`swift package add-dependency` will add the same dependency multiple times causing build failures

[euanh]

Is it reproducible with SwiftPM command-line tools: swift build, swift test, swift package etc?

• Confirmed reproduction steps with SwiftPM CLI. The description text must include reproduction steps with either of command-line SwiftPM commands, swift build, swift test, swift package etc.

Description

swift package add-dependency is not idempotent. It will add a dependency entry for a package to Package.swift even if that package is already a dependency. This makes the package fail to build.

Expected behavior

swift package add-dependency should refuse to add an existing dependency.

Actual behavior

swift package add-dependency adds duplicate dependencies, leading to build failures.

Steps to reproduce

1. Create a new package

% swift package init --type executable
Creating executable package: dependencies
Creating Package.swift
Creating Sources/
Creating Sources/main.swift
% cat Package.swift 
// swift-tools-version: 6.1
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "dependencies",
    targets: [
        // Targets are the basic building blocks of a package, defining a module or a test suite.
        // Targets can depend on other targets in this package and products from dependencies.
        .executableTarget(
            name: "dependencies"),
    ]
)
% swift build
Building for debugging...
[8/8] Applying dependencies
Build complete! (1.41s)

2. Add a dependency on swift-argument-parser and rebuild - this succeeds.

% swift package add-dependency https://github.com/apple/swift-argument-parser --from 1.5.0      
Updating package manifest at Package.swift... done.
% cat Package.swift
// swift-tools-version: 6.1
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "dependencies",
    dependencies: [
        .package(url: "https://github.com/apple/swift-argument-parser", from: "1.5.0"),
    ],
    targets: [
        // Targets are the basic building blocks of a package, defining a module or a test suite.
        // Targets can depend on other targets in this package and products from dependencies.
        .executableTarget(
            name: "dependencies"),
    ]
)
% swift build
Fetching https://github.com/apple/swift-argument-parser from cache
Fetched https://github.com/apple/swift-argument-parser from cache (0.56s)
Computing version for https://github.com/apple/swift-argument-parser
Computed https://github.com/apple/swift-argument-parser at 1.5.0 (1.00s)
Creating working copy for https://github.com/apple/swift-argument-parser
Working copy of https://github.com/apple/swift-argument-parser resolved at 1.5.0
[1/1] Planning build
Building for debugging...
[2/2] Write swift-version-1AF32027EA501FB9.txt
Build complete! (2.61s)

3. Add the dependency on swift-argument-parser again. The add-dependency command does not return an error, but swift build now fails immediately with a duplicate dependency error.

% swift package add-dependency https://github.com/apple/swift-argument-parser --from 1.5.0      
Updating package manifest at Package.swift... done.
% cat Package.swift
// swift-tools-version: 6.1
// The swift-tools-version declares the minimum version of Swift required to build this package.

import PackageDescription

let package = Package(
    name: "dependencies",
    dependencies: [
        .package(url: "https://github.com/apple/swift-argument-parser", from: "1.5.0"),
        .package(url: "https://github.com/apple/swift-argument-parser", from: "1.5.0"),
    ],
    targets: [
        // Targets are the basic building blocks of a package, defining a module or a test suite.
        // Targets can depend on other targets in this package and products from dependencies.
        .executableTarget(
            name: "dependencies"),
    ]
)
% swift build
error: 'dependencies': 'dependencies' dependency on 'https://github.com/apple/swift-argument-parser' conflicts with dependency on 'https://github.com/apple/swift-argument-parser' which has the same identity 'swift-argument-parser'
error: ExitCode(rawValue: 1)
[0/1] Planning build

Swift Package Manager version/commit hash

Swift 6.1.0

Swift & OS version (output of swift --version ; uname -a)

The example above comes from Swift 6.1 but it also occurred on 6.0.3 and other builds. I believe this problem has existed since the add-dependency command was introduced.

% swift --version
Apple Swift version 6.1 (swift-6.1-RELEASE)
Target: arm64-apple-macosx15.0
% uname -a
Darwin computermabob.local 24.3.0 Darwin Kernel Version 24.3.0

[euanh]

I've run into this problem a few times by accident, but it has also just come up in the context of documentation and tutorials apple/swift-container-plugin#99 (comment). The documentation says "use this command to install the plugin" but the tutorial example already has the plugin installed, so a new user will end up with a broken build after following the instructions.

[euanh]

testPackageAddURLDependency actually relies on the current behaviour. It adds the same dependency multiple times with different constraints and checks that all the constraints end up correctly in Package.swift.

swift-package-manager/Tests/CommandsTests/PackageCommandTests.swift

Line 878 in 2c4c785

func testPackageAddURLDependency() async throws {

If we run the same sequence of commands in the terminal we find that the package which it produces won't build:

% cat >Package.swift <<EOF
heredoc> // swift-tools-version: 5.9
heredoc> import PackageDescription
heredoc> let package = Package(
heredoc>     name: "client",
heredoc>     targets: [ .target(name: "client", dependencies: [ "library" ]) ]
heredoc> )
heredoc> EOF
% swift package add-dependency https://github.com/swiftlang/swift-syntax.git --exact 1.0.0
Updating package manifest at Package.swift... done.
% swift package add-dependency https://github.com/swiftlang/swift-syntax.git --branch main
Updating package manifest at Package.swift... done.
% swift package add-dependency https://github.com/swiftlang/swift-syntax.git --revision 58e9de4e7b79e67c72a46e164158e3542e570ab6
Updating package manifest at Package.swift... done.
% swift package add-dependency https://github.com/swiftlang/swift-syntax.git --from 1.0.0
Updating package manifest at Package.swift... done.
% swift package add-dependency https://github.com/swiftlang/swift-syntax.git --from 2.0.0 --to 2.2.0
Updating package manifest at Package.swift... done.
% swift package add-dependency https://github.com/swiftlang/swift-syntax.git --up-to-next-minor-from 1.0.0 
Updating package manifest at Package.swift... done.
% swift package add-dependency https://github.com/swiftlang/swift-syntax.git --up-to-next-minor-from 3.0.0 --to 3.3.0
Updating package manifest at Package.swift... done.
% cat Package.swift 
// swift-tools-version: 5.9
import PackageDescription
let package = Package(
    name: "client",
    dependencies: [
        .package(url: "https://github.com/swiftlang/swift-syntax.git", exact: "1.0.0"),
        .package(url: "https://github.com/swiftlang/swift-syntax.git", branch: "main"),
        .package(url: "https://github.com/swiftlang/swift-syntax.git", revision: "58e9de4e7b79e67c72a46e164158e3542e570ab6"),
        .package(url: "https://github.com/swiftlang/swift-syntax.git", from: "1.0.0"),
        .package(url: "https://github.com/swiftlang/swift-syntax.git", "2.0.0" ..< "2.2.0"),
        .package(url: "https://github.com/swiftlang/swift-syntax.git", "1.0.0" ..< "1.1.0"),
        .package(url: "https://github.com/swiftlang/swift-syntax.git", "3.0.0" ..< "3.3.0"),
    ],
    targets: [ .target(name: "client", dependencies: [ "library" ]) ]
)
% swift build
Fetching https://github.com/swiftlang/swift-syntax.git from cache
Fetched https://github.com/swiftlang/swift-syntax.git from cache (0.90s)
error: swift-syntax is required using two different revision-based requirements (main and 58e9de4e7b79e67c72a46e164158e3542e570ab6), which is not supported
warning: 'dependencies': Source files for target client should be located under 'Sources/client', 'Sources', or a custom sources path can be set with the 'path' property in Package.swift
error: ExitCode(rawValue: 1)
[0/1] Planning build
warning: 'dependencies': Source files for target client should be located under 'Sources/client', 'Sources', or a custom sources path can be set with the 'path' property in Package.swift
% 

[heckj]

Hit this recently when suggesting using the add-dependency command to add swift-docc-plugin to a project to get the generate-documentation command from the docc plugin operational as well.

[plemarquand]

Fixed in #8534