Allow indexer bindings to be at the start of a binding path.

src/Markup/Avalonia.Markup/Data/Parsers/ArgumentListParser.cs

@@ -51,10 +51,10 @@ namespace Avalonia.Markup.Data.Parsers
                     }
                 }
 
-                throw new ExpressionParseException(r.Position, "Expected ']'.");
+                throw new ExpressionParseException(r.Position, $"Expected '{close}'.");
             }
 
-            return null;
+            throw new ExpressionParseException(r.Position, $"Expected '{open}'.");
         }
     }
 }

src/Markup/Avalonia.Markup/Data/Parsers/ExpressionParser.cs

@@ -40,6 +40,10 @@ namespace Avalonia.Markup.Data.Parsers
                     case State.AttachedProperty:
                         state = ParseAttachedProperty(r, nodes);
                         break;
+
+                    case State.Indexer:
+                        state = ParseIndexer(r, nodes);
+                        break;
                 }
             }
 
@@ -67,6 +71,10 @@ namespace Avalonia.Markup.Data.Parsers
             {
                 return State.AttachedProperty;
             }
+            else if (PeekOpenBracket(r))
+            {
+                return State.Indexer;
+            }
             else
             {
                 var identifier = IdentifierParser.Parse(r);
@@ -92,20 +100,9 @@ namespace Avalonia.Markup.Data.Parsers
                 nodes.Add(new StreamNode());
                 return State.AfterMember;
             }
-            else
+            else if (PeekOpenBracket(r))
             {
-                var args = ArgumentListParser.Parse(r, '[', ']');
-
-                if (args != null)
-                {
-                    if (args.Count == 0)
-                    {
-                        throw new ExpressionParseException(r.Position, "Indexer may not be empty.");
-                    }
-
-                    nodes.Add(new IndexerNode(args));
-                    return State.AfterMember;
-                }
+                return State.Indexer;
             }
 
             return State.End;
@@ -151,6 +148,19 @@ namespace Avalonia.Markup.Data.Parsers
             return State.AfterMember;
         }
 
+        private State ParseIndexer(Reader r, List<ExpressionNode> nodes)
+        {
+            var args = ArgumentListParser.Parse(r, '[', ']');
+
+            if (args.Count == 0)
+            {
+                throw new ExpressionParseException(r.Position, "Indexer may not be empty.");
+            }
+
+            nodes.Add(new IndexerNode(args));
+            return State.AfterMember;
+        }
+        
         private static bool ParseNot(Reader r)
         {
             return !r.End && r.TakeIf('!');
@@ -166,6 +176,11 @@ namespace Avalonia.Markup.Data.Parsers
             return !r.End && r.TakeIf('(');
         }
 
+        private static bool PeekOpenBracket(Reader r)
+        {
+            return !r.End && r.Peek == '[';
+        }
+
         private static bool ParseStreamOperator(Reader r)
         {
             return !r.End && r.TakeIf('^');
@@ -177,6 +192,7 @@ namespace Avalonia.Markup.Data.Parsers
             AfterMember,
             BeforeMember,
             AttachedProperty,
+            Indexer,
             End,
         }
     }

tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_Indexer.cs

@@ -338,6 +338,18 @@ namespace Avalonia.Markup.UnitTests.Data
             GC.KeepAlive(data);
         }
 
+        [Fact]
+        public async Task Indexer_Only_Binding_Works()
+        {
+            var data = new[] { 1, 2, 3 };
+
+            var target = new ExpressionObserver(data, "[1]");
+
+            var value = await target.Take(1);
+
+            Assert.Equal(data[1], value);
+        }
+
         private class NonIntegerIndexer : NotifyingBase
         {
             private readonly Dictionary<string, string> _storage = new Dictionary<string, string>();