[Compiler Bug]: math operators prevent caching

[lstkz]

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/#N4Igzg9grgTgxgUxALhAMygOzgFwJYSYAEAYjHgpgCYAyeYOAFMEWuZVWEQL4CURwADrEiAGwQ4iADyIBeIgikAHSmDwA3BIzYVqYXgG5hRaQGpTRkTAmxijYyaIAeKhoB8DxwKndPzgPSu6h4ihsLcINxAA

Repro steps

  let x = expensive(friends);
  x++;

It's not memoized, and expensive is called on every render.

---

https://playground.react.dev/#N4Igzg9grgTgxgUxALhAMygOzgFwJYSYAEAYjHgpgCYAyeYOAFMEWuZVWADREQ4AWCGEQC+ASiLAAOsSIAbBDiIAPIgF4iCZQAdKYPADcEjNhWpgxAbhlEieNEUZ9BMCdNm3lAai-XZImyIYRVhiRkDbAB4qQwA+CNtJZQCPIkiAehiDeNkrGREQESA

  let x = expensive(friends);
  if (other) {
    x++;
  }

Similar. Not cached.

---

https://playground.react.dev/#N4Igzg9grgTgxgUxALhAMygOzgFwJYSYAEAYjHgpgCYAyeYOAFMEWuZVWADREQ4AWCGEQC+ASiLAAOsSIAbBDiIAPIgF4iCZQAdKYPADcEjNhWpgxAbhlEieNEUZ9BMCdNm3VGgIw3RfmEVYYkY-WwAeKkMAPjDbSWUROPCAeiiDWNkrGREQESA

  let x = expensive(friends);
  if (other) {
    x = 1
  }

Cached properly.

How often does this bug happen?

Every time

What version of React are you using?

latest from playground

What version of React Compiler are you using?

latest from playground

[guillaumebrunerie]

I think this is a duplicate of #29172. The compiler does not cache expensive function calls. It only caches values that risk triggering a useless rerender, and by doing x++ you’re telling the compiler that x is a number which cannot trigger useless rerenders.

[josephsavona]

@guillaumebrunerie yup this is a duplicate, thanks for flagging! Great explanation.