fix #2257

src/System.CommandLine.Tests/OptionTests.cs

@@ -512,5 +512,24 @@ public void Custom_parser_can_be_used_to_implement_int_binding_based_on_token_co
 
             result.GetValue(option).Should().Be(3);
         }
+
+        [Fact] // https://github.com/dotnet/command-line-api/issues/2257
+        public void Default_value_is_used_when_option_with_ZeroOrOne_arity_is_parsed_without_an_argument()
+        {
+            var option = new Option<int>("-o")
+            {
+                Arity = ArgumentArity.ZeroOrOne,
+                DefaultValueFactory = _ => 42
+            };
+
+            var rootCommand = new RootCommand
+            {
+                option
+            };
+
+            var parseResult = rootCommand.Parse("-o");
+
+            parseResult.GetValue(option).Should().Be(42);
+        }
     }
 }

src/System.CommandLine/Parsing/ArgumentResult.cs

@@ -153,7 +153,7 @@ private ArgumentConversionResult ValidateAndConvert(bool useValidators)
                 }
             }
 
-            if (Parent!.UseDefaultValueFor(this))
+            if (Argument.HasDefaultValue && Parent!.UseDefaultValueFor(this))
             {
                 var defaultValue = Argument.GetDefaultValue(this);
 

src/System.CommandLine/Parsing/OptionResult.cs

@@ -64,6 +64,16 @@ internal bool IsArgumentLimitReached
         internal ArgumentConversionResult ArgumentConversionResult
             => _argumentConversionResult ??= GetResult(Option.Argument)!.GetArgumentConversionResult();
 
-        internal override bool UseDefaultValueFor(ArgumentResult argument) => Implicit;
+        internal override bool UseDefaultValueFor(ArgumentResult argumentResult)
+        {
+            if (Implicit)
+            {
+                return true;
+            }
+
+            return Tokens.Count is 0 &&
+                   Option.Arity is { MinimumNumberOfValues: 0, MaximumNumberOfValues: > 0 } &&
+                   !argumentResult.Argument.IsBoolean();
+        }
     }
 }