add CliOption.HasDefaultValue

Author: adamsitnik

src/System.CommandLine.ApiCompatibility.Tests/ApiCompatibilityApprovalTests.System_CommandLine_api_is_not_changed.approved.txt

@@ -85,6 +85,7 @@ System.CommandLine
     public System.Boolean AllowMultipleArgumentsPerToken { get; set; }
     public ArgumentArity Arity { get; set; }
     public System.Collections.Generic.List<System.Func<System.CommandLine.Completions.CompletionContext,System.Collections.Generic.IEnumerable<System.CommandLine.Completions.CompletionItem>>> CompletionSources { get; }
+    public System.Boolean HasDefaultValue { get; }
     public System.String HelpName { get; set; }
     public System.Boolean Recursive { get; set; }
     public System.Boolean Required { get; set; }

src/System.CommandLine.Tests/OptionTests.cs

@@ -4,14 +4,30 @@
 using System.CommandLine.Parsing;
 using FluentAssertions;
 using System.Linq;
-using System.Threading;
-using System.Threading.Tasks;
 using Xunit;
 
 namespace System.CommandLine.Tests
 {
     public partial class OptionTests
     {
+        [Fact]
+        public void By_default_there_is_no_default_value()
+        {
+            CliOption<string> option = new("name");
+
+            option.HasDefaultValue.Should().BeFalse();
+        }
+
+        [Fact]
+        public void When_default_value_factory_is_set_then_HasDefaultValue_is_true()
+        {
+            CliOption<string[]> option = new("name");
+
+            option.DefaultValueFactory = (_) => Array.Empty<string>();
+
+            option.HasDefaultValue.Should().BeTrue();
+        }
+
         [Fact]
         public void When_an_option_has_only_name_then_it_has_no_aliases()
         {

src/System.CommandLine/CliOption.cs

@@ -34,6 +34,11 @@ private protected CliOption(string name, string[] aliases) : base(name)
         /// </summary>
         internal abstract CliArgument Argument { get; }
 
+        /// <summary>
+        /// Specifies if a default value is defined for the option.
+        /// </summary>
+        public bool HasDefaultValue => Argument.HasDefaultValue;
+
         /// <summary>
         /// Gets or sets the name of the Option when displayed in help.
         /// </summary>