Dependency Check

A project to study Swift Package Manager dependency behavior across different Swift language versions.

Project Structure

Diamond Dependency Pattern

This project uses a diamond dependency structure to test Swift language version compatibility:

        Main App
            |
            v
        PackageA
        /       \
       v         v
PackageACore   PackageB
       ↑         |
       |         v
       |    PackageBCore
       |         |
       +---------+

Dependency relationships:

• Main App depends on PackageA
• PackageA depends on PackageACore (internal target) and PackageB
• PackageB depends on PackageBCore (internal target)
• PackageBCore depends on PackageACore

This creates a diamond where PackageACore is reached through two paths:

1. Main App → PackageA → PackageACore
2. Main App → PackageA → PackageB → PackageBCore → PackageACore

Package Configuration

Package	Swift Tools Version	Swift Language Mode
PackageA	6.0	v6
PackageB	6.0	v5
PackageC	5.9	(default)

Findings

Swift Language Version Compatibility Issue

When setting PackageA to Swift 5 language mode (swiftLanguageModes: [.v5]) while its dependencies (PackageB) remain on Swift 6, the test target fails with:

PackageATests: 2 issues
- 5 duplicate symbols
- Linker command failed with exit code 1

Root Cause

Swift 6 has stricter module boundaries and symbol isolation by default. When dropping to Swift 5 mode, symbols that were properly isolated in Swift 6 may be exported/visible in ways that cause duplication at link time.