fix #1629

Author: jonsequitur

src/System.CommandLine.Tests/Binding/TypeConversionTests.cs

@@ -864,7 +864,10 @@ public void Enum_values_that_cannot_be_parsed_result_in_an_informative_error()
             var value = option.Parse("-x Notaday");
 
             value.Errors
-                 .Select(e => e.Message)
+                 .Should()
+                 .ContainSingle()
+                 .Which
+                 .Message
                  .Should()
                  .Contain("Cannot parse argument 'Notaday' for option '-x' as expected type 'System.DayOfWeek'.");
         }

src/System.CommandLine.Tests/CompletionTests.cs

@@ -1,7 +1,6 @@
 // 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.Collections.Generic;
 using System.CommandLine.Builder;
 using System.CommandLine.Parsing;
 using System.CommandLine.Tests.Utility;
@@ -10,6 +9,7 @@
 using FluentAssertions;
 using Xunit;
 using Xunit.Abstractions;
+using static System.Environment;
 
 namespace System.CommandLine.Tests
 {
@@ -974,5 +974,22 @@ public void Completions_for_subcommands_provide_a_description()
                        .Should()
                        .Be(description);
         }
+
+        [Fact] // https://github.com/dotnet/command-line-api/issues/1629
+        public void When_option_completions_are_available_then_they_are_suggested_when_a_validation_error_occurs()
+        {
+            var option = new Option<DayOfWeek>("--day");
+
+            var result = option.Parse("--day SleepyDay");
+
+            result.Errors
+                  .Should()
+                  .ContainSingle()
+                  .Which
+                  .Message
+                  .Should()
+                  .Be(
+                      $"Cannot parse argument 'SleepyDay' for option '--day' as expected type 'System.DayOfWeek'. Did you mean one of the following?{NewLine}Friday{NewLine}Monday{NewLine}Saturday{NewLine}Sunday{NewLine}Thursday{NewLine}Tuesday{NewLine}Wednesday");
+        }
     }
 }

src/System.CommandLine/Parsing/ParseResultVisitor.cs

@@ -483,22 +483,31 @@ private void ValidateAndConvertOptionResult(OptionResult optionResult)
 
         private void ValidateAndConvertArgumentResult(ArgumentResult argumentResult)
         {
-            if (argumentResult.Argument is { } argument)
-            {
-                var parseError =
-                    argumentResult.Parent?.UnrecognizedArgumentError(argument) ??
-                    argumentResult.CustomError(argument);
+            var argument = argumentResult.Argument;
 
-                if (parseError is { })
-                {
-                    AddErrorToResult(argumentResult, parseError);
-                    return;
-                }
+            var parseError =
+                argumentResult.Parent?.UnrecognizedArgumentError(argument) ??
+                argumentResult.CustomError(argument);
+
+            if (parseError is { })
+            {
+                AddErrorToResult(argumentResult, parseError);
+                return;
             }
 
-            if (argumentResult.GetArgumentConversionResult() is FailedArgumentConversionResult failed 
+            if (argumentResult.GetArgumentConversionResult() is FailedArgumentConversionResult failed
                 and not FailedArgumentConversionArityResult)
             {
+                if (argument.Parents.FirstOrDefault() is Option option)
+                {
+                    var completions = option.GetCompletions().ToArray();
+
+                    if (completions.Length > 0)
+                    {
+                        failed.ErrorMessage += " Did you mean one of the following?" + Environment.NewLine + string.Join(Environment.NewLine, completions.Select(c => c.Label));
+                    }
+                }
+
                 AddErrorToResult(argumentResult, new ParseError(failed.ErrorMessage!, argumentResult));
             }
         }