feat(auth): legacyFetchSignInWithEmail config option for users still wishing to use auth.fetchSignInWithEmail()

[russellwheatley]

Description

• Adds an opt-in legacyFetchSignInWithEmail auth mode that looks up a user’s previous sign-in methods and offers a recovery path when email/password sign-in fails or hits an account conflict.
• A LegacySignInRecoveryView sheet in the SwiftUI auth flow, with localized copy and buttons for the previously used provider(s), including email/password, email link, phone, and enabled OAuth providers.
• Prefills the recovered email address when the user is routed back into email/password or email-link sign-in, and suppresses the generic error alert for this handled recovery case.
• Updates conflict/error handling across auth entry points so account conflicts can route into the legacy recovery flow instead of always surfacing a default error.
• Adds test coverage for the new recovery flow with unit tests, UI tests, emulator helpers for email-link-only accounts, and test launch flags to preview the recovery sheet.

This is the modal that pops up when the user has set legacyFetchSignInWithEmail to true and they have previously signed in with email/password:

Simulator.Screen.Recording.-.iPhone.17.-.2026-04-22.at.17.18.03.mov

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a legacy sign-in recovery feature to FirebaseUI, allowing users to recover their accounts when they attempt to sign in with an incorrect provider. It adds a new configuration option to enable this feature, modifies error handling to accommodate the new recovery flow, and introduces a new UI component to display the recovery options. Additionally, it includes new tests to ensure the feature functions as expected.

Highlights

• Legacy Sign-In Recovery: Introduces a new feature to help users recover their accounts when they attempt to sign in with the wrong provider. This is achieved by presenting a recovery modal with a list of available sign-in methods for their email.
• AuthConfiguration Option: Adds a legacyFetchSignInWithEmail option to AuthConfiguration to enable or disable the legacy sign-in recovery feature.
• Error Handling: Modifies error handling in various views to accommodate the new legacySignInRecoveryPresented error, preventing the display of error alerts when the recovery modal is presented.
• UI Enhancements: Adds a new LegacySignInRecoveryView to display the recovery options to the user.
• Testing: Adds new tests to verify the behavior of the legacy sign-in recovery feature.

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[gemini-code-assist]

Warning

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[russellwheatley]

/gemini review

[gemini-code-assist]

Code Review

The pull request introduces a new legacy sign-in recovery feature for Firebase SwiftUI authentication. This includes adding a legacySignInRecoveryPresented error type, new data structures (LegacySignInOption, LegacySignInRecoveryContext), and methods within AuthService to manage and present recovery options. The AuthConfiguration now includes a legacyFetchSignInWithEmail flag to enable this feature. A new LegacySignInRecoveryView is added to display these options, and AuthPickerView is updated to present this view as a sheet. Error handling in various sign-in flows is adjusted to correctly trigger this recovery mechanism. A critical fix ensures that the legacySignInRecoveryPresented error, which is an internal signal, does not trigger a user-facing error alert.