Incremental build skipping changed file after adding new compiler flag

Resolution

To force Xcode to recognize a new compiler flag and recompile modified source files, you must invalidate the build graph for the affected target. The most reliable method is to perform a Clean Build or manually purge the Derived Data folder for the project.

Likely Cause: Build Graph Invalidation

Xcode's build system tracks inputs for each CompileSwift action, including the source file timestamp and the OTHER_SWIFT_FLAGS setting. While a file change usually triggers a rebuild, adding a compiler flag changes the parameters of the build action. If the build system fails to propagate this setting change to the specific target's dependency graph, it may incorrectly determine that the existing object file is still valid despite the source change.

Note on Versioning: There is a likely mismatch in your environment description; Swift 5.9 is typically bundled with Xcode 15. Xcode 14 generally uses Swift 5.8. If you are using a custom toolchain (Swift 5.9 inside Xcode 14), the build system may experience synchronization issues between the IDE's project settings and the external compiler's expectations.

Steps to Force Recompilation

1. Verify Target Scope: Ensure the flag is added to the Target settings under Swift Compiler > Custom Flags, rather than just the Project level, to ensure the dependency graph is updated for that specific binary.
2. Perform a Clean: Use Product > Clean Build Folder (Cmd+Shift+K) to remove intermediate object files.
3. Purge Derived Data: If the issue persists, delete the project's folder within ~/Library/Developer/Xcode/DerivedData. This forces a complete recalculation of the module cache and dependency graph.
4. Command Line Force: Use the following scoped command to ensure a fresh build state:

   xcodebuild clean build -scheme YourSchemeName -configuration Debug

Verification

To confirm the fix, check the build logs for the specific file:

• Open the Report Navigator (Cmd+9).
• Locate the CompileSwift action for the modified file.
• Expand the log to verify that the new compiler flag is explicitly present in the command-line arguments passed to the Swift compiler.

Diagnostic Requirement

Are you using a custom Swift toolchain or a .xcconfig file to manage these flags? If flags are injected via .xcconfig, Xcode may not always trigger a target-wide invalidation upon file modification unless the .xcconfig file itself is saved/timestamped.