fix #1294

Author: jonsequitur

src/System.CommandLine/Argument.cs

@@ -178,6 +178,8 @@ public void SetDefaultValueFactory(Func<ArgumentResult, object?> getDefaultValue
         /// </summary>
         public bool HasDefaultValue => _defaultValueFactory is not null;
 
+        internal virtual bool HasCustomParser => false;
+
         internal static Argument None() => new()
         {
             Arity = ArgumentArity.Zero,

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

@@ -9,6 +9,8 @@ namespace System.CommandLine
     /// <inheritdoc cref="Argument" />
     public class Argument<T> : Argument, IValueDescriptor<T>
     {
+        private bool _hasCustomParser;
+
         /// <summary>
         /// Initializes a new instance of the Argument class.
         /// </summary>
@@ -110,6 +112,8 @@ public Argument(
                 }
             };
 
+            _hasCustomParser = true;
+
             Description = description;
         }
 
@@ -122,6 +126,8 @@ public Argument(ParseArgument<T> parse, bool isDefault = false) : this(null, par
         {
         }
 
+        internal override bool HasCustomParser => _hasCustomParser;
+
         /// <inheritdoc />
         public override Type ValueType
         {

src/System.CommandLine/Parsing/ParseOperation.cs

@@ -192,7 +192,7 @@ private void ParseOptionArguments(OptionNode optionNode)
             var argument = optionNode.Option.Argument;
 
             var contiguousTokens = 0;
-            var  continueProcessing = true;
+            var continueProcessing = true;
 
             while (More() &&
                    CurrentToken.Type == TokenType.Argument &&

src/System.CommandLine/Parsing/ParseResultVisitor.cs

@@ -372,12 +372,27 @@ private void ValidateAndConvertOptionResult(OptionResult optionResult)
                 }
             }
 
-            for (var i = 0; i < optionResult.Children.Count; i++)
+            if (optionResult.Children.Count == 0)
             {
-                var result = optionResult.Children[i];
-                if (result is ArgumentResult argumentResult)
+                if (optionResult.Option.Argument is Argument { HasCustomParser: true })
                 {
-                    ValidateAndConvertArgumentResult(argumentResult);
+                    if (optionResult.Option is Option opt)
+                    {
+                        var argResult = optionResult.GetOrCreateDefaultArgumentResult(opt.Argument);
+                        optionResult.Children.Add(argResult);
+                        ValidateAndConvertArgumentResult(argResult);
+                    }
+                }
+            }
+            else
+            {
+                for (var i = 0; i < optionResult.Children.Count; i++)
+                {
+                    var result = optionResult.Children[i];
+                    if (result is ArgumentResult argumentResult)
+                    {
+                        ValidateAndConvertArgumentResult(argumentResult);
+                    }
                 }
             }
         }