From 472352558f24c7be59c9bfd86a6129d600ae6e82 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Thu, 6 Aug 2020 17:13:34 +0200
Subject: [PATCH 1/2] Added failing test for #4403.

---
 tests/Avalonia.Styling.UnitTests/SetterTests.cs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/tests/Avalonia.Styling.UnitTests/SetterTests.cs b/tests/Avalonia.Styling.UnitTests/SetterTests.cs
index 46cde30b8a..901233801e 100644
--- a/tests/Avalonia.Styling.UnitTests/SetterTests.cs
+++ b/tests/Avalonia.Styling.UnitTests/SetterTests.cs
@@ -35,6 +35,21 @@ namespace Avalonia.Styling.UnitTests
             Assert.Equal("foo", control.Text);
         }
 
+        [Fact]
+        public void Setter_Should_Handle_Binding_Producing_UnsetValue()
+        {
+            var control = new TextBlock();
+            var subject = new BehaviorSubject<object>(AvaloniaProperty.UnsetValue);
+            var descriptor = InstancedBinding.OneWay(subject);
+            var binding = Mock.Of<IBinding>(x => x.Initiate(control, TextBlock.TagProperty, null, false) == descriptor);
+            var style = Mock.Of<IStyle>();
+            var setter = new Setter(TextBlock.TagProperty, binding);
+
+            setter.Instance(control).Start(false);
+
+            Assert.Equal("", control.Text);
+        }
+
         [Fact]
         public void Setter_Should_Materialize_Template_To_Property()
         {

From de8171f8ac2221095bd9129636d754194992884b Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Thu, 6 Aug 2020 17:13:49 +0200
Subject: [PATCH 2/2] Correctly handle UnsetValue/DoNothing in setters.

Just need to call `BindingValue<T>.FromUntyped` instead of doing a conditional, because otherwise we run the risk of passing `UnsetValue` to the `BindingValue<T>` ctor which will throw in that case.
---
 src/Avalonia.Styling/Styling/PropertySetterBindingInstance.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/Avalonia.Styling/Styling/PropertySetterBindingInstance.cs b/src/Avalonia.Styling/Styling/PropertySetterBindingInstance.cs
index e177993d13..929f7142bb 100644
--- a/src/Avalonia.Styling/Styling/PropertySetterBindingInstance.cs
+++ b/src/Avalonia.Styling/Styling/PropertySetterBindingInstance.cs
@@ -164,7 +164,7 @@ namespace Avalonia.Styling
 
         private void ConvertAndPublishNext(object? value)
         {
-            _value = value is T v ? v : BindingValue<T>.FromUntyped(value);
+            _value = BindingValue<T>.FromUntyped(value);
 
             if (_isActive)
             {