Fix for #1248: Hosting with BindCommandLine throws TargetParameterCountException (#1277)

* fix #1248

* Add TestCase

* review comment

Author: BobSilent

src/System.CommandLine.Tests/Binding/ModelBinderTests.cs

@@ -265,6 +265,20 @@ public void Parse_result_can_be_used_to_modify_an_existing_instance_without_doin
             instance.IntOption.Should().Be(123);
         }
 
+        [Fact]
+        public void Modify_an_existing_instance_should_keep_all_default_values_if_no_argument_matches_option()
+        {
+            var parser = new Parser(new Command("the-command"));
+
+            var instance = new ClassWithComplexTypes();
+            var bindingContext = new BindingContext(parser.Parse("the-command"));
+            var binder = new ModelBinder(typeof(ClassWithComplexTypes));
+
+            binder.UpdateInstance(instance, bindingContext);
+
+            instance.Should().BeEquivalentTo(new ClassWithComplexTypes());
+        }
+
         [Fact]
         public void Values_from_options_on_parent_commands_are_bound_by_name_by_default()
         {
@@ -475,6 +489,7 @@ public void PropertyExpression_can_be_bound_to_argument()
             instance.IntOption.Should().Be(42);
         }
 
+
         [Fact]
         public void Option_argument_is_bound_to_longest_constructor()
         {

src/System.CommandLine.Tests/Binding/TestModels.cs

@@ -30,6 +30,15 @@ public class ClassWithMultiLetterSetters
         public bool BoolOption { get; set; }
     }
 
+    public class ClassWithComplexTypes
+    {
+        public int IntOption { get; set; }
+        public string StringOption { get; set; }
+        public bool BoolOption { get; set; }
+        public List<ClassWithMultiLetterSetters> ListOptionDefaultNull { get; set; }
+        public List<ClassWithCtorParameter<string>> ListOptionDefaultEmpty { get; set; } = new ();
+    }
+
     public class ClassWithListTypePropertiesAndDefaultCtor
     {
         public List<string> Strings { get; set; }

src/System.CommandLine/Binding/ModelBinder.cs

@@ -265,23 +265,23 @@ internal static (BoundValue? boundValue, bool usedNonDefault) GetBoundValue(
                 return (boundValue, true);
             }
 
-            if (valueDescriptor.HasDefaultValue)
+            if (includeMissingValues)
             {
-                return (BoundValue.DefaultForValueDescriptor(valueDescriptor), false);
-            }
+                if (valueDescriptor.HasDefaultValue)
+                {
+                    return (BoundValue.DefaultForValueDescriptor(valueDescriptor), false);
+                }
 
-            if (valueDescriptor.ValueType != parentType) // Recursive models aren't allowed
-            {
-                var binder = bindingContext.GetModelBinder(valueDescriptor);
-                var (success, newInstance, usedNonDefaults) = binder.CreateInstanceInternal(bindingContext);
-                if (success)
+                if (valueDescriptor.ValueType != parentType) // Recursive models aren't allowed
                 {
-                    return (new BoundValue(newInstance, valueDescriptor, valueSource), usedNonDefaults);
+                    var binder = bindingContext.GetModelBinder(valueDescriptor);
+                    var (success, newInstance, usedNonDefaults) = binder.CreateInstanceInternal(bindingContext);
+                    if (success)
+                    {
+                        return (new BoundValue(newInstance, valueDescriptor, valueSource), usedNonDefaults);
+                    }
                 }
-            }
 
-            if (includeMissingValues)
-            {
                 if (valueDescriptor is ParameterDescriptor parameterDescriptor && parameterDescriptor.AllowsNull)
                 {
                     return (new BoundValue(parameterDescriptor.GetDefaultValue(), valueDescriptor, valueSource), false);