[compiler] Add useFormContext to known incompatible libraries

[everettbu]

Mirror of facebook/react#35465
Original author: reyronald

---

The original implementation in #34027 added useForm from react-hook-form to the known incompatible libraries list, but missed useFormContext. useFormContext returns the same API surface as useForm, including the watch() function that cannot be memoized safely.

This PR adds useFormContext with the same configuration as useForm

I repurposed @Maxou44 's original repro repo from react-hook-form/react-hook-form#11910 to demonstrate that the issue can be reproduced with useFormContext too, you can play around with the updated demo if you want to confirm, repo link below

https://github.com/reyronald/issue-react-compiler-react-hook-form

Repro details

import {
  useForm,
  useWatch,
  useFormContext,
  FormProvider,
} from "react-hook-form";

export default function App() {
  const methods = useForm();
  const { register, handleSubmit, watch, control } = methods;
  const onSubmit = (data) => console.log(data);

  const test = useWatch({ control, name: "test" });

  return (
    <div>
      <Headers description="Performant, flexible and extensible forms with easy-to-use validation." />

      <FormProvider {...methods}>
        <form onSubmit={handleSubmit(onSubmit)}>
          <input {...register("test")} placeholder="Test field" />

          <div>useForm.watch is {watch("test")}</div>

          <div>useForm.useWatch value is {test}</div>

          <Component />

          <input type="submit" />
        </form>
      </FormProvider>
    </div>
  );
}

function Component() {
  const { watch, control } = useFormContext();

  const test = useWatch({ control, name: "test" });

  return (
    <>
      <div>useFormContext.watch is {watch("test")}</div>

      <div>useForm.useWatch value is {test}</div>
    </>
  );
}

References

1. issue: Watch don't work with React Compiler (React 19) react-hook-form/react-hook-form#11910
2. #34027
3. #34493

[greptile-apps]

Greptile Overview

Greptile Summary

This PR correctly adds useFormContext to the list of known incompatible React Hook Form APIs. The implementation mirrors the existing useForm configuration, marking only the watch() function as incompatible with memoization.

Key Changes:

• Added useFormContext hook configuration with identical structure to useForm
• Properly marked the watch() function with ValueKind.Mutable and knownIncompatible message
• Used consistent configuration: Effect.Read for restParam and calleeEffect, matching the existing pattern

The change is minimal, focused, and correctly addresses the gap identified in the original implementation. Since useFormContext returns the same API surface as useForm (including the problematic watch() function), this addition is necessary for complete coverage.

Confidence Score: 5/5

• This PR is safe to merge with no identified issues
• The change is a straightforward addition that perfectly follows the established pattern. The configuration is identical to the existing useForm setup, correctly addresses a documented gap, and includes no logical errors or inconsistencies. The implementation is well-documented with inline comments explaining the incompatibility.
• No files require special attention

Important Files Changed

File Analysis

Filename	Score	Overview
compiler/packages/babel-plugin-react-compiler/src/HIR/DefaultModuleTypeProvider.ts	5/5	Added useFormContext hook configuration identical to useForm, correctly marking the watch() function as incompatible with memoization

[greptile-apps]

Greptile Summary

This PR adds useFormContext from react-hook-form to the React Compiler's known-incompatible libraries list, complementing the existing useForm entry added in #34027. Since useFormContext exposes the same API surface as useForm — including the memoization-unsafe watch() function — the omission was a bug that caused the compiler to silently produce incorrect memoization for components that consumed the context variant.

Changes:

• Adds a useFormContext entry under the react-hook-form case in DefaultModuleTypeProvider.ts, with identical configuration to useForm (hook kind, watch property marked ValueKind.Mutable / knownIncompatible).
• The knownIncompatible message is correctly scoped to useFormContext() rather than useForm().

Notes:

• No test fixtures are added for useFormContext. The repository currently has no dedicated fixture specifically exercising useForm's incompatibility either, so this is consistent with the existing test coverage model, but adding a fixture (similar to the existing error.invalid-known-incompatible-hook-return-property pattern) would improve confidence.

Confidence Score: 5/5

• This PR is safe to merge — it is a minimal, correctly structured addition that mirrors an existing, well-tested pattern.
• The change is a single-entry addition to a known-incompatible registry, structurally identical to the already-merged useForm entry. The knownIncompatible message, hook kind, property shape, ValueKind.Mutable, and Effect.Read settings are all consistent with the existing entry. The motivation is clearly documented and supported by a reproducible demo. No existing logic is modified.
• No files require special attention.

Important Files Changed

Filename	Overview
compiler/packages/babel-plugin-react-compiler/src/HIR/DefaultModuleTypeProvider.ts	Adds useFormContext to the react-hook-form known-incompatible block, mirroring the existing useForm entry exactly — same hook shape, same watch property marked mutable/incompatible, and a correctly updated knownIncompatible message.

_{Last reviewed commit: c0951ba}