fix #817

Author: jonsequitur

src/System.CommandLine.Tests/ParsingValidationTests.cs

@@ -702,6 +702,22 @@ public void A_command_with_subcommands_is_invalid_to_invoke_if_it_has_no_handler
                            e.SymbolResult.Symbol.Name.Equals("inner"));
         }
 
+        [Fact]
+        public void A_root_command_is_invalid_if_it_has_no_handler()
+        {
+            var rootCommand = new RootCommand();
+            var inner = new Command("inner");
+            rootCommand.Add(inner);
+
+            var result = rootCommand.Parse("");
+
+            result.Errors
+                  .Should()
+                  .ContainSingle(
+                      e => e.Message.Equals(ValidationMessages.Instance.RequiredCommandWasNotProvided()) &&
+                           e.SymbolResult.Symbol == rootCommand);
+        }
+
         [Fact]
         public void A_command_with_subcommands_is_valid_to_invoke_if_it_has_a_handler()
         {

src/System.CommandLine.Tests/UseParseErrorReportingTests.cs

@@ -0,0 +1,38 @@
+// 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.
+
+// 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 System.CommandLine.Builder;
+using System.CommandLine.Parsing;
+using FluentAssertions;
+using Xunit;
+
+namespace System.CommandLine.Tests
+{
+    public class UseParseErrorReportingTests
+    {
+        [Fact] // https://github.com/dotnet/command-line-api/issues/817
+        public void Parse_error_reporting_reports_error_when_help_is_used_and_required_subcommand_is_missing()
+        {
+            var root = new RootCommand
+            {
+                new Command("inner")
+            };
+
+            var parser = new CommandLineBuilder(root)
+                         .UseParseErrorReporting()
+                         .UseHelp()
+                         .Build();
+
+            var parseResult = parser.Parse("");
+
+            parseResult.Errors.Should().NotBeEmpty();
+
+            var result = parser.Invoke("");
+
+            result.Should().Be(1);
+        }
+    }
+}

src/System.CommandLine/Parsing/ParseResultVisitor.cs

@@ -226,10 +226,20 @@ private void ValidateCommandResult()
 
         private void ValidateCommandHandler()
         {
-            if (_innermostCommandResult.Command is Command cmd &&
-                cmd.Handler == null &&
-                cmd.Children.OfType<ICommand>().Any() && 
-                !cmd.Options.OfType<HelpOption>().Any())
+            if (!(_innermostCommandResult.Command is Command cmd) || 
+                cmd.Handler != null)
+            {
+                return;
+            }
+
+            if (!cmd.Children.OfType<ICommand>().Any())
+            {
+                return;
+            }
+
+            if (!_innermostCommandResult
+                 .Children
+                 .Select(o => o.Symbol is HelpOption).Any())
             {
                 _errors.Insert(0,
                                new ParseError(