Swift 6

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.8
+// swift-tools-version:6.0
 import PackageDescription
 
 let package = Package(
@@ -32,7 +32,7 @@ extension Array where Element == SwiftSetting {
     static var upcomingFeatures: [SwiftSetting] {
         [
             .enableUpcomingFeature("ExistentialAny"),
-            .enableExperimentalFeature("StrictConcurrency")
+            .swiftLanguageMode(.v6)
         ]
     }
 }

Package@swift-5.9.swift

@@ -0,0 +1,38 @@
+// swift-tools-version:5.9
+import PackageDescription
+
+let package = Package(
+    name: "KeyValueCoder",
+    platforms: [
+        .macOS(.v13), .iOS(.v15), .tvOS(.v15), .watchOS(.v8)
+    ],
+    products: [
+        .library(
+            name: "KeyValueCoder",
+            targets: ["KeyValueCoder"]
+        ),
+    ],
+    targets: [
+        .target(
+            name: "KeyValueCoder",
+            path: "Sources",
+            swiftSettings: .upcomingFeatures
+        ),
+        .testTarget(
+            name: "KeyValueCoderTests",
+            dependencies: ["KeyValueCoder"],
+            path: "Tests",
+            swiftSettings: .upcomingFeatures
+        )
+    ]
+)
+
+extension Array where Element == SwiftSetting {
+
+    static var upcomingFeatures: [SwiftSetting] {
+        [
+            .enableUpcomingFeature("ExistentialAny"),
+            .enableExperimentalFeature("StrictConcurrency")
+        ]
+    }
+}