diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 55f0e483..c2de655a 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -45,16 +45,29 @@ jobs:
       - name: Test example tvOS
         run: scripts/test.sh example-cross-platform tvos
 
-  upcoming_feature:
-    name: Upcoming features
+  test_upcoming_features:
+    name: Test upcoming features with Xcode 15
     runs-on: macos-14
     env:
       DEVELOPER_DIR: /Applications/Xcode_15.4.app
     steps:
       - uses: actions/checkout@v4
-      - name: Test with upcoming features
+      - name: Test upcoming features
         run: ENABLE_UPCOMING_FEATURES=1 scripts/test.sh library ios
 
+  test_language_mode:
+    name: Test Swift 5 language mode
+    runs-on: macos-14
+    strategy:
+      matrix:
+        enable_upcoming_features:
+          - 0
+          - 1
+    steps:
+      - uses: actions/checkout@v4
+      - name: Test Swift 5 language mode
+        run: ENABLE_UPCOMING_FEATURES=${{ matrix.enable_upcoming_features }} scripts/test.sh library ios SWIFT_VERSION=5
+
   benchmark:
     name: Benchmark
     runs-on: macos-14
diff --git a/Package.swift b/Package.swift
index e8c74627..de290413 100644
--- a/Package.swift
+++ b/Package.swift
@@ -2,9 +2,22 @@
 
 import PackageDescription
 
-let swiftSettings: [SwiftSetting] = [
-    .enableUpcomingFeature("ExistentialAny")
-]
+let swiftSettings: [SwiftSetting]
+
+if Context.environment["ENABLE_UPCOMING_FEATURES"] == "1" {
+    swiftSettings = [
+        .enableUpcomingFeature("DisableOutwardActorInference"),
+        .enableUpcomingFeature("InferSendableFromCaptures"),
+        .enableUpcomingFeature("IsolatedDefaultValues"),
+        .enableUpcomingFeature("StrictConcurrency"),
+        .enableUpcomingFeature("ExistentialAny"),
+    ]
+}
+else {
+    swiftSettings = [
+        .enableUpcomingFeature("ExistentialAny")
+    ]
+}
 
 let package = Package(
     name: "swiftui-atom-properties",
diff --git a/Package@swift-5.swift b/Package@swift-5.swift
index 641530b8..fa62d687 100644
--- a/Package@swift-5.swift
+++ b/Package@swift-5.swift
@@ -4,10 +4,11 @@ import PackageDescription
 
 let swiftSettings: [SwiftSetting]
 
-if Context.environment["ENABLE_UPCOMING_FEATURES"] != nil {
+if Context.environment["ENABLE_UPCOMING_FEATURES"] == "1" {
     swiftSettings = [
-        .enableUpcomingFeature("ExistentialAny"),
         .enableUpcomingFeature("DisableOutwardActorInference"),
+        .enableUpcomingFeature("IsolatedDefaultValues"),
+        .enableUpcomingFeature("ExistentialAny"),
     ]
 }
 else {
diff --git a/Sources/Atoms/Core/SubscriberState.swift b/Sources/Atoms/Core/SubscriberState.swift
index 07b1fd3c..7dcd6be2 100644
--- a/Sources/Atoms/Core/SubscriberState.swift
+++ b/Sources/Atoms/Core/SubscriberState.swift
@@ -4,6 +4,10 @@ import Foundation
 internal final class SubscriberState {
     let token = SubscriberKey.Token()
 
+    #if !hasFeature(IsolatedDefaultValues)
+        nonisolated init() {}
+    #endif
+
     #if compiler(>=6)
         nonisolated(unsafe) var subscribing = Set<AtomKey>()
         nonisolated(unsafe) var unsubscribe: ((Set<AtomKey>) -> Void)?
@@ -34,10 +38,6 @@ internal final class SubscriberState {
             _modify { yield &_unsubscribe.value }
         }
 
-        #if hasFeature(DisableOutwardActorInference)
-            nonisolated init() {}
-        #endif
-
         deinit {
             if Thread.isMainThread {
                 _unsubscribe.value?(_subscribing.value)
diff --git a/scripts/test.sh b/scripts/test.sh
index 74f68232..7a905f57 100755
--- a/scripts/test.sh
+++ b/scripts/test.sh
@@ -4,6 +4,7 @@ set -eu
 
 TARGET=$1
 PLATFORM=$2
+ARGS=${@:3}
 
 pushd "$(cd $(dirname $0)/.. && pwd)" &>/dev/null
 
@@ -22,20 +23,24 @@ watchos)
     ;;
 esac
 
+clean_test() {
+    xcodebuild clean test "$@" $ARGS
+}
+
 case $TARGET in
 library)
-    xcodebuild clean test -scheme swiftui-atom-properties -destination platform="$platform"
+    clean_test -scheme swiftui-atom-properties -destination platform="$platform"
     ;;
 example-ios)
     cd Examples/Packages/iOS
-    xcodebuild clean test -scheme iOSExamples -destination platform="$platform"
+    clean_test -scheme iOSExamples -destination platform="$platform"
     ;;
 example-cross-platform)
     cd Examples/Packages/CrossPlatform
-    xcodebuild clean test -scheme CrossPlatformExamples -destination platform="$platform"
+    clean_test -scheme CrossPlatformExamples -destination platform="$platform"
     ;;
 benchmark)
     cd Benchmarks
-    xcodebuild clean test -scheme BenchmarkTests -destination platform="$platform"
+    clean_test -scheme BenchmarkTests -destination platform="$platform"
     ;;
 esac