fix #792 (again)

Author: jonsequitur

src/System.CommandLine.Tests/ParserTests.MultipleArguments.cs

@@ -144,6 +144,31 @@ public void Multiple_arguments_of_unspecified_type_are_parsed_correctly()
                       .Be("dest.txt");
             }
 
+
+            [Fact]
+            public void When_multiple_arguments_are_defined_but_not_provided_then_option_parses_correctly()
+            {
+                var option = new Option<string>("-e");
+                var command = new Command("the-command") { option };
+
+                command.AddArgument(new Argument<string>
+                {
+                    Name = "arg1",
+                });
+
+                command.AddArgument(new Argument<string>
+                {
+                    Name = "arg2",
+                });
+
+                var result = command.Parse("-e foo");
+
+                var optionResult = result.ValueForOption(option);
+
+                optionResult.Should().Be("foo");
+            }
+
+
             [Fact]
             public void tokens_that_cannot_be_converted_by_multiple_arity_argument_flow_to_next_multiple_arity_argument()
             {

src/System.CommandLine/Parsing/OptionResult.cs

@@ -48,12 +48,17 @@ internal ArgumentConversionResult ArgumentConversionResult
             {
                 if (_argumentConversionResult is null)
                 {
-                    var results = Children
-                                  .OfType<ArgumentResult>()
-                                  .Select(r => r.GetArgumentConversionResult());
+                    for (var i = 0; i < Children.Count; i++)
+                    {
+                        var child = Children[i];
 
-                    _argumentConversionResult = results.SingleOrDefault() ??
-                                                ArgumentConversionResult.None(Option.Argument);
+                        if (child is ArgumentResult argumentResult)
+                        {
+                            return _argumentConversionResult = argumentResult.GetArgumentConversionResult();
+                        }
+                    }
+
+                    return _argumentConversionResult = ArgumentConversionResult.None(Option.Argument);
                 }
 
                 return _argumentConversionResult;

src/System.CommandLine/Parsing/ParseResultVisitor.cs

@@ -199,7 +199,10 @@ protected override void Stop(SyntaxNode node)
 
                         argumentResults.Add(nextArgumentResult);
 
-                        previousArgumentResult.Parent!.Children.Add(nextArgumentResult);
+                        if (previousArgumentResult.Parent is CommandResult commandResult)
+                        {
+                             commandResult.Children.Add(nextArgumentResult);
+                        }
 
                         _rootCommandResult.AddToSymbolMap(nextArgumentResult);
                     }