Fix #2621

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 preactionWasInvoked = false;
+            var option = new Option<bool>("--quiet")
+            {
+                DefaultValueFactory = _ => true,
+                Action = new SynchronousTestAction(_ => preactionWasInvoked = true, terminating: false)
+            };
+
+            var rootCommand = new RootCommand
+            {
+                option
+            };
+            rootCommand.SetAction(_ => { });
+
+            var parseResult = rootCommand.Parse("");
+            parseResult.Invoke();
+
+            preactionWasInvoked.Should().BeTrue();
+        }
     }
 }

src/System.CommandLine/Parsing/ParseOperation.cs

@@ -333,7 +333,7 @@ void ParseDirective()
         }
 
         private void AddPreAction(CommandLineAction action)
-        {
+        {   
             if (_preActions is null)
             {
                 _preActions = new();
@@ -342,6 +342,18 @@ private void AddPreAction(CommandLineAction action)
             _preActions.Add(action);
         }
 
+        private void AddPreActionsForImplicitOptions()
+        {
+            foreach (var kvp in _symbolResultTree)
+            {
+                if (kvp is { Key: Option { Action: { Terminating: false } action }, Value: OptionResult { Implicit: true } } && 
+                    _primaryAction != action)
+                {
+                    AddPreAction(action);
+                }
+            }
+        }
+
         private void AddCurrentTokenToUnmatched()
         {
             if (CurrentToken.Type == TokenType.DoubleDash)
@@ -435,6 +447,8 @@ private void ValidateAndAddDefaultResults()
                     }
                 }
             }
+
+            AddPreActionsForImplicitOptions();
         }
     }
 }