A DESCRIPTION OF THE PROBLEM :
The compiler doesnt recognize my logically correct generic wildcard.

REGRESSION : Last worked in version 12.0.2

ACTUAL -
Compiler Output:
```
Information:Kotlin: kotlinc-jvm 1.3.21 (JRE 12.0.2+10)
Information:java: Errors occurred while compiling module 'CrystalShard.api'
Information:javac 12.0.2 was used to compile java sources
Information:31.08.2019 15:20 - Build completed with 1 error and 0 warnings in 7 s 606 ms
A:\Workspaces\CrystalShard\api\src\main\java\de\kaleidox\crystalshard\core\api\gateway\Gateway.java
    Error:Error:line (11)java: type argument de.kaleidox.crystalshard.core.api.gateway.GatewayListener is not within bounds of type-variable AL
A:\Workspaces\CrystalShard\api\src\main\java\de\kaleidox\crystalshard\util\Util.java
    Information:Information:java: /A:/Workspaces/CrystalShard/api/src/main/java/de/kaleidox/crystalshard/util/Util.java uses unchecked or unsafe operations.
    Information:Information:java: Recompile with -Xlint:unchecked for details.
```

---------- BEGIN SOURCE ----------
package de.kaleidox.crystalshard.core.api.gateway;

import java.util.Collection;
import java.util.concurrent.CompletableFuture;

import de.kaleidox.crystalshard.api.listener.AttachableListener;
import de.kaleidox.crystalshard.api.listener.Listener;
import de.kaleidox.crystalshard.api.listener.ListenerManager;
import de.kaleidox.crystalshard.core.api.gateway.event.GatewayEvent;

public interface Gateway extends ListenerAttachable<Gateway, GatewayListener /* this declaration does not compile with error:
    Error:Error:line (11)java: type argument de.kaleidox.crystalshard.core.api.gateway.GatewayListener is not within bounds of type-variable AL
 */> {
    CompletableFuture<Void> sendRequest(OpCode code, String payload);
}


interface GatewayListener<E extends GatewayEvent> extends AttachableListener<Gateway>, Listener {
}

interface ListenerAttachable<S, AL extends AttachableListener<S> & Listener> {
    <TL extends AL> ListenerManager<TL> attachListener(TL listener);

    <TL extends AL> boolean detachListener(TL listener);

    Collection<ListenerManager<? extends AL>> getAttachedListenerManagers();

    default <TL extends AL> boolean detachListener(ListenerManager<TL> listenerManager) {
        return detachListener(listenerManager.getListener());
    }

    default int detachAllListeners() {
        int c = 0;

        for (ListenerManager<? extends AL> attachedListenerManager : getAttachedListenerManagers())
            if (detachListener(attachedListenerManager))
                c++;

        return c;
    }
}

---------- END SOURCE ----------

FREQUENCY : always