Improve message of AmbiguousNodeException

cloud-core/src/main/java/org/incendo/cloud/CommandTree.java

@@ -1170,7 +1170,8 @@ private void checkAmbiguity(final @NonNull CommandNode<C> node) throws Ambiguous
                     node.children()
                             .stream()
                             .filter(n -> n.component() != null)
-                            .collect(Collectors.toList())
+                            .collect(Collectors.toList()),
+                    null
             );
         }
 
@@ -1194,7 +1195,8 @@ private void checkAmbiguity(final @NonNull CommandNode<C> node) throws Ambiguous
                             node.children()
                                     .stream()
                                     .filter(n -> n.component() != null)
-                                    .collect(Collectors.toList())
+                                    .collect(Collectors.toList()),
+                            nameOrAlias
                     );
                 }
             }

cloud-core/src/main/java/org/incendo/cloud/exception/AmbiguousNodeException.java

@@ -45,23 +45,27 @@ public final class AmbiguousNodeException extends IllegalStateException {
     private final CommandNode<?> parentNode;
     private final CommandNode<?> ambiguousNode;
     private final List<CommandNode<?>> children;
+    private final String ambiguousName;
 
     /**
      * Construct a new ambiguous node exception
      *
      * @param parentNode    Parent node
      * @param ambiguousNode Node that caused exception
+     * @param ambiguousName Ambiguous name or alias
      * @param children      All children of the parent
      */
     @API(status = API.Status.INTERNAL, consumers = "org.incendo.cloud.*")
     public AmbiguousNodeException(
             final @Nullable CommandNode<?> parentNode,
             final @NonNull CommandNode<?> ambiguousNode,
-            final @NonNull List<@NonNull CommandNode<?>> children
+            final @NonNull List<@NonNull CommandNode<?>> children,
+            final @Nullable String ambiguousName
     ) {
         this.parentNode = parentNode;
         this.ambiguousNode = ambiguousNode;
         this.children = children;
+        this.ambiguousName = ambiguousName;
     }
 
     /**
@@ -91,13 +95,28 @@ public AmbiguousNodeException(
         return Collections.unmodifiableList(this.children);
     }
 
+    /**
+     * Returns the ambiguous name or alias of the node.
+     * Might be null if the reason for this exception is not an ambiguous name or alias (e.g. multiple child nodes with a 
+     * variable argument).
+     * 
+     * @return the ambiguous name or alias of the node
+     */
+    public @Nullable String getAmbiguousName() {
+        return this.ambiguousName;
+    }
+
     @Override
     public String getMessage() {
         final StringBuilder stringBuilder = new StringBuilder("Ambiguous Node: ")
                 .append(this.ambiguousNode.component().name())
                 .append(" cannot be added as a child to ")
-                .append(this.parentNode == null ? "<root>" : this.parentNode.component().name())
-                .append(" (All children: ");
+                .append(this.parentNode == null ? "<root>" : this.parentNode.component().name());
+        if (this.ambiguousName != null) {
+            stringBuilder.append(" because of ambiguous name ")
+            .append(this.ambiguousName);
+        }
+        stringBuilder.append(" (All children: ");
         final Iterator<CommandNode<?>> childIterator = this.children.iterator();
         while (childIterator.hasNext()) {
             stringBuilder.append(childIterator.next().component().name());

cloud-core/src/test/java/org/incendo/cloud/CommandTreeTest.java

@@ -446,6 +446,14 @@ void testAmbiguousNodes() {
                 this.commandManager.command(this.commandManager.commandBuilder("ambiguous")
                         .literal("literal")));
         this.setup();
+
+        // Two literals with the same alias can not co-exist, causes AmbiguousNodeException
+        this.commandManager.command(this.commandManager.commandBuilder("ambiguous")
+                .literal("literal", "alias"));
+        assertThrows(AmbiguousNodeException.class, () ->
+                this.commandManager.command(this.commandManager.commandBuilder("ambiguous")
+                        .literal("literal2", "alias")));
+        this.setup();
     }
 
     @Test