Allow casting to a nested type in a binding path (#22173)

The binding path parser built type names with ParseIdentifier, which doesn't
accept '+', so a cast like ((local:Outer+Nested)DataContext) stopped parsing at
the '+' and then failed expecting a ')'. Parse type names with a variant that
accepts '+'. Ordinary identifiers are unchanged.

src/Avalonia.Base/Data/Core/Parsers/BindingExpressionGrammar.cs

@@ -377,12 +377,12 @@ namespace Avalonia.Data.Core.Parsers
         {
             ReadOnlySpan<char> ns, typeName;
             ns = ReadOnlySpan<char>.Empty;
-            var typeNameOrNamespace = r.ParseIdentifier();
+            var typeNameOrNamespace = r.ParseTypeIdentifier();
 
             if (!r.End && r.TakeIf(':'))
             {
                 ns = typeNameOrNamespace;
-                typeName = r.ParseIdentifier();
+                typeName = r.ParseTypeIdentifier();
             }
             else
             {

src/Avalonia.Base/Utilities/IdentifierParser.cs

@@ -21,6 +21,25 @@ namespace Avalonia.Utilities
             }
         }
 
+        /// <summary>
+        /// Parses an identifier that may name a nested type, e.g. "Outer+Inner".
+        /// </summary>
+        public static ReadOnlySpan<char> ParseTypeIdentifier(this
+#if NET7SDK
+            scoped
+#endif
+            ref CharacterReader r)
+        {
+            if (IsValidIdentifierStart(r.Peek))
+            {
+                return r.TakeWhile(c => IsValidIdentifierChar(c) || c == '+');
+            }
+            else
+            {
+                return ReadOnlySpan<char>.Empty;
+            }
+        }
+
         private static bool IsValidIdentifierStart(char c)
         {
             return char.IsLetter(c) || c == '_';

tests/Avalonia.Base.UnitTests/Data/Core/Parsers/BindingExpressionGrammarTests.cs

@@ -244,6 +244,26 @@ namespace Avalonia.Base.UnitTests.Data.Core.Parsers
             Assert.IsType<BindingExpressionGrammar.StreamNode>(result[1]);
         }
 
+        [Fact]
+        public void Should_Parse_Cast_To_Nested_Type()
+        {
+            var result = Parse("((ns:Outer+Inner)Foo).Bar");
+
+            var cast = Assert.IsType<BindingExpressionGrammar.TypeCastNode>(result[1]);
+            Assert.Equal("ns", cast.Namespace);
+            Assert.Equal("Outer+Inner", cast.TypeName);
+        }
+
+        [Fact]
+        public void Should_Parse_Cast_To_Non_Nested_Type()
+        {
+            var result = Parse("((ns:Outer)Foo).Bar");
+
+            var cast = Assert.IsType<BindingExpressionGrammar.TypeCastNode>(result[1]);
+            Assert.Equal("ns", cast.Namespace);
+            Assert.Equal("Outer", cast.TypeName);
+        }
+
         private static void AssertIsProperty(
             BindingExpressionGrammar.INode node,
             string name,

tests/Avalonia.Markup.Xaml.UnitTests/MarkupExtensions/BindingExtensionTests.cs

@@ -11,6 +11,27 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
 {
     public class BindingExtensionTests : XamlTestBase
     {
+        [Fact]
+        public void SupportCastToNestedTypeInExpression()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests.MarkupExtensions;assembly=Avalonia.Markup.Xaml.UnitTests'
+        >
+    <ContentControl Content='{Binding $parent.((local:ReflectionOuter+Nested)DataContext).NestedProperty}' Name='contentControl' />
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var contentControl = window.GetControl<ContentControl>("contentControl");
+
+                window.DataContext = new ReflectionOuter.Nested { NestedProperty = "hello" };
+
+                Assert.Equal("hello", contentControl.Content);
+            }
+        }
+
         [Fact]
         public void BindingExtension_Binds_To_Source()
         {
@@ -167,4 +188,12 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
             };
         }
     }
+
+    public class ReflectionOuter
+    {
+        public class Nested
+        {
+            public string NestedProperty { get; set; } = "nested";
+        }
+    }
 }

tests/Avalonia.Markup.Xaml.UnitTests/MarkupExtensions/CompiledBindingExtensionTests.cs

@@ -1456,6 +1456,26 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
             }
         }
 
+        [Fact]
+        public void SupportCastToNestedTypeInExpression()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='using:Avalonia.Markup.Xaml.UnitTests.MarkupExtensions'>
+    <ContentControl Content='{CompiledBinding $parent.((local:OuterClass+NestedClass)DataContext).NestedProperty}' Name='contentControl' />
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var contentControl = window.GetControl<ContentControl>("contentControl");
+
+                window.DataContext = new OuterClass.NestedClass { NestedProperty = "hello" };
+
+                Assert.Equal("hello", contentControl.Content);
+            }
+        }
+
         [Fact]
         public void SupportCastToTypeInExpression_DifferentTypeEvaluatesToNull()
         {
@@ -2825,6 +2845,14 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
         public string? StringProperty { get; set; }
     }
 
+    public class OuterClass
+    {
+        public class NestedClass
+        {
+            public string NestedProperty { get; set; } = "nested value";
+        }
+    }
+
     public class TestDataContextBaseClass {}
     
     public class TestItemsCollectionDataContext : TestDataContextBaseClass