Adding proposed fix for FromAmong

The behavior of the method now matches its description.

Author: Keboo

src/System.CommandLine.Tests/ArgumentTests.cs

@@ -765,6 +765,24 @@ public void OnlyTake_can_pass_on_all_tokens_from_a_single_arity_argument_to_anot
             }
         }
 
+        [Fact]
+        public void Argument_of_enum_can_limit_enum_members_as_valid_values()
+        {
+            var argument = new Argument<ConsoleColor>()
+                .FromAmong(ConsoleColor.Red.ToString(), ConsoleColor.Green.ToString());
+            Command command = new("set-color")
+            {
+                argument
+            };
+
+            var result = command.Parse("set-color Fuschia");
+
+            result.Errors
+                .Select(e => e.Message)
+                .Should()
+                .BeEquivalentTo(new[] { $"Argument 'Fuschia' not recognized. Must be one of:\n\t'Red'\n\t'Green'" });
+        }
+
         protected override Symbol CreateSymbol(string name)
         {
             return new Argument<string>(name);

src/System.CommandLine.Tests/OptionTests.cs

@@ -1,9 +1,9 @@
 // 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 FluentAssertions;
 using System.CommandLine.Parsing;
 using System.Linq;
-using FluentAssertions;
 using Xunit;
 
 namespace System.CommandLine.Tests
@@ -370,6 +370,20 @@ public void Option_of_boolean_defaults_to_false_when_not_specified()
                 .Should()
                 .BeFalse();
         }
+
+        [Fact]
+        public void Option_of_enum_can_limit_enum_members_as_valid_values()
+        {
+            var option = new Option<ConsoleColor>("--color")
+                .FromAmong(ConsoleColor.Red.ToString(), ConsoleColor.Green.ToString());
+
+            var result = option.Parse("--color Fuschia");
+
+            result.Errors
+                .Select(e => e.Message)
+                .Should()
+                .BeEquivalentTo(new[] { $"Argument 'Fuschia' not recognized. Must be one of:\n\t'Red'\n\t'Green'" });
+        }
 
         protected override Symbol CreateSymbol(string name) => new Option<string>(name);
     }

src/System.CommandLine/ArgumentExtensions.cs

@@ -76,7 +76,9 @@ public static TArgument FromAmong<TArgument>(
             params string[] values)
             where TArgument : Argument
         {
+            argument.AllowedValues?.Clear();
             argument.AddAllowedValues(values);
+            argument.Completions.Clear();
             argument.Completions.Add(values);
 
             return argument;

src/System.CommandLine/OptionExtensions.cs

@@ -25,7 +25,9 @@ public static TOption FromAmong<TOption>(
             params string[] values)
             where TOption : Option
         {
+            option.Argument.AllowedValues?.Clear();
             option.Argument.AddAllowedValues(values);
+            option.Argument.Completions.Clear();
             option.Argument.Completions.Add(values);
 
             return option;