[MSVC] Flutterfire failed due to warning C4238 on Windows with VS2026

[QuellaZhang]

Is there an existing issue for this?

• I have searched the existing issues.

Which plugins are affected?

No response

Which platforms are affected?

No response

Description

Hi All,

The MSVC team regularly builds popular open-source projects, including yours, with development versions of the build tools in order to find and fix regressions in the compiler and libraries before they can ship and cause trouble for the world. This also allows us to provide advance notice of breaking changes, which is the case here.

Flutterfire failed due to warning C4238 on Windows with VS2026. This is a new problem, could you please help take a look? Thanks in advance!

Reproducing the issue

1. Open VS2026 x64 CMD.
2. git clone https://github.com/firebase/flutterfire.git C:\gitP\firebase\flutterfire
3. cd C:\gitP\firebase\flutterfire
4. Download https://storage.googleapis.com/flutter_infra_release/releases/stable/windows/flutter_windows_3.41.3-stable.zip
5. set PATH=C:\gitP\firebase\flutterfire\tools\flutter\bin;%PATH%
6. dart pub cache clean --force
7. dart run melos bootstrap
8. cd /d C:\gitP\firebase\flutterfire\tests\windows\
9. flutter pub get

Firebase Core version

latest source code

Flutter Version

latest source code

Relevant Log Output

firebase_auth_plugin.cpp
C:\gitP\firebase\flutterfire\packages\firebase_auth\firebase_auth\example\windows\flutter\ephemeral\.plugin_symlinks\firebase_auth\windows\firebase_auth_plugin.cpp(821): error C2220: the following warning is treated as an error
C:\gitP\firebase\flutterfire\packages\firebase_auth\firebase_auth\example\windows\flutter\ephemeral\.plugin_symlinks\firebase_auth\windows\firebase_auth_plugin.cpp(821): warning C4238: nonstandard extension used: class rvalue used as lvalue
C:\gitP\firebase\flutterfire\packages\firebase_auth\firebase_auth\example\windows\flutter\ephemeral\.plugin_symlinks\firebase_auth\windows\firebase_auth_plugin.cpp(1022): warning C4238: nonstandard extension used: class rvalue used as lvalue
C:\gitP\firebase\flutterfire\packages\firebase_auth\firebase_auth\example\windows\flutter\ephemeral\.plugin_symlinks\firebase_auth\windows\firebase_auth_plugin.cpp(1066): warning C4238: nonstandard extension used: class rvalue used as lvalue

Flutter dependencies

Expand Flutter dependencies snippet

Replace this line with the contents of your `flutter pub deps -- --style=compact`.

Additional context and comments

Build.log

[SelaseKay]

Hi @QuellaZhang, thanks for the report. We'll look into this further.

[SelaseKay]

Hi @QuellaZhang,

Thanks again for the report. SignInWithProvider, LinkWithProvider, and ReauthenticateWithProvider were taking the address of the temporary returned by getProviderFromArguments. That is the MSVC extension behind C4238, and Flutter compiles the Windows plugin with /W4 /WX, so the warning fails the build.

Those three calls now keep the FederatedOAuthProvider in a named local and pass that pointer. The change is in #18702.

Please rebuild the same Windows app with the VS2026 toolset that produced the warning, and point firebase_auth at that branch:

dependency_overrides:
  firebase_auth:
    git:
      url: https://github.com/firebase/flutterfire.git
      ref: auth_18698
      path: packages/firebase_auth/firebase_auth

Then flutter pub get and rebuild. The C4238 warnings in firebase_auth_plugin.cpp should be gone.