Add default values to HelpBuilder output

.gitignore

@@ -151,4 +151,7 @@ nCrunchTemp*
 TestResults/
 
 .fake
-.ionide
+.ionide
+
+# ApprovalTests
+*.received.txt

src/System.CommandLine.Tests/ApprovalTests/ApprovalTests.Config.cs

@@ -0,0 +1,9 @@
+using ApprovalTests.Reporters;
+using ApprovalTests.Reporters.TestFrameworks;
+
+// Use globally defined Reporter for ApprovalTests. Please see
+// https://github.com/approvals/ApprovalTests.Net/blob/master/docs/ApprovalTests/Reporters.md
+
+[assembly: UseReporter(typeof(FrameworkAssertReporter))]
+
+[assembly: ApprovalTests.Namers.UseApprovalSubdirectory("Approvals")]

src/System.CommandLine.Tests/ApprovalTests/Help/Approvals/HelpBuilderTests.Help_describes_default_values_for_complex_root_command_scenario.approved.txt

@@ -0,0 +1,21 @@
+the-root-command:
+  Test description
+
+Usage:
+  the-root-command [options] <the-root-arg-no-description-no-default> [<the-root-arg-no-description-default> <the-root-arg-no-default> [<the-root-arg> [<the-root-arg-enum-default>]]]
+
+Arguments:
+  <the-root-arg-no-description-no-default>    
+  <the-root-arg-no-description-default>       [default: the-root-arg-no-description-default-value]
+  <the-root-arg-no-default>                   the-root-arg-no-default-description
+  <the-root-arg>                              the-root-arg-description [default: the-root-arg-one-value]
+  <Read|ReadWrite|Write>                      the-root-arg-enum-default-description [default: Read]
+
+Options:
+  -trna, --the-root-option-no-arg (REQUIRED)                                                             the-root-option-no-arg-description
+  -trondda, --the-root-option-no-description-default-arg <the-root-option-no-description-default-arg>    [default: the-root-option--no-description-default-arg-value]
+  -tronda, --the-root-option-no-default-arg <the-root-option-arg-no-default-arg> (REQUIRED)              the-root-option-no-default-description
+  -troda, --the-root-option-default-arg <the-root-option-arg>                                            the-root-option-default-arg-description [default: the-root-option-arg-value]
+  -troea, --the-root-option-enum-arg <Read|ReadWrite|Write>                                              the-root-option-description [default: Read]
+  -trorea, --the-root-option-required-enum-arg <Read|ReadWrite|Write> (REQUIRED)                         the-root-option-description [default: Read]
+

src/System.CommandLine.Tests/ApprovalTests/Help/HelpBuilderTests.Approval.cs

@@ -0,0 +1,84 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Xunit;
+using System.CommandLine.Help;
+using System.IO;
+using ApprovalTests;
+
+namespace System.CommandLine.Tests.Help
+{
+    public partial class HelpBuilderTests
+    {
+        [Fact]
+        public void Help_describes_default_values_for_complex_root_command_scenario()
+        {
+            var command = new RootCommand(description: "Test description")
+            {
+                new Argument<string>("the-root-arg-no-description-no-default"),
+                new Argument<string>("the-root-arg-no-description-default",
+                    argResult => "the-root-arg-no-description-default-value",
+                    isDefault: true),
+                new Argument<string>("the-root-arg-no-default")
+                {
+                    Description = "the-root-arg-no-default-description",
+                },
+                new Argument<string>("the-root-arg", () => "the-root-arg-one-value")
+                {
+                    Description = "the-root-arg-description"
+                },
+                new Argument<FileAccess>("the-root-arg-enum-default", () => FileAccess.Read)
+                {
+                    Description = "the-root-arg-enum-default-description",
+                    ArgumentType = typeof(FileAccess)
+                },
+                new Option(aliases: new string[] {"--the-root-option-no-arg", "-trna"}) {
+                    Description = "the-root-option-no-arg-description",
+                    Required = true
+                },
+                new Option<string>(
+                    aliases: new string[] {"--the-root-option-no-description-default-arg", "-trondda"}, 
+                    parseArgument: _ => "the-root-option--no-description-default-arg-value",
+                    isDefault: true
+                ),
+                new Option(aliases: new string[] {"--the-root-option-no-default-arg", "-tronda"}) {
+                    Description = "the-root-option-no-default-description",
+                    Argument = new Argument<string>("the-root-option-arg-no-default-arg")
+                    {
+                        Description = "the-root-option-arg-no-default-description"
+                    },
+                    Required = true
+                },
+                new Option(aliases: new string[] {"--the-root-option-default-arg", "-troda"}) {
+                    Description = "the-root-option-default-arg-description",
+                    Argument = new Argument<string>("the-root-option-arg", () => "the-root-option-arg-value")
+                    {
+                        Description = "the-root-option-arg-description"
+                    },
+                },
+                new Option(aliases: new string[] {"--the-root-option-enum-arg", "-troea"}) {
+                    Description = "the-root-option-description",
+                    Argument = new Argument<FileAccess>("the-root-option-arg", () => FileAccess.Read)
+                    {
+                        Description = "the-root-option-arg-description",
+                    },
+                },
+                new Option(aliases: new string[] {"--the-root-option-required-enum-arg", "-trorea"}) {
+                    Description = "the-root-option-description",
+                    Argument = new Argument<FileAccess>("the-root-option-arg", () => FileAccess.Read)
+                    {
+                        Description = "the-root-option-arg-description",
+                    },
+                    Required = true
+                }
+            };
+            command.Name = "the-root-command";
+
+            HelpBuilder helpBuilder = GetHelpBuilder(LargeMaxWidth);
+            helpBuilder.Write(command);
+            var output = _console.Out.ToString();
+            Approvals.Verify(output);
+        }
+
+    }
+}

src/System.CommandLine.Tests/Help/HelpBuilderTests.cs

@@ -16,7 +16,7 @@
 
 namespace System.CommandLine.Tests.Help
 {
-    public class HelpBuilderTests
+    public partial class HelpBuilderTests
     {
         private const int SmallMaxWidth = 70;
         private const int LargeMaxWidth = 200;
@@ -842,6 +842,55 @@ public void Option_argument_descriptor_indicates_enums_values(Type type)
             _console.Out.ToString().Should().Contain($"--opt <Read|ReadWrite|Write>{_columnPadding}{description}");
         }
 
+        [Fact]
+        public void Help_describes_default_value_for_defaultable_argument()
+        {
+            var argument = new Argument
+            {
+                Name = "the-arg",
+                Description = "Help text from HelpDetail",
+            };
+            argument.SetDefaultValue("the-arg-value");
+
+            var command = new Command("the-command",
+                "Help text from description") { argument };
+
+            HelpBuilder helpBuilder = GetHelpBuilder(SmallMaxWidth);
+
+            helpBuilder.Write(command);
+
+            var help = _console.Out.ToString();
+
+            help.Should().Contain($"[default: the-arg-value]");
+        }
+
+        [Fact]
+        public void Command_arguments_default_value_provided()
+        {
+            var argument = new Argument
+            {
+                Name = "the-arg",
+            };
+
+            var otherArgument = new Argument
+            {
+                Name = "the-other-arg",
+            };
+            argument.SetDefaultValue("the-arg-value");
+            otherArgument.SetDefaultValue("the-other-arg-value");
+            var command = new Command("the-command",
+                "Help text from description") { argument, otherArgument };
+
+            HelpBuilder helpBuilder = GetHelpBuilder(SmallMaxWidth);
+
+            helpBuilder.Write(command);
+
+            var help = _console.Out.ToString();
+
+            help.Should().Contain($"[default: the-arg-value]")
+                .And.Contain($"[default: the-other-arg-value]");
+        }
+
         #endregion Arguments
 
         #region Options
@@ -1188,6 +1237,58 @@ public void Option_aliases_are_shown_before_long_names_regardless_of_alphabetica
                 .ToString().Should().Contain("-a, -z, --aaa, --zzz");
         }
 
+        [Fact]
+        public void Help_describes_default_value_for_option_with_defaultable_argument()
+        {
+            var argument = new Argument
+            {
+                Name = "the-arg",
+            };
+            argument.SetDefaultValue("the-arg-value");
+
+            var command = new Command("the-command", "command help")
+            {
+                new Option(new[] { "-arg"})
+                {
+                    Argument = argument
+                }
+            };
+
+            HelpBuilder helpBuilder = GetHelpBuilder(SmallMaxWidth);
+
+            helpBuilder.Write(command);
+
+            var help = _console.Out.ToString();
+
+            help.Should().Contain($"[default: the-arg-value]");
+        }
+
+        [Fact]
+        public void Help_should_not_contain_default_value_for_hidden_argument_defined_for_option ()
+        {
+            var argument = new Argument
+            {
+                Name = "the-arg",
+                IsHidden = true
+            };
+            argument.SetDefaultValue("the-arg-value");
+            var command = new Command("the-command", "command help")
+            {
+                new Option(new[] { "-arg"})
+                {
+                    Argument = argument
+                }
+            };
+
+            HelpBuilder helpBuilder = GetHelpBuilder(LargeMaxWidth);
+
+            helpBuilder.Write(command);
+
+            var help = _console.Out.ToString();
+
+            help.Should().NotContain($"[default: the-arg-value]");
+        }
+
         #endregion Options
 
         #region Subcommands
@@ -1391,6 +1492,82 @@ public void Help_text_can_be_added_after_default_text_by_inheriting_HelpBuilder(
             console.Out.ToString().Should().EndWith("The text to add");
         }
 
+        [Fact]
+        public void Help_describes_default_value_for_subcommand_with_arguments_and_only_defaultable_is_shown()
+        {
+            var argument = new Argument
+            {
+                Name = "the-arg",
+            };
+            var otherArgumentHidden = new Argument
+            {
+                Name = "the-other-hidden-arg",
+                IsHidden = true
+            };
+            argument.SetDefaultValue("the-arg-value");
+            otherArgumentHidden.SetDefaultValue("the-other-hidden-arg-value");
+
+            var command = new Command("outer", "outer command help")
+                {
+                    new Argument<string>
+                    {
+                        Name = "outer-args"
+                    },
+                    new Command("inner", $"inner command help")
+                    {
+                        argument,
+                        otherArgumentHidden,
+                        new Argument<string>
+                        {
+                            Name = "inner-other-arg-no-default"
+                        }
+                    }
+                };
+
+            HelpBuilder helpBuilder = GetHelpBuilder(LargeMaxWidth);
+
+            helpBuilder.Write(command);
+
+            var help = _console.Out.ToString();
+
+            help.Should().Contain($"[default: the-arg-value]");
+        }
+
+        [Fact]
+        public void Help_describes_default_values_for_subcommand_with_multiple_defaultable_arguments ()
+        {
+            var argument = new Argument
+            {
+                Name = "the-arg",
+            };
+            var otherArgument = new Argument
+            {
+                Name = "the-other-arg"
+            };
+            argument.SetDefaultValue("the-arg-value");
+            otherArgument.SetDefaultValue("the-other-arg-value");
+
+            var command = new Command("outer", "outer command help")
+                {
+                    new Argument<string>
+                    {
+                        Name = "outer-args"
+                    },
+                    new Command("inner", $"inner command help")
+                    {
+                        argument, otherArgument
+                    }
+                };
+
+            HelpBuilder helpBuilder = GetHelpBuilder(LargeMaxWidth);
+
+            helpBuilder.Write(command);
+
+            var help = _console.Out.ToString();
+
+            help.Should().Contain($"[the-arg: the-arg-value, the-other-arg: the-other-arg-value]");
+        }
+
         private class CustomHelpBuilderThatAddsTextAfterDefaultText : HelpBuilder
         {
             private readonly string _theTextToAdd;

src/System.CommandLine.Tests/System.CommandLine.Tests.csproj

@@ -19,9 +19,10 @@
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="ApprovalTests" Version="5.2.2" />
     <PackageReference Include="FluentAssertions" Version="5.10.3" />
     <PackageReference Include="Newtonsoft.Json" Version="10.0.3" />
-	<PackageReference Include="Microsoft.DotNet.PlatformAbstractions" Version="2.1.0" />
+    <PackageReference Include="Microsoft.DotNet.PlatformAbstractions" Version="2.1.0" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(DisableArcade)' == '1'">