diff --git a/Package@swift-5.8.swift b/Package@swift-5.8.swift
index fb860aff1..9b4d0aa6c 100644
--- a/Package@swift-5.8.swift
+++ b/Package@swift-5.8.swift
@@ -146,9 +146,7 @@ for target in package.targets {
     // https://github.com/swiftlang/swift-evolution/blob/main/proposals/0444-member-import-visibility.md
     settings.append(.enableUpcomingFeature("MemberImportVisibility"))
     target.swiftSettings = settings
-  case .macro, .plugin, .system, .binary:
+  default:
     break  // not applicable
-  @unknown default:
-    break  // we don't know what to do here, do nothing
   }
 }
diff --git a/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift b/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift
index 97573656c..a7a069d77 100644
--- a/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift	
+++ b/Sources/ArgumentParser/Parsable Properties/CompletionKind.swift	
@@ -43,8 +43,10 @@ public struct CompletionKind {
     case custom(@Sendable ([String], Int, String) -> [String])
     #if !canImport(Dispatch)
     @available(*, unavailable, message: "DispatchSemaphore is unavailable")
-    #endif
     case customAsync(@Sendable ([String], Int, String) async -> [String])
+    #else
+    case customAsync(@Sendable ([String], Int, String) async -> [String])
+    #endif
     case customDeprecated(@Sendable ([String]) -> [String])
   }
 
@@ -186,17 +188,20 @@ public struct CompletionKind {
   /// except that the closure is asynchronous.
   #if !canImport(Dispatch)
   @available(*, unavailable, message: "DispatchSemaphore is unavailable")
-  #endif
   @available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *)
   public static func custom(
     _ completion: @Sendable @escaping ([String], Int, String) async -> [String]
   ) -> CompletionKind {
-    #if !canImport(Dispatch)
     fatalError("DispatchSemaphore is unavailable")
-    #else
+  }
+  #else
+  @available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *)
+  public static func custom(
+    _ completion: @Sendable @escaping ([String], Int, String) async -> [String]
+  ) -> CompletionKind {
     CompletionKind(kind: .customAsync(completion))
-    #endif
   }
+  #endif
 
   /// Deprecated; only kept for backwards compatibility.
   ///
diff --git a/Sources/ArgumentParser/Parsing/CommandParser.swift b/Sources/ArgumentParser/Parsing/CommandParser.swift
index b3296d675..81ad49441 100644
--- a/Sources/ArgumentParser/Parsing/CommandParser.swift
+++ b/Sources/ArgumentParser/Parsing/CommandParser.swift
@@ -531,15 +531,19 @@ private func parseCustomCompletionArguments(
 
 #if !canImport(Dispatch)
 @available(*, unavailable, message: "DispatchSemaphore is unavailable")
-#endif
 @available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *)
 private func asyncCustomCompletions(
   from args: [String],
   complete: @escaping @Sendable ([String], Int, String) async -> [String]
 ) throws -> [String] {
-  #if !canImport(Dispatch)
   throw ParserError.invalidState
-  #else
+}
+#else
+@available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *)
+private func asyncCustomCompletions(
+  from args: [String],
+  complete: @escaping @Sendable ([String], Int, String) async -> [String]
+) throws -> [String] {
   let (args, completingArgumentIndex, completingPrefix) =
     try parseCustomCompletionArguments(from: args)
 
@@ -557,8 +561,8 @@ private func asyncCustomCompletions(
 
   semaphore.wait()
   return completionsBox.withLock { $0 }
-  #endif
 }
+#endif
 
 // MARK: Building Command Stacks
 
diff --git a/Sources/ArgumentParser/Utilities/Mutex.swift b/Sources/ArgumentParser/Utilities/Mutex.swift
index 129d0433c..6efb2a55c 100644
--- a/Sources/ArgumentParser/Utilities/Mutex.swift
+++ b/Sources/ArgumentParser/Utilities/Mutex.swift
@@ -10,10 +10,17 @@
 //===----------------------------------------------------------------------===//
 
 #if canImport(os)
+#if compiler(>=6.0)
 internal import os
 #if canImport(C.os.lock)
 internal import C.os.lock
 #endif
+#else
+import os
+#if canImport(C.os.lock)
+import C.os.lock
+#endif
+#endif
 #elseif canImport(Bionic)
 @preconcurrency import Bionic
 #elseif canImport(Glibc)