[X] throw XPE on missing element .ctor

In case of runtime parse, wrap the MissingMethodException in a
XamlParseException, to get lineInfo. In case of XamlC, detect missing
constructor and fail early.

- fixes #4751

Xamarin.Forms.Build.Tasks/CreateObjectVisitor.cs

@@ -148,6 +148,9 @@ public void Visit(ElementNode node, INode parentNode)
 				md.IsSpecialName &&
 				md.Name == "op_Implicit" && md.Parameters [0].ParameterType.FullName == "System.String");
 
+			if (!typedef.IsValueType && ctorInfo == null && factoryMethodInfo == null)
+				throw new XamlParseException($"Missing default constructor for '{typedef.FullName}'.", node);
+
 			if (ctorinforef != null || factorymethodinforef != null || typedef.IsValueType) {
 				VariableDefinition vardef = new VariableDefinition(typeref);
 				Context.Variables [node] = vardef;

Xamarin.Forms.Xaml.UnitTests/Issues/Gh4751.xaml

@@ -0,0 +1,10 @@
+<?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.Gh4751">
+	<ContentPage.BindingContext>
+		<local:Gh4751VM />
+	</ContentPage.BindingContext>
+</ContentPage>

Xamarin.Forms.Xaml.UnitTests/Issues/Gh4751.xaml.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+
+using NUnit.Framework;
+
+using Xamarin.Forms.Core.UnitTests;
+
+namespace Xamarin.Forms.Xaml.UnitTests
+{
+	public class Gh4751VM
+	{
+		public string Title { get; }
+		public Gh4751VM(string title = null) => Title = title; //a .ctor with a default value IS NOT a default .ctor
+	}
+
+	[XamlCompilation(XamlCompilationOptions.Skip)]
+	public partial class Gh4751 : ContentPage
+	{
+		public Gh4751() => InitializeComponent();
+		public Gh4751(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 ErrorOnMissingDefaultCtor([Values (false, true)]bool useCompiledXaml)
+			{
+				if (useCompiledXaml)
+					Assert.Throws<XamlParseException>(() => MockCompiler.Compile(typeof(Gh4751)));
+				else
+					Assert.Throws<XamlParseException>(() => new Gh4751(useCompiledXaml));
+			}
+		}
+	}
+}

Xamarin.Forms.Xaml/CreateValuesVisitor.cs

@@ -83,10 +83,11 @@ public void Visit(ElementNode node, INode parentNode)
 					if (value == null)
 						value = Activator.CreateInstance(type);
 				}
-				catch (TargetInvocationException e) {
-					if (e.InnerException is XamlParseException || e.InnerException is XmlException)
-						throw e.InnerException;
-					throw;
+				catch (TargetInvocationException e) when (e.InnerException is XamlParseException || e.InnerException is XmlException) {
+					throw e.InnerException;
+				}
+				catch (MissingMethodException mme) {
+					throw new XamlParseException(mme.Message, node, mme);
 				}
 			}