Fix OnPlatform + Setter when no match for current platform

src/Controls/samples/Controls.Sample.UITests/Issues/Issue12064.xaml

@@ -0,0 +1,44 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage
+    x:Class="Maui.Controls.Sample.Issues.Issue12604"
+    xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml">
+    <ContentPage.Resources>
+        <Style x:Key="ButtonStyle1" TargetType="Button">
+            <Setter Property="FontSize" Value="{OnPlatform iOS=36}" />
+            <Setter Property="TextColor" Value="{OnPlatform iOS=Red}" />
+        </Style>
+        <Style x:Key="ButtonStyle2" TargetType="Button">
+            <Setter Property="FontSize">
+                <OnPlatform x:TypeArguments="x:Double">
+                    <On Platform="iOS">36</On>
+                </OnPlatform>
+            </Setter>
+            <Setter Property="TextColor">
+                <OnPlatform x:TypeArguments="Color">
+                    <On Platform="iOS">Red</On>
+                </OnPlatform>
+            </Setter>
+        </Style>
+    </ContentPage.Resources>
+
+    <ScrollView WidthRequest="400">
+        <VerticalStackLayout
+            Padding="10,0"
+            Spacing="15"
+            VerticalOptions="Center">
+            <Label Text="FontSize for all buttons below should be the same: 36 on iOS, default on other platforms"
+                   AutomationId="TestLabel" />
+            <Button Style="{StaticResource ButtonStyle1}" Text="Button label" />
+            <Button Style="{StaticResource ButtonStyle2}" Text="Button label" />
+            <Button FontSize="{OnPlatform iOS=36}" Text="Button label" />
+            <Button Text="Button label">
+                <Button.FontSize>
+                    <OnPlatform x:TypeArguments="x:Double">
+                        <On Platform="iOS">36</On>
+                    </OnPlatform>
+                </Button.FontSize>
+            </Button>
+        </VerticalStackLayout>
+    </ScrollView>
+</ContentPage>

src/Controls/samples/Controls.Sample.UITests/Issues/Issue12064.xaml.cs

@@ -0,0 +1,24 @@
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+using Microsoft.Maui.Platform;
+using Microsoft.Maui;
+using System.Linq;
+using System;
+
+#if IOS
+using UIKit;
+using CoreGraphics;
+#endif
+
+namespace Maui.Controls.Sample.Issues
+{
+	//[XamlCompilation(XamlCompilationOptions.Compile)]
+	[Issue(IssueTracker.Github, 12604, "Setter showing a Build Error when using XAML OnPlatform Markup Extension", PlatformAffected.All)]
+	public partial class Issue12604 : ContentPage
+	{
+		public Issue12604()
+		{
+			InitializeComponent();
+		}
+	}
+}

src/Controls/src/Build.Tasks/CompiledValueProviders/SetterValueProvider.cs

@@ -23,8 +23,9 @@ public IEnumerable<Instruction> ProvideValue(VariableDefinitionReference vardefr
 			if (SetterValueIsCollection(bpRef, module, node, context))
 				yield break;
 
+			// valueNode is null, for example, when OnPlatform doesn't have a match for the current platform, so the property should not be set
 			if (valueNode == null)
-				throw new XamlParseException("Missing Value for Setter", (IXmlLineInfo)node);
+				yield break;
 
 			//if it's an elementNode, there's probably no need to convert it
 			if (valueNode is IElementNode)

src/Controls/src/Xaml/MarkupExtensions/OnPlatformExtension.cs

@@ -65,7 +65,13 @@ public object ProvideValue(IServiceProvider serviceProvider)
 			if (!TryGetValueForPlatform(out var value))
 			{
 				if (bp != null)
-					return bp.GetDefaultValue(valueProvider.TargetObject as BindableObject);
+				{
+					object targetObject = valueProvider.TargetObject;
+
+					if (targetObject is Setter)
+						return null;
+					else return bp.GetDefaultValue(targetObject as BindableObject);
+				}
 				if (propertyType.IsValueType)
 					return Activator.CreateInstance(propertyType);
 				return null;

src/Controls/tests/UITests/Tests/Issues/Issue12064.cs

@@ -0,0 +1,21 @@
+using Microsoft.Maui.Appium;
+using NUnit.Framework;
+
+namespace Microsoft.Maui.AppiumTests.Issues
+{
+	public class Issue12064 : _IssuesUITest
+	{
+		public Issue12064(TestDevice device)
+			: base(device)
+		{ }
+
+		public override string Issue => "Setter showing a Build Error when using XAML OnPlatform Markup Extension";
+
+		[Test]
+		public void Issue12064Test()
+		{
+			App.WaitForElement("TestLabel");
+			VerifyScreenshot();
+		}
+	}
+}