unspecified option with preaction and default value has preaction invoked

Author: Jon Sequeira

src/System.CommandLine.Tests/OptionTests.cs

@@ -531,5 +531,27 @@ public void Default_value_is_used_when_option_with_ZeroOrOne_arity_is_parsed_wit
 
             parseResult.GetValue(option).Should().Be(42);
         }
+
+        [Fact] // https://github.com/dotnet/command-line-api/issues/2621
+        public void Option_with_default_value_has_Action_added_to_PreActions_when_not_specified_but_has_default()
+        {
+            var actionInvoked = false;
+            var option = new Option<bool>("--quiet")
+            {
+                DefaultValueFactory = _ => true,
+                Action = new SynchronousTestAction(_ => actionInvoked = true, terminating: false)
+            };
+
+            var rootCommand = new RootCommand
+            {
+                option
+            };
+            rootCommand.SetAction(_ => { });
+
+            var parseResult = rootCommand.Parse("");
+            parseResult.Invoke();
+
+            actionInvoked.Should().BeTrue();
+        }
     }
 }

src/System.CommandLine/Parsing/ParseOperation.cs

@@ -342,6 +342,24 @@ private void AddPreAction(CommandLineAction action)
             _preActions.Add(action);
         }
 
+        private void AddActionsForImplicitOptions()
+        {
+            // Check for options with default values and actions that were implicitly added
+            foreach (var kvp in _symbolResultTree)
+            {
+                if (kvp.Key is Option { Action: { } action } &&
+                    kvp.Value is OptionResult optionResult &&
+                    optionResult.Implicit)
+                {
+                    if (!action.Terminating &&
+                        (_primaryAction is null || _primaryAction != action))
+                    {
+                        AddPreAction(action);
+                    }
+                }
+            }
+        }
+
         private void AddCurrentTokenToUnmatched()
         {
             if (CurrentToken.Type == TokenType.DoubleDash)
@@ -366,6 +384,9 @@ private void ValidateAndAddDefaultResults()
                 currentResult = currentResult.Parent as CommandResult;
             }
 
+            // Add actions for options with default values that were not specified on the command line
+            AddActionsForImplicitOptions();
+
             if (_primaryAction is null)
             {
                 if (_innermostCommandResult is { Command: { Action: null, HasSubcommands: true } })