Fix 1137 (for real this time) (#1336)

Author: jonsequitur

src/System.CommandLine.Hosting/InvocationLifetime.cs

@@ -1,4 +1,7 @@
-using System.CommandLine.Invocation;
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.CommandLine.Invocation;
 using System.Threading;
 using System.Threading.Tasks;
 

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

@@ -5,7 +5,6 @@
 using System.CommandLine.Binding;
 using System.CommandLine.Builder;
 using System.CommandLine.Invocation;
-using System.CommandLine.IO;
 using System.CommandLine.Parsing;
 using System.IO;
 using FluentAssertions;
@@ -794,5 +793,43 @@ public class ComplexType
         {
             public decimal OptDecimal { get; set; }
         }
+
+#if NETCOREAPP2_1_OR_GREATER
+        [Theory]
+        [InlineData("--class-with-span-ctor a51ca309-84fa-452f-96be-51e47702ffb4 --int-value 1234")]
+        [InlineData("--class-with-span-ctor a51ca309-84fa-452f-96be-51e47702ffb4")]
+        [InlineData("--int-value 1234")]
+        public void When_only_available_constructor_is_span_then_null_is_passed(string commandLine)
+        {
+            var root = new RootCommand
+            {
+                new Option<ClassWithSpanConstructor>("--class-with-span-ctor"),
+                new Option<int>("--int-value"),
+            };
+
+            var handlerWasCalled = false;
+
+            root.Handler = CommandHandler.Create<ClassWithSpanConstructor, int>((spanCtor, intValue) =>
+            {
+                handlerWasCalled = true;
+            });
+
+            root.Invoke(commandLine);
+
+            handlerWasCalled.Should().BeTrue();
+        }
+
+        public class ClassWithSpanConstructor
+        {
+            private Guid value;
+
+            public ClassWithSpanConstructor(ReadOnlySpan<byte> guid)
+            {
+                value = new Guid(guid);
+            }
+
+            public override string ToString() => value.ToString();
+        }
+#endif
     }
 }

src/System.CommandLine/Binding/ModelBinder.cs

@@ -67,7 +67,7 @@ public void BindMemberFromValue(PropertyInfo property, IValueDescriptor valueDes
 
         private (bool success, object? newInstance, bool anyNonDefaults) CreateInstanceInternal(BindingContext bindingContext)
         {
-            if (DisallowedBindingType())
+            if (IsModelTypeUnbindable())
             {
                 throw new InvalidOperationException($"The type {ModelDescriptor.ModelType} cannot be bound");
             }
@@ -93,11 +93,11 @@ public void BindMemberFromValue(PropertyInfo property, IValueDescriptor valueDes
             }
         }
 
-        private bool DisallowedBindingType()
+        private bool IsModelTypeUnbindable()
         {
             var modelType = ModelDescriptor.ModelType;
-            return modelType.IsConstructedGenericTypeOf(typeof(Span<>)) || 
-                modelType.IsConstructedGenericTypeOf(typeof(ReadOnlySpan<>));
+            return modelType.IsConstructedGenericTypeOf(typeof(Span<>)) ||
+                   modelType.IsConstructedGenericTypeOf(typeof(ReadOnlySpan<>));
         }
 
         private bool ShortCutTheBinding()
@@ -258,6 +258,8 @@ internal static (BoundValue? boundValue, bool usedNonDefault) GetBoundValue(
                     bool includeMissingValues,
                     Type? parentType)
         {
+
+
             if (bindingContext.TryBindToScalarValue(
                     valueDescriptor,
                     valueSource,
@@ -276,7 +278,14 @@ internal static (BoundValue? boundValue, bool usedNonDefault) GetBoundValue(
                 if (valueDescriptor.ValueType != parentType) // Recursive models aren't allowed
                 {
                     var binder = bindingContext.GetModelBinder(valueDescriptor);
+
+                    if (binder.IsModelTypeUnbindable())
+                    {
+                        return (null, false);
+                    }
+
                     var (success, newInstance, usedNonDefaults) = binder.CreateInstanceInternal(bindingContext);
+
                     if (success)
                     {
                         return (new BoundValue(newInstance, valueDescriptor, valueSource), usedNonDefaults);
@@ -287,14 +296,13 @@ internal static (BoundValue? boundValue, bool usedNonDefault) GetBoundValue(
                 {
                     return (new BoundValue(parameterDescriptor.GetDefaultValue(), valueDescriptor, valueSource), false);
                 }
-                // Logic dropped here - misnamed and purpose unclear: ShouldPassNullToConstructor(constructorDescriptor.Parent, constructorDescriptor))
+
                 return (BoundValue.DefaultForType(valueDescriptor), false);
             }
 
             return (null, false);
         }
 
-
         protected ConstructorDescriptor FindModelConstructorDescriptor(ConstructorInfo constructorInfo)
         {
             var constructorParameters = constructorInfo.GetParameters();