为什么不能使用Swift Package Manager将SwiftPM用作依赖项？

Question

我有以下Package.swift：

// swift-tools-version:5.2
import PackageDescription

let package = Package(
    name: "my-vapor-project",
    platforms: [
       .macOS(.v10_15)
    ],
    dependencies: [
        // 💧 A server-side Swift web framework.
        .package(url: "https://github.com/vapor/vapor.git", from: "4.0.0-rc"),
        .package(url: "https://github.com/vapor/fluent.git", from: "4.0.0-rc"),
        .package(url: "https://github.com/vapor/fluent-mysql-driver.git", from: "4.0.0-rc"),
        .package(url: "https://github.com/apple/swift-package-manager", .branch("master")),
    ],
    targets: [
        .target(name: "App", dependencies: [
            .product(name: "Fluent", package: "fluent"),
            .product(name: "FluentMySQLDriver", package: "fluent-mysql-driver"),
            .product(name: "Vapor", package: "vapor"),
            .product(name: "SwiftPM", package: "swift-package-manager")
        ]),
        .target(name: "Run", dependencies: ["App"]),
        .testTarget(name: "AppTests", dependencies: [
            .target(name: "App"),
            .product(name: "XCTVapor", package: "vapor"),
        ])
    ]
)

当我保存文件时，Xcode尝试自动解决依赖关系，并且出现以下错误和警告：

显示所有消息/Users/sluther/Projects/my-vapor-project/Package.swift：软件包“ swift-package-manager”中的产品依赖性“ SwiftPM”找到

：任何目标都未使用依赖项'SwiftPM'

Answer 1

事实证明，在编写本文时，使用Swift Tools版本5.2删除了用于定义产品依赖项的旧语法时，不能将Swift Package Manager作为依赖项包括在内。将Swift Tools版本切换到5.1可解决此问题。

// swift-tools-version:5.1
import PackageDescription

let package = Package(
    name: "my-vapor-project",
    platforms: [
       .macOS(.v10_15)
    ],
    dependencies: [
        // 💧 A server-side Swift web framework.
        .package(url: "https://github.com/vapor/vapor.git", from: "4.0.0-rc"),
        .package(url: "https://github.com/vapor/fluent.git", from: "4.0.0-rc"),
        .package(url: "https://github.com/vapor/fluent-mysql-driver.git", from: "4.0.0-rc"),
        .package(url: "https://github.com/apple/swift-package-manager", .branch("master")),
    ],
    targets: [
        .target(name: "App", dependencies: [
            .product(name: "Fluent", package: "fluent"),
            .product(name: "FluentMySQLDriver", package: "fluent-mysql-driver"),
            .product(name: "Vapor", package: "vapor"),
            "SwiftPM"
        ]),
        .target(name: "Run", dependencies: ["App"]),
        .testTarget(name: "AppTests", dependencies: [
            .target(name: "App"),
            .product(name: "XCTVapor", package: "vapor"),
        ])
    ]
)