDMEIAPACMIEADYJ+mAHy00-AxPoBuVqyJEYk2MS0ATPADdTjp0wCQmBEAGShpGQAkjgIALYMAL7+TrpQODiERIQAwgZ4cADWSsAMyqbikhrBtDgwUAj0iaYUHh5aAPQ66Zlm-p1evqzJmKwY2PhZ5DHxZSxsLkIw7lSm1cLIRHUNnavDIIlAA

Repro steps

I am using conditional rendering ({status && MyItem}) in my JSX. When status is changed from false to true, the application crashes due to hooks not matching between renders. It works as expected without the compiler.

To reproduce, simply press the button in the example.

How often does this bug happen?

Every time

What version of React are you using?

18.2.0

What version of React Compiler are you using?

19.1.0-rc.1

[josephsavona]

Hey, thanks for posting. What happening here is that you're calling a component as a function — specifically, MyItem() instead of <MyItem />. In React components are not safe to call directly precisely because it can mean that you call hooks conditionally. The same issue can happen if you were to put this code inside a useMemo() ({useMemo(status && MyItem(), [])}.

The fix is to ensure that you're always calling components with JSX: https://playground.react.dev/#N4Igzg9grgTgxgUxALhAMygOzgFwJYSYAEAsgJ4CCADlQBQCURwAOsUXIWDkQNpcCGOKGAA0RMAhwBlHIOEBdIgF4iwhDMEIAPACMIEADYJ+mAHy00-AxPoBuVqyJEYk2MS0ATPADdTjp0wCQmBEAGShRFrkAJI4CAC2RAD0pgC+-k66UDg4hESEAMIGeHAA1krADMqm4pIawbQ4MFAI9KmmFB4eWkk62blm-j1evqzpmKwY2Ph5MXHxVSxsLkIw7lSm9cLIRE0tPRtjIKlAA

See https://react.dev/reference/rules/react-calls-components-and-hooks#never-call-component-functions-directly for more.

[josephsavona]

Reopening to see if we can make this easier to detect/debug. We ran into this same issue internally and the compiler has an option to detect potential cases of components being invoked as a call instead of with jsx, which would flag this case (playground). However the validation can have false positives — by default it just looks for calls to functions where the function name is capitalized like a component. But of course a developer might have non-component functions that are capitalized.

cc @mofeiZ and @jbrown215 who have thought about this a bunch as well.