https://github.com/swiftlang/swift-package-manager/blob/392a2a99510f391cb172ceef510d7aefce5c7c62/Sources/PackageLoading/PackageBuilder.swift#L1269-L1276 ここでtools-versionよりもswiftLanguageVersionsを優先してる。

swift-package-manager/Sources/PackageLoading/PackageBuilder.swift a...

The Package Manager for the Swift Programming Language - swiftlang/swift-package-manager

7:38 AM

 // Figure out the swift version from declared list in the manifest. let declaredSwiftVersions = try declaredSwiftVersions() let computedSwiftVersion: SwiftLanguageVersion = if let declaredSwiftVersion = declaredSwiftVersions.first { declaredSwiftVersion } else { // Otherwise, use the version depending on the manifest version. self.manifest.toolsVersion.swiftLanguageVersion }