[Xaml] don't swallow exceptions from converters (#4100)

Compiled converters are invoked, by reflection, at compile time. Any
exception thrown there will be wrapped in a TargetInvocationException.
When that happens, we still want to expose the inner XamlParseException
to the user.

- fixes #4099

Xamarin.Forms.Build.Tasks/NodeILExtensions.cs

@@ -103,8 +103,13 @@ namespace Xamarin.Forms.Build.Tasks
 			if (compiledConverterName != null && (compiledConverterType = Type.GetType (compiledConverterName)) != null) {
 				var compiledConverter = Activator.CreateInstance (compiledConverterType);
 				var converter = typeof(ICompiledTypeConverter).GetMethods ().FirstOrDefault (md => md.Name == "ConvertFromString");
-				var instructions = (IEnumerable<Instruction>)converter.Invoke (compiledConverter, new object[] {
+				IEnumerable<Instruction> instructions;
+				try {
+					instructions = (IEnumerable<Instruction>)converter.Invoke(compiledConverter, new object[] {
 					node.Value as string, context, node as BaseNode});
+				} catch (System.Reflection.TargetInvocationException tie) when (tie.InnerException is XamlParseException) {
+					throw tie.InnerException;
+				}
 				foreach (var i in instructions)
 					yield return i;
 				if (targetTypeRef.IsValueType && boxValueTypes)

Xamarin.Forms.Xaml.UnitTests/Issues/Gh4099.xaml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+			 xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+			 x:Class="Xamarin.Forms.Xaml.UnitTests.Gh4099">
+	<StackLayout Padding="2 2 2 5" />
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh4099.xaml.cs

@@ -0,0 +1,54 @@
+using System;
+using System.Collections.Generic;
+
+using NUnit.Framework;
+
+using Xamarin.Forms;
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	[XamlCompilation(XamlCompilationOptions.Skip)]
+	public partial class Gh4099 : ContentPage
+	{
+		public Gh4099()
+		{
+			InitializeComponent();
+		}
+
+		public Gh4099(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)]
+			public void BetterExceptionReport(bool useCompiledXaml)
+			{
+				if(useCompiledXaml) {
+					try {
+						MockCompiler.Compile(typeof(Gh4099));
+					} catch (XamlParseException xpe) {
+						Assert.That(xpe.XmlInfo.LineNumber, Is.EqualTo(5));
+						Assert.Pass();
+					}
+					Assert.Fail();
+				}
+			}
+		}
+	}
+}

Xamarin.Forms.Xaml.UnitTests/Xamarin.Forms.Xaml.UnitTests.csproj

@@ -652,6 +652,9 @@
     <Compile Include="Issues\Gh3821View.xaml.cs">
       <DependentUpon>Gh3821View.xaml</DependentUpon>
     </Compile>
+    <Compile Include="Issues\Gh4099.xaml.cs">
+      <DependentUpon>Gh4099.xaml</DependentUpon>
+    </Compile>
   </ItemGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
   <PropertyGroup>
@@ -1200,6 +1203,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
+    <EmbeddedResource Include="Issues\Gh4099.xaml">
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+      <SubType>Designer</SubType>
+    </EmbeddedResource>
   </ItemGroup>
   <ItemGroup>
     <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />