HOW TO fix 'swift build' not working with a new project

fix-swift-build-broken.md

Problem

Swift build does not work. Not even on brand new project. Example:

1. Create new proejct

$ mkdir swift-app-test2; cd swift-app-test2
$ swift package init

2. Attempt to build it

$ swift build
  
error: 'swift-app-test2': Invalid manifest (compiled with: ["/Library/Developer/CommandLineTools/usr/bin/swiftc", "-vfsoverlay", "/var/folders/82/248qxrjd3md0lkf4lz8r0hcc0000gn/T/TemporaryDirectory.Hn4O79/vfs.yaml", "-L", "/Library/Developer/CommandLineTools/usr/lib/swift/pm/ManifestAPI", "-lPackageDescription", "-Xlinker", "-rpath", "-Xlinker", "/Library/Developer/CommandLineTools/usr/lib/swift/pm/ManifestAPI", "-target", "arm64-apple-macosx14.0", "-F", "/Library/Developer/CommandLineTools/Library/Developer/Frameworks", "-sdk", "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk", "-swift-version", "6", "-I", "/Library/Developer/CommandLineTools/usr/lib/swift/pm/ManifestAPI", "-sdk", "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk", "-package-description-version", "6.1.0", "/Users/markus/dev/swift-app-test2/Package.swift", "-o", "/var/folders/82/248qxrjd3md0lkf4lz8r0hcc0000gn/T/TemporaryDirectory.UZS9vv/swift-app-test2-manifest"])
error: link command failed with exit code 1 (use -v to see invocation)
 Undefined symbols for architecture arm64:
   "PackageDescription.Package.__allocating_init(name: Swift.String, defaultLocalization: PackageDescription.LanguageTag?, platforms: [PackageDescription.SupportedPlatform]?, pkgConfig: Swift.String?, providers: [PackageDescription.SystemPackageProvider]?, products: [PackageDescription.Product], dependencies: [PackageDescription.Package.Dependency], targets: [PackageDescription.Target], swiftLanguageVersions: [PackageDescription.SwiftVersion]?, cLanguageStandard: PackageDescription.CLanguageStandard?, cxxLanguageStandard: PackageDescription.CXXLanguageStandard?) -> PackageDescription.Package", referenced from:

Like huh???? The error message is not at all helpful

Solution

This happens likely because you upgraded MacOS version recently. This seems to usually break CommandLineTools.

To fix is, reinstall CommandLineTools:

$ sudo rm -rf /Library/Developer/CommandLineTools
$ xcode-select --install   # NOTE: opens a GUI dialog where you can finish the install

After install finishes, build succeeds as expected:

$ swift build
Building for debugging...
[4/4] Compiling swift_app_test2 swift_app_test2.swift
Build complete! (22.68s)