diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index b4fd148..c94528c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -6,10 +6,10 @@ on:
   workflow_dispatch:
 
 jobs:
-  xcode_14_3_1:
+  xcode_15_0_1:
     runs-on: macos-13
     env:
-      DEVELOPER_DIR: /Applications/Xcode_14.3.1.app/Contents/Developer
+      DEVELOPER_DIR: /Applications/Xcode_15.0.1.app/Contents/Developer
     steps:
       - name: Checkout
         uses: actions/checkout@v3
@@ -26,10 +26,10 @@ jobs:
         with:
           files: ./coverage_report.lcov
 
-  xcode_15:
+  xcode_14_3_1:
     runs-on: macos-13
     env:
-      DEVELOPER_DIR: /Applications/Xcode_15.0.app/Contents/Developer
+      DEVELOPER_DIR: /Applications/Xcode_14.3.1.app/Contents/Developer
     steps:
       - name: Checkout
         uses: actions/checkout@v3
@@ -40,9 +40,9 @@ jobs:
       - name: Test
         run: swift test --skip-build
 
-  linux_swift_5_9:
+  linux_swift_5_9_2:
     runs-on: ubuntu-latest
-    container: swift:5.9
+    container: swift:5.9.2
     steps:
       - name: Checkout
         uses: actions/checkout@v3
@@ -65,16 +65,3 @@ jobs:
         run: swift build --build-tests
       - name: Test
         run: swift test --skip-build
-
-  linux_swift_5_7:
-    runs-on: ubuntu-latest
-    container: swift:5.7
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v3
-      - name: Version
-        run: swift --version
-      - name: Build
-        run: swift build --build-tests
-      - name: Test
-        run: swift test --skip-build
diff --git a/Package.swift b/Package.swift
index 84e215f..a35182b 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version:5.7
+// swift-tools-version:5.8
 import PackageDescription
 
 let package = Package(
@@ -15,12 +15,23 @@ let package = Package(
     targets: [
         .target(
             name: "KeyValueCoder",
-            path: "Sources"
+            path: "Sources",
+            swiftSettings: .upcomingFeatures
         ),
         .testTarget(
             name: "KeyValueCoderTests",
             dependencies: ["KeyValueCoder"],
-            path: "Tests"
+            path: "Tests",
+            swiftSettings: .upcomingFeatures
         )
     ]
 )
+
+extension Array where Element == SwiftSetting {
+
+    static var upcomingFeatures: [SwiftSetting] {
+        [
+            .enableUpcomingFeature("ExistentialAny")
+        ]
+    }
+}
diff --git a/README.md b/README.md
index 58bd27e..c561967 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 [![Build](https://github.com/swhitty/KeyValueCoder/actions/workflows/build.yml/badge.svg)](https://github.com/swhitty/KeyValueCoder/actions/workflows/build.yml)
 [![CodeCov](https://codecov.io/gh/swhitty/KeyValueCoder/branch/main/graphs/badge.svg)](https://codecov.io/gh/swhitty/KeyValueCoder/branch/main)
-[![Swift 5.9](https://img.shields.io/badge/swift-5.7%20–%205.9-red.svg?style=flat)](https://developer.apple.com/swift)
+[![Swift 5.9](https://img.shields.io/badge/swift-5.8%20–%205.9-red.svg?style=flat)](https://developer.apple.com/swift)
 [![License](https://img.shields.io/badge/license-MIT-lightgrey.svg)](https://opensource.org/licenses/MIT)
 [![Twitter](https://img.shields.io/badge/twitter-@simonwhitty-blue.svg)](http://twitter.com/simonwhitty)
 
diff --git a/Sources/KeyValueDecoder.swift b/Sources/KeyValueDecoder.swift
index 2a952dc..6c08430 100644
--- a/Sources/KeyValueDecoder.swift
+++ b/Sources/KeyValueDecoder.swift
@@ -78,7 +78,7 @@ private extension KeyValueDecoder {
     struct Decoder: Swift.Decoder {
 
         private let container: SingleContainer
-        let codingPath: [CodingKey]
+        let codingPath: [any CodingKey]
         let userInfo: [CodingUserInfoKey: Any]
 
         init(container: SingleContainer) {
@@ -97,25 +97,25 @@ private extension KeyValueDecoder {
             return KeyedDecodingContainer(keyed)
         }
 
-        func unkeyedContainer() throws -> UnkeyedDecodingContainer {
+        func unkeyedContainer() throws -> any UnkeyedDecodingContainer {
             let storage = try container.decode([Any].self)
             return UnkeyedContainer(codingPath: codingPath, storage: storage, userInfo: userInfo, nilDecodingStrategy: container.nilDecodingStrategy)
         }
 
-        func singleValueContainer() throws -> SingleValueDecodingContainer {
+        func singleValueContainer() throws -> any SingleValueDecodingContainer {
             container
         }
     }
 
     struct SingleContainer: SingleValueDecodingContainer {
 
-        let codingPath: [CodingKey]
+        let codingPath: [any CodingKey]
         let userInfo: [CodingUserInfoKey: Any]
         let nilDecodingStrategy: NilDecodingStrategy
 
         private var value: Any
 
-        init(value: Any, codingPath: [CodingKey], userInfo: [CodingUserInfoKey: Any], nilDecodingStrategy: NilDecodingStrategy) {
+        init(value: Any, codingPath: [any CodingKey], userInfo: [CodingUserInfoKey: Any], nilDecodingStrategy: NilDecodingStrategy) {
             self.value = value
             self.codingPath = codingPath
             self.userInfo = userInfo
@@ -297,11 +297,11 @@ private extension KeyValueDecoder {
     struct KeyedContainer<Key: CodingKey>: KeyedDecodingContainerProtocol {
 
         let storage: [String: Any]
-        let codingPath: [CodingKey]
+        let codingPath: [any CodingKey]
         private let userInfo: [CodingUserInfoKey: Any]
         private let nilDecodingStrategy: NilDecodingStrategy
 
-        init(codingPath: [CodingKey], storage: [String: Any], userInfo: [CodingUserInfoKey: Any], nilDecodingStrategy: NilDecodingStrategy) {
+        init(codingPath: [any CodingKey], storage: [String: Any], userInfo: [CodingUserInfoKey: Any], nilDecodingStrategy: NilDecodingStrategy) {
             self.codingPath = codingPath
             self.storage = storage
             self.userInfo = userInfo
@@ -407,7 +407,7 @@ private extension KeyValueDecoder {
             return KeyedDecodingContainer<NestedKey>(keyed)
         }
 
-        func nestedUnkeyedContainer(forKey key: Key) throws -> UnkeyedDecodingContainer {
+        func nestedUnkeyedContainer(forKey key: Key) throws -> any UnkeyedDecodingContainer {
             let container = try container(for: key)
             return try UnkeyedContainer(
                 codingPath: container.codingPath,
@@ -417,25 +417,25 @@ private extension KeyValueDecoder {
             )
         }
 
-        func superDecoder() throws -> Swift.Decoder {
+        func superDecoder() throws -> any Swift.Decoder {
             let container = SingleContainer(value: storage, codingPath: codingPath, userInfo: userInfo, nilDecodingStrategy: nilDecodingStrategy)
             return Decoder(container: container)
         }
 
-        func superDecoder(forKey key: Key) throws -> Swift.Decoder {
+        func superDecoder(forKey key: Key) throws -> any Swift.Decoder {
             try Decoder(container: container(for: key))
         }
     }
 
     struct UnkeyedContainer: UnkeyedDecodingContainer {
 
-        let codingPath: [CodingKey]
+        let codingPath: [any CodingKey]
 
         let storage: [Any]
         private let userInfo: [CodingUserInfoKey: Any]
         private let nilDecodingStrategy: NilDecodingStrategy
 
-        init(codingPath: [CodingKey], storage: [Any], userInfo: [CodingUserInfoKey: Any], nilDecodingStrategy: NilDecodingStrategy) {
+        init(codingPath: [any CodingKey], storage: [Any], userInfo: [CodingUserInfoKey: Any], nilDecodingStrategy: NilDecodingStrategy) {
             self.codingPath = codingPath
             self.storage = storage
             self.userInfo = userInfo
@@ -541,13 +541,13 @@ private extension KeyValueDecoder {
             return result
         }
 
-        mutating func nestedUnkeyedContainer() throws -> UnkeyedDecodingContainer {
+        mutating func nestedUnkeyedContainer() throws -> any UnkeyedDecodingContainer {
             let result = try Decoder(container: nextContainer()).unkeyedContainer()
             currentIndex = storage.index(after: currentIndex)
             return result
         }
 
-        mutating func superDecoder() -> Swift.Decoder {
+        mutating func superDecoder() -> any Swift.Decoder {
             let container = SingleContainer(value: storage, codingPath: codingPath, userInfo: userInfo, nilDecodingStrategy: nilDecodingStrategy)
             return Decoder(container: container)
         }
diff --git a/Sources/KeyValueEncoder.swift b/Sources/KeyValueEncoder.swift
index 683001b..69a2b90 100644
--- a/Sources/KeyValueEncoder.swift
+++ b/Sources/KeyValueEncoder.swift
@@ -145,11 +145,11 @@ private extension KeyValueEncoder {
 
     final class Encoder: Swift.Encoder {
 
-        let codingPath: [CodingKey]
+        let codingPath: [any CodingKey]
         let userInfo: [CodingUserInfoKey: Any]
         let nilEncodingStrategy: NilEncodingStrategy
 
-        init(codingPath: [CodingKey] = [], userInfo: [CodingUserInfoKey: Any], nilEncodingStrategy: NilEncodingStrategy) {
+        init(codingPath: [any CodingKey] = [], userInfo: [CodingUserInfoKey: Any], nilEncodingStrategy: NilEncodingStrategy) {
             self.codingPath = codingPath
             self.userInfo = userInfo
             self.nilEncodingStrategy = nilEncodingStrategy
@@ -174,13 +174,13 @@ private extension KeyValueEncoder {
             return KeyedEncodingContainer(keyed)
         }
 
-        func unkeyedContainer() -> UnkeyedEncodingContainer {
+        func unkeyedContainer() -> any UnkeyedEncodingContainer {
             let unkeyed = UnkeyedContainer(codingPath: codingPath, userInfo: userInfo, nilEncodingStrategy: nilEncodingStrategy)
             container = .provider(unkeyed.getEncodedValue)
             return unkeyed
         }
 
-        func singleValueContainer() -> SingleValueEncodingContainer {
+        func singleValueContainer() -> any SingleValueEncodingContainer {
             let single = SingleContainer(codingPath: codingPath, userInfo: userInfo, nilEncodingStrategy: nilEncodingStrategy)
             container = .provider(single.getEncodedValue)
             return single
@@ -201,11 +201,11 @@ private extension KeyValueEncoder {
     final class KeyedContainer<K: CodingKey>: KeyedEncodingContainerProtocol {
         typealias Key = K
 
-        let codingPath: [CodingKey]
+        let codingPath: [any CodingKey]
         private let userInfo: [CodingUserInfoKey: Any]
         private let nilEncodingStrategy: NilEncodingStrategy
 
-        init(codingPath: [CodingKey], userInfo: [CodingUserInfoKey: Any], nilEncodingStrategy: NilEncodingStrategy) {
+        init(codingPath: [any CodingKey], userInfo: [CodingUserInfoKey: Any], nilEncodingStrategy: NilEncodingStrategy) {
             self.codingPath = codingPath
             self.storage = [:]
             self.userInfo = userInfo
@@ -307,18 +307,18 @@ private extension KeyValueEncoder {
             return KeyedEncodingContainer(keyed)
         }
 
-        func nestedUnkeyedContainer(forKey key: K) -> UnkeyedEncodingContainer {
+        func nestedUnkeyedContainer(forKey key: K) -> any UnkeyedEncodingContainer {
             let path = codingPath.appending(key: key)
             let unkeyed = UnkeyedContainer(codingPath: path, userInfo: userInfo, nilEncodingStrategy: nilEncodingStrategy)
             storage[key.stringValue] = .provider(unkeyed.getEncodedValue)
             return unkeyed
         }
 
-        func superEncoder() -> Swift.Encoder {
+        func superEncoder() -> any Swift.Encoder {
             return superEncoder(forKey: Key(stringValue: "super")!)
         }
 
-        func superEncoder(forKey key: Key) -> Swift.Encoder {
+        func superEncoder(forKey key: Key) -> any Swift.Encoder {
             let path = codingPath.appending(key: key)
             let encoder = Encoder(codingPath: path, userInfo: userInfo, nilEncodingStrategy: nilEncodingStrategy)
             storage[key.stringValue] = .provider(encoder.getEncodedValue)
@@ -328,11 +328,11 @@ private extension KeyValueEncoder {
 
     final class UnkeyedContainer: Swift.UnkeyedEncodingContainer {
 
-        let codingPath: [CodingKey]
+        let codingPath: [any CodingKey]
         private let userInfo: [CodingUserInfoKey: Any]
         private let nilEncodingStrategy: NilEncodingStrategy
 
-        init(codingPath: [CodingKey], userInfo: [CodingUserInfoKey: Any], nilEncodingStrategy: NilEncodingStrategy) {
+        init(codingPath: [any CodingKey], userInfo: [CodingUserInfoKey: Any], nilEncodingStrategy: NilEncodingStrategy) {
             self.codingPath = codingPath
             self.userInfo = userInfo
             self.nilEncodingStrategy = nilEncodingStrategy
@@ -437,14 +437,14 @@ private extension KeyValueEncoder {
             return KeyedEncodingContainer(keyed)
         }
 
-        func nestedUnkeyedContainer() -> UnkeyedEncodingContainer {
+        func nestedUnkeyedContainer() -> any UnkeyedEncodingContainer {
             let path = codingPath.appending(index: count)
             let unkeyed = UnkeyedContainer(codingPath: path, userInfo: userInfo, nilEncodingStrategy: nilEncodingStrategy)
             storage.append(.provider(unkeyed.getEncodedValue))
             return unkeyed
         }
 
-        func superEncoder() -> Swift.Encoder {
+        func superEncoder() -> any Swift.Encoder {
             let path = codingPath.appending(index: count)
             let encoder = Encoder(codingPath: path, userInfo: userInfo, nilEncodingStrategy: nilEncodingStrategy)
             storage.append(.provider(encoder.getEncodedValue))
@@ -454,11 +454,11 @@ private extension KeyValueEncoder {
 
     final class SingleContainer: SingleValueEncodingContainer {
 
-        let codingPath: [CodingKey]
+        let codingPath: [any CodingKey]
         private let userInfo: [CodingUserInfoKey: Any]
         private let nilEncodingStrategy: NilEncodingStrategy
 
-        init(codingPath: [CodingKey], userInfo: [CodingUserInfoKey: Any], nilEncodingStrategy: NilEncodingStrategy) {
+        init(codingPath: [any CodingKey], userInfo: [CodingUserInfoKey: Any], nilEncodingStrategy: NilEncodingStrategy) {
             self.codingPath = codingPath
             self.userInfo = userInfo
             self.nilEncodingStrategy = nilEncodingStrategy
@@ -547,21 +547,21 @@ private extension KeyValueEncoder {
     }
 }
 
-extension Array where Element == CodingKey {
+extension Array where Element == any CodingKey {
 
-    func appending(key codingKey: CodingKey) -> [CodingKey] {
+    func appending(key codingKey: any CodingKey) -> [any CodingKey] {
         var path = self
         path.append(codingKey)
         return path
     }
 
-    func appending(index: Int) -> [CodingKey] {
+    func appending(index: Int) -> [any CodingKey] {
         var path = self
         path.append(AnyCodingKey(intValue: index))
         return path
     }
 
-    func makeKeyPath(appending key: CodingKey? = nil) -> String {
+    func makeKeyPath(appending key: (any CodingKey)? = nil) -> String {
         var path = map(\.keyPath)
         if let key = key {
             path.append(key.keyPath)
diff --git a/Tests/KeyValueDecoderTests.swift b/Tests/KeyValueDecoderTests.swift
index 066571d..e381e1b 100644
--- a/Tests/KeyValueDecoderTests.swift
+++ b/Tests/KeyValueDecoderTests.swift
@@ -809,7 +809,7 @@ private extension KeyValueDecoder {
         return proxy.result!
     }
 
-    static func decodeUnkeyedValue<T>(from value: [Any], with closure: @escaping (inout UnkeyedDecodingContainer) throws -> T) throws -> T {
+    static func decodeUnkeyedValue<T>(from value: [Any], with closure: @escaping (inout any UnkeyedDecodingContainer) throws -> T) throws -> T {
         let proxy = StubDecoder.Proxy { decoder in
             var container = try decoder.unkeyedContainer()
             return try closure(&container)
@@ -835,20 +835,20 @@ private extension CodingUserInfoKey {
 private struct StubDecoder: Decodable {
 
     final class Proxy<T> {
-        private let closure: (Decoder) throws -> T
+        private let closure: (any Decoder) throws -> T
         private(set) var result: T?
 
-        init(_ closure: @escaping (Decoder) throws -> T) {
+        init(_ closure: @escaping (any Decoder) throws -> T) {
             self.closure = closure
         }
 
-        func decode(from decoder: Decoder) throws {
+        func decode(from decoder: any Decoder) throws {
             self.result = try closure(decoder)
         }
     }
 
-    init(from decoder: Decoder) throws {
-        let closure = decoder.userInfo[.decoder] as! (Decoder) throws -> Void
+    init(from decoder: any Decoder) throws {
+        let closure = decoder.userInfo[.decoder] as! (any Decoder) throws -> Void
         try closure(decoder)
     }
 }
diff --git a/Tests/KeyValueEncoderTests.swift b/Tests/KeyValueEncoderTests.swift
index 20056f4..c9dfd9a 100644
--- a/Tests/KeyValueEncoderTests.swift
+++ b/Tests/KeyValueEncoderTests.swift
@@ -668,7 +668,7 @@ final class KeyValueEncodedTests: XCTestCase {
 private extension KeyValueEncoder {
 
     static func encodeSingleValue(nilEncodingStrategy: NilEncodingStrategy = .default,
-                                  with closure: (inout SingleValueEncodingContainer) throws -> Void) throws -> EncodedValue {
+                                  with closure: (inout any SingleValueEncodingContainer) throws -> Void) throws -> EncodedValue {
         let encoder = KeyValueEncoder()
         encoder.nilEncodingStrategy = nilEncodingStrategy
         return try encoder.encodeValue {
@@ -677,7 +677,7 @@ private extension KeyValueEncoder {
         }
     }
 
-    static func encodeUnkeyedValue(with closure: (inout UnkeyedEncodingContainer) throws -> Void) throws -> EncodedValue {
+    static func encodeUnkeyedValue(with closure: (inout any UnkeyedEncodingContainer) throws -> Void) throws -> EncodedValue {
         try KeyValueEncoder().encodeValue {
             var container = $0.unkeyedContainer()
             try closure(&container)
@@ -695,7 +695,7 @@ private extension KeyValueEncoder {
         }
     }
 
-    func encodeValue(with closure: (Encoder) throws -> Void) throws -> EncodedValue {
+    func encodeValue(with closure: (any Encoder) throws -> Void) throws -> EncodedValue {
         try withoutActuallyEscaping(closure) {
             try self.encodeValue(StubEncoder(closure: $0))
         }
@@ -709,9 +709,9 @@ private extension KeyValueEncoder {
 }
 
 private struct StubEncoder: Encodable {
-    var closure: (Encoder) throws -> Void
+    var closure: (any Encoder) throws -> Void
 
-    func encode(to encoder: Encoder) throws {
+    func encode(to encoder: any Encoder) throws {
         try closure(encoder)
     }
 }
@@ -783,11 +783,11 @@ private extension JSONDecoder {
 }
 
 private struct Empty: Encodable {
-    func encode(to encoder: Encoder) throws { }
+    func encode(to encoder: any Encoder) throws { }
 }
 
 private struct Null: Encodable {
-    func encode(to encoder: Encoder) throws {
+    func encode(to encoder: any Encoder) throws {
         var container = encoder.singleValueContainer()
         try container.encodeNil()
     }