Fixes binding to nullable types (#6860)

-fixes #6280

Xamarin.Forms.Core/BindingExpression.cs

@@ -449,6 +449,8 @@ namespace Xamarin.Forms
 					return false;
 				}
 
+				convertTo = Nullable.GetUnderlyingType(convertTo) ?? convertTo;
+
 				value = Convert.ChangeType(value, convertTo, CultureInfo.InvariantCulture);
 				return true;
 			}

Xamarin.Forms.Xaml.UnitTests/Issues/Issue6280.xaml

@@ -0,0 +1,13 @@
+<?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:d="http://xamarin.com/schemas/2014/forms/design"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             mc:Ignorable="d"
+             xmlns:local="clr-namespace:Xamarin.Forms.Xaml.UnitTests"
+             x:Class="Xamarin.Forms.Xaml.UnitTests.Issue6280"
+             x:DataType="local:Issue6280ViewModel">
+    <Entry x:Name="_entry"
+           Text="{Binding NullableInt}"
+           VerticalOptions="Start" />
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Issue6280.xaml.cs

@@ -0,0 +1,35 @@
+using NUnit.Framework;
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	public partial class Issue6280 : ContentPage
+	{
+		public Issue6280() => InitializeComponent();
+		public Issue6280(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;
+
+			[Test]
+			public void BindingToNullable([Values(false, true)]bool useCompiledXaml)
+			{
+				var vm = new Issue6280ViewModel();
+				var page = new Issue6280(useCompiledXaml) { BindingContext = vm };
+				page._entry.SetValueFromRenderer(Entry.TextProperty, 1);
+				Assert.AreEqual(vm.NullableInt, 1);
+			}
+		}
+	}
+
+	public class Issue6280ViewModel
+	{
+		public int? NullableInt { get; set; }
+	}
+}