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swiftLanguageMode adds -swift-version to OTHER_SWIFT_FLAGS, which triggers a warning in Xcode 27

GitHub issue · Open

Metadata
Source
tuist/tuist #13752
Updated
Oct 1, 2026
Details

What happened?

When a Swift package target declares .swiftLanguageMode(...), Tuist sets both SWIFT_VERSION and a -swift-version flag in OTHER_SWIFT_FLAGS. Xcode 27 warns about the flag:

language mode was overridden by extra Swift flags and may be inconsistent with code generated during the build

For example, FirebaseAuth (firebase-ios-sdk 12.19.2) declares .swiftLanguageMode(.v5). The generated project contains:

SWIFT_VERSION = 5

OTHER_SWIFT_FLAGS = "$(inherited) -swift-version 5 ..."

The flag comes from SettingsMapper.swift:

case (.swift, .swiftLanguageMode):

    // TODO: Use -language-mode instead of -swift-version when Xcode 15 support is removed.

    swiftFlags.append("-swift-version \(setting.value[0])")

    settingsDictionary["SWIFT_VERSION"] = "\(setting.value[0])"

Expected

Only SWIFT_VERSION is set. The extra -swift-version flag isn’t needed and causes the warning.

How to reproduce

  1. Add firebase-ios-sdk 12.19.2 as an external dependency in Tuist/Package.swift and use FirebaseAuth.
  2. Run tuist install and tuist generate.
  3. Build with Xcode 27. The FirebaseAuth target shows the warning above.

Environment

  • Tuist 4.210.0
  • Xcode 27, Swift 6.4
  • macOS 27.0
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