remove Option.Argument setter

Author: jonsequitur

src/System.CommandLine.Tests/Utility/ConsoleAssertions.cs

@@ -1,9 +1,6 @@
 // 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.
 
-// 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 FluentAssertions.Primitives;
 
 namespace System.CommandLine.Tests.Utility

src/System.CommandLine/DebugAssert.cs

@@ -0,0 +1,25 @@
+// 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.Diagnostics;
+
+namespace System.CommandLine
+{
+    internal static class DebugAssert
+    {
+        [Conditional("DEBUG")]
+        public static void ThrowIf(bool condition, string message)
+        {
+            if (condition)
+            {
+                Throw(message);
+            }
+        }
+        
+        [Conditional("DEBUG")]
+        public static void Throw(string message)
+        {
+            throw new Exception(message);
+        }
+    }
+}

src/System.CommandLine/Help/HelpOption.cs

@@ -20,7 +20,6 @@ public HelpOption() : base(new[]
         internal override Argument Argument
         {
             get => Argument.None;
-            set { }
         }
 
         public override bool Equals(object obj)

src/System.CommandLine/Option.cs

@@ -30,7 +30,7 @@ public Option(
             Type? argumentType = null,
             Func<object?>? getDefaultValue = null,
             IArgumentArity? arity = null)
-            : this(aliases, description, GetArgument(argumentType, getDefaultValue, arity))
+            : this(aliases, description, CreateArgument(argumentType, getDefaultValue, arity))
         { }
 
         internal Option(
@@ -54,13 +54,14 @@ internal Option(
                 var alias = aliases[i];
                 AddAlias(alias);
             }
+
             if (argument != null)
             {
-                Argument = argument;
+                AddArgumentInner(argument);
             }
         }
 
-        private static Argument? GetArgument(Type? argumentType, Func<object?>? getDefaultValue, IArgumentArity? arity)
+        private static Argument? CreateArgument(Type? argumentType, Func<object?>? getDefaultValue, IArgumentArity? arity)
         {
             if (argumentType is null &&
                 getDefaultValue is null &&
@@ -87,8 +88,20 @@ getDefaultValue is null &&
 
         internal virtual Argument Argument
         {
-            get => Arguments.FirstOrDefault() ?? Argument.None;
-            set => AddArgumentInner(value); // FIX: delete?
+            get
+            {
+                switch (Children.Arguments.Count)
+                {
+                    case 0:
+                        var none = Argument.None;
+                        Children.Add(none);
+                        return none;
+
+                    default:
+                        DebugAssert.ThrowIf(Children.Arguments.Count > 1, $"Unexpected number of option arguments: {Children.Arguments.Count}");
+                        return Children.Arguments[0];
+                }
+            }
         }
 
         //TODO: Guard against Argument.None?
@@ -98,8 +111,6 @@ public string ArgumentHelpName
             set => Argument.Name = value;
         }
 
-        private IEnumerable<Argument> Arguments => Children.OfType<Argument>();
-
         internal bool DisallowBinding { get; set; } 
 
         public override string Name

src/System.CommandLine/Option{T}.cs

@@ -51,19 +51,5 @@ public Option(
             string? description = null) 
             : base(aliases, description, new Argument<T>(getDefaultValue ?? throw new ArgumentNullException(nameof(getDefaultValue))))
         { }
-
-        internal override Argument Argument
-        {
-            set
-            {
-                if (!(value is Argument<T>))
-                {
-
-                    throw new ArgumentException($"{nameof(Argument)} must be of type {typeof(Argument<T>)} but was {value?.GetType().ToString() ?? "null"}");
-                }
-
-                base.Argument = value;
-            }
-        }
     }
 }

src/System.CommandLine/TypeExtensions.cs

@@ -1,9 +1,7 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
+// 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.Runtime.CompilerServices;
-using System.Text;
-using System.Threading.Tasks;
 
 namespace System.CommandLine
 {