[XamlC] comp binding to generic properties (#4483)

- fixes #4438

Xamarin.Forms.Build.Tasks/SetPropertiesVisitor.cs

@@ -392,7 +392,11 @@ namespace Xamarin.Forms.Build.Tasks
 				yield break; //throw
 
 			var properties = ParsePath(path, tSourceRef, node as IXmlLineInfo, module);
-			var tPropertyRef = properties != null && properties.Any() ? properties.Last().Item1.PropertyType : tSourceRef;
+			TypeReference tPropertyRef = tSourceRef;
+			if (properties != null && properties.Count > 0) {
+				var lastProp = properties[properties.Count - 1];
+				tPropertyRef = lastProp.property.ResolveGenericPropertyType(lastProp.propDeclTypeRef, module);
+			}
 			tPropertyRef = module.ImportReference(tPropertyRef);
 
 			var funcRef = module.ImportReference(module.ImportReference(("mscorlib", "System", "Func`2")).MakeGenericInstanceType(new [] { tSourceRef, tPropertyRef }));

Xamarin.Forms.Xaml.UnitTests/Issues/Gh4438.xaml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage
+		xmlns="http://xamarin.com/schemas/2014/forms"
+		xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+		xmlns:local="using:Xamarin.Forms.Xaml.UnitTests"
+		x:Class="Xamarin.Forms.Xaml.UnitTests.Gh4438"
+		x:DataType="local:Gh4438VM">
+	<Label x:Name="label" Text="{Binding SelectedItem}" />
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh4438.xaml.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Linq;
+using NUnit.Framework;
+using Xamarin.Forms;
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	public class Gh4438VM : Gh4438VMBase<string>
+	{
+		public Gh4438VM()
+		{
+			Add("test");
+			SelectedItem = this.First();
+		}
+	}
+
+	public class Gh4438VMBase<T> : Collection<string>
+	{
+		public virtual T SelectedItem { get; set; }
+	}
+
+	public partial class Gh4438 : ContentPage
+	{
+		public Gh4438()
+		{
+			InitializeComponent();
+		}
+
+		public Gh4438(bool useCompiledXaml)
+		{
+			//this stub will be replaced at compile time
+		}
+
+		[TestFixture]
+		class Tests
+		{
+			[SetUp]
+			public void Setup()
+			{
+				Device.PlatformServices = new MockPlatformServices();
+			}
+
+			[TearDown]
+			public void TearDown()
+			{
+				Device.PlatformServices = null;
+			}
+
+			[TestCase(true), TestCase(false)]
+			public void GenericBaseClassResolution(bool useCompiledXaml)
+			{
+				if (useCompiledXaml)
+					Assert.DoesNotThrow(() => MockCompiler.Compile(typeof(Gh4438)));
+				var layout = new Gh4438(useCompiledXaml) { BindingContext = new Gh4438VM() };
+				Assert.That(layout.label.Text, Is.EqualTo("test"));
+			}
+		}
+	}
+}