Under RN_STRICT_API no feature-flag header is public, so a library cannot read a flag and an app cannot override one #59982

Description

Every header in react/featureflags/ but ReactNativePublicFeatureFlags.h includes react/cxxstableapi/PrivateGuard.h, which is an #error once a build defines RN_STRICT_API. ReactNativePublicFeatureFlags.h declares ReactNativeFeatureFlags_DO_NOT_USE with seven flags and no way to override one. So with the strict C++ API on:

• a library whose Fabric code must agree with a flag (preventShadowTreeCommitExhaustion, enableFabricCommitBranching, updateRuntimeShadowNodeReferencesOnCommit, cxxNativeAnimatedEnabled, enableFlexboxAutoMinSizeInStrictMode) has no header it may include to read it;
• an app has no header it may include to override a flag through ReactNativeFeatureFlagsOverridesOSSExperimental or ReactNativeFeatureFlags::override.

0.88.1 has the <React/FeatureFlags.h> umbrella for both; #58744 removed it and made the module private, from 0.89.0-rc.0.

Expected: a consumer that defines RN_STRICT_API can read every flag and override flags through a header the guards accept. Builds without RN_STRICT_API stay as they are.

Steps to reproduce

1. Clone the reproducer below. ReproducerApp/cpp/FlagReader.cpp reads the five flags above through <react/featureflags/ReactNativeFeatureFlags.h>; ReproducerApp/cpp/FlagOverrides.cpp overrides one with a ReactNativeFeatureFlagsOverridesOSSExperimental subclass and ReactNativeFeatureFlags::override.
2. Run its repro.sh:

   docker run --rm -v "$PWD:/repro:ro" gcc:15 bash /repro/repro.sh

   It sparse-clones react/react-native at main and compiles both files against its ReactCommon with -fsyntax-only, once plain and once with -DRN_STRICT_API. bash repro.sh v0.89.0-rc.1 does the same on the release candidate.
3. Expected: both files compile with -DRN_STRICT_API, or a public header offers the same reads and the override. Actual: both compile without it and stop on the private-header #error with it, and the public header declares none of the five flags:

   react/react-native de7e643aa30b4e87376898b74037bd9712e9d4d6
   == which flags ReactNativePublicFeatureFlags.h declares
   class ReactNativeFeatureFlags_DO_NOT_USE {
   preventShadowTreeCommitExhaustion: not declared
   enableFabricCommitBranching: not declared
   updateRuntimeShadowNodeReferencesOnCommit: not declared
   cxxNativeAnimatedEnabled: not declared
   enableFlexboxAutoMinSizeInStrictMode: not declared
   == FlagReader.cpp without RN_STRICT_API
   compiles
   == FlagReader.cpp with -DRN_STRICT_API
   react/cxxstableapi/PrivateGuard.h:32:2: error: #error "This is a private React Native header and is not part of the public API. Do not include it directly; it has no stability guarantee."
   == FlagOverrides.cpp without RN_STRICT_API
   compiles
   == FlagOverrides.cpp with -DRN_STRICT_API
   react/cxxstableapi/PrivateGuard.h:32:2: error: #error "This is a private React Native header and is not part of the public API. Do not include it directly; it has no stability guarantee."
   SUMMARY: under RN_STRICT_API 2 of 2 files stop on the private-header #error: no feature-flag header is public (react/react-native de7e643aa3)
   

   The script exits 1 while a strict compile stops on that #error and 0 once both compile.

React Native Version

0.89.0-rc.1

Affected Platforms

Build - Linux, Build - Windows

Areas

Fabric - The New Renderer

Output of npx @react-native-community/cli info

System:
  OS: Linux 6.18 Debian GNU/Linux 13 (trixie) 13 (trixie)
  CPU: (24) x64 AMD Ryzen 9 9950X3D 16-Core Processor
  Memory: 120.35 GB / 125.63 GB
  Shell:
    version: 5.2.37
    path: /bin/bash
Binaries:
  Node:
    version: 24.22.3
    path: /usr/bin/node
  Yarn: Not Found
  npm:
    version: 11.19.1
    path: /usr/bin/npm
  Watchman: Not Found
SDKs:
  Android SDK: Not Found
IDEs:
  Android Studio: Not Found
Languages:
  Java: Not Found
  Ruby: Not Found
npmPackages:
  "@react-native-community/cli":
    installed: 21.0.0
    wanted: ^21.0.0
  react: Not Found
  react-native:
    installed: 0.89.0-rc.1
    wanted: ^0.89.0-rc.1
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: Not found
  newArchEnabled: Not found
iOS:
  hermesEnabled: Not found
  newArchEnabled: Not found

Stacktrace or Logs

react/cxxstableapi/PrivateGuard.h:32:2: error: #error "This is a private React Native header and is not part of the public API. Do not include it directly; it has no stability guarantee."

MANDATORY Reproducer

https://github.com/YevheniiKotyrlo/react-native-feature-flags-under-strict-cxx-api

Screenshots and Videos

No response