React Compiler doesn't memoize class instances or their dependents without explicit useMemo

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Summary

React Compiler doesn't memoize:

1. Class instance creation (new Class())
2. Any expression that depends on that class instance

This means that a single un-memoized class instance "poisons" the entire dependency chain —
even expressions that could theoretically be safely memoized are skipped.

Reproduction

Playground link (with useMemo):
https://playground.react.dev/#N4Igzg9grgTgxgUxALhAgHgBwjALgAjggDswCBZATwAkIIBrfAXnwAoBKZgPn2AB1i+IYRJl8kALYIAwgBsAhmDABJUrnnFEzfFDAJyCCRFYdu+YggDu+OYrAcANPgDaAXXYDhIteIm4AFtqSMgpKqmQaiAB0AGZ0rOgegsIwCLiwgmB+-gIAvgDcIA4gRMQxAJYA5igg5RLYePi4lJgIvPgACrJQleXEAPKYuOWi+Ln4MTAQEvgA5ABG8vMIsgC0mN29xKup8nC4q0T15bIIMAD0ACblZLP5AgKs-Mnn50eYJ-LDJOQQlwjIfB8EDyWSyYF5cRfG4VBBgTqbPqDb6kdiFYpgfwQSyqXBnYigsAoGKEhC5IA

With useMemo, the compiler memoizes both someClassInstance and smth variables. Once useMemo is removed, all memoization is gone.

Questions

1. Is this expected behavior or a limitation?
2. What determines whether an expression's dependencies are "stable" enough for memoization?
3. Is there documentation about this?

[aniruddhaadak80]

Hi there! Taking a look at your issue regarding React Compiler doesn't memoize class instances or their dependents without explicit useMemo in the context of reactwg/react-compiler...

When working with the React Compiler (React Forget), this type of behavior usually stems from how the compiler analyzes hook dependencies and component boundaries. Make sure that:

1. Your code strictly adheres to the Rules of React. The compiler bails out or behaves unexpectedly when it detects rule violations.
2. In certain cases, explicitly defining memoization boundaries or isolating the hook/component into its own tightly-scoped file helps the Babel plugin successfully parse the AST.
3. Verify your ESLint eslint-plugin-react-compiler rules are green before compilation, as the compiler relies heavily on those guarantees.

Hope this helps clarify the expected boundaries! Let me know if you have a minimal repro I can test against.