Fix java 8 build

tdlight-java/src/main/java/it/tdlight/client/SimpleTelegramClient.java

@@ -8,6 +8,7 @@ import it.tdlight.ResultHandler;
 import it.tdlight.TelegramClient;
 import it.tdlight.jni.TdApi.Message;
 import it.tdlight.jni.TdApi.Update;
+import it.tdlight.util.FutureSupport;
 import it.tdlight.util.UnsupportedNativeLibraryException;
 import it.tdlight.jni.TdApi;
 import it.tdlight.jni.TdApi.ChatListArchive;
@@ -359,7 +360,7 @@ public final class SimpleTelegramClient implements Authenticable, MutableTelegra
 	 * Wait until TDLight is closed
 	 */
 	public CompletableFuture<Void> waitForExitAsync() {
-		return closed.copy();
+		return FutureSupport.copy(closed);
 	}
 
 	private void onCloseUpdate() {

tdlight-java/src/main/java/it/tdlight/util/FutureSupport.java

@@ -0,0 +1,10 @@
+package it.tdlight.util;
+
+import java.util.concurrent.CompletableFuture;
+
+public class FutureSupport {
+
+	public static <T> CompletableFuture<T> copy(CompletableFuture<T> future) {
+		return CompletableFuture.completedFuture(true).thenCompose(ignored -> future);
+	}
+}

tdlight-java/src/main/java11/it/tdlight/util/FutureSupport.java

@@ -0,0 +1,10 @@
+package it.tdlight.util;
+
+import java.util.concurrent.CompletableFuture;
+
+public class FutureSupport {
+
+	public static <T> CompletableFuture<T> copy(CompletableFuture<T> future) {
+		return future.copy();
+	}
+}