set the parent CommandResult for implicit tokens

Author: jonsequitur

src/System.CommandLine.Tests/ArgumentTests.cs

@@ -190,7 +190,7 @@ public void custom_parsing_of_scalar_value_from_an_argument_with_multiple_tokens
             }
 
             [Fact]
-            public void ArgumentResult_Parent_is_set_correctly_when_token_is_present()
+            public void Option_ArgumentResult_Parent_is_set_correctly_when_token_is_implicit()
             {
                 ArgumentResult argumentResult = null;
 
@@ -202,19 +202,20 @@ public void ArgumentResult_Parent_is_set_correctly_when_token_is_present()
                         {
                             argumentResult = argResult;
                             return null;
-                        })
+                        }, isDefault: true)
                 };
 
-                command.Parse("-x abc");
+                command.Parse("");
 
                 argumentResult
                     .Parent
+                    .Symbol
                     .Should()
-                    .NotBeNull();
+                    .Be(command.Options.Single());
             }
 
             [Fact]
-            public void Option_ArgumentResult_Parent_is_set_correctly_when_token_is_implicit()
+            public void Option_ArgumentResult_parentage_to_root_symbol_is_set_correctly_when_token_is_implicit()
             {
                 ArgumentResult argumentResult = null;
 
@@ -232,10 +233,11 @@ public void Option_ArgumentResult_Parent_is_set_correctly_when_token_is_implicit
                 command.Parse("");
 
                 argumentResult
+                    .Parent
                     .Parent
                     .Symbol
                     .Should()
-                    .Be(command.Options.Single());
+                    .Be(command);
             }
 
             [Fact]

src/System.CommandLine/Parsing/OptionResult.cs

@@ -13,7 +13,7 @@ public class OptionResult : SymbolResult
         internal OptionResult(
             IOption option,
             Token token,
-            CommandResult parent = null) :
+            CommandResult parent) :
             base(option ?? throw new ArgumentNullException(nameof(option)),
                  parent)
         {

src/System.CommandLine/Parsing/ParseResultVisitor.cs

@@ -320,7 +320,8 @@ private void PopulateDefaultValues()
 
                                 var optionResult = new OptionResult(
                                     option,
-                                    option.CreateImplicitToken());
+                                    option.CreateImplicitToken(),
+                                    commandResult);
 
                                 var token = new ImplicitToken(
                                     optionResult.GetDefaultValueFor(option.Argument),

src/System.CommandLine/Parsing/SymbolResult.cs

@@ -2,7 +2,6 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using System.Collections.Generic;
-using System.CommandLine.Binding;
 using System.Linq;
 
 namespace System.CommandLine.Parsing
@@ -15,7 +14,7 @@ public abstract class SymbolResult
 
         private protected SymbolResult(
             ISymbol symbol, 
-            SymbolResult parent = null)
+            SymbolResult parent)
         {
             Symbol = symbol ?? throw new ArgumentNullException(nameof(symbol));