Argument.AcceptOnlyFromAmong should return void

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

@@ -23,7 +23,7 @@ System.CommandLine
     public System.Type ValueType { get; }
     public Argument<T> AcceptLegalFileNamesOnly()
     public Argument<T> AcceptLegalFilePathsOnly()
-    public Argument<T> AcceptOnlyFromAmong(System.String[] values)
+    public System.Void AcceptOnlyFromAmong(System.String[] values)
     public System.Void SetDefaultValue(T value)
     public System.Void SetDefaultValueFactory(Func<T> defaultValueFactory)
     public System.Void SetDefaultValueFactory(Func<System.CommandLine.Parsing.ArgumentResult,T> defaultValueFactory)

src/System.CommandLine.Tests/ArgumentTests.cs

@@ -769,8 +769,9 @@ 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>()
-                .AcceptOnlyFromAmong(ConsoleColor.Red.ToString(), ConsoleColor.Green.ToString());
+            var argument = new Argument<ConsoleColor>();
+            argument.AcceptOnlyFromAmong(ConsoleColor.Red.ToString(), ConsoleColor.Green.ToString());
+
             Command command = new("set-color")
             {
                 argument
@@ -788,7 +789,6 @@ public void Argument_of_enum_can_limit_enum_members_as_valid_values()
         public void Argument_of_T_fluent_APIs_return_Argument_of_T()
         {
             Argument<string> argument = new Argument<string>("--path")
-                .AcceptOnlyFromAmong("text")
                 .AcceptLegalFileNamesOnly()
                 .AcceptLegalFilePathsOnly();
 

src/System.CommandLine.Tests/CompletionTests.cs

@@ -698,15 +698,15 @@ public void When_parsing_from_text_then_argument_completions_are_based_on_the_pr
             {
                 new Command("one")
                 {
-                    new Argument<string>().AcceptOnlyFromAmong("one-a", "one-b", "one-c")
+                    CreateArgumentWithAcceptOnlyFromAmong("one-a", "one-b", "one-c")
                 },
                 new Command("two")
                 {
-                    new Argument<string>().AcceptOnlyFromAmong("two-a", "two-b", "two-c")
+                    CreateArgumentWithAcceptOnlyFromAmong("two-a", "two-b", "two-c")
                 },
                 new Command("three")
                 {
-                    new Argument<string>().AcceptOnlyFromAmong("three-a", "three-b", "three-c")
+                    CreateArgumentWithAcceptOnlyFromAmong("three-a", "three-b", "three-c")
                 }
             };
 
@@ -725,15 +725,15 @@ public void When_parsing_from_array_then_argument_completions_are_based_on_the_p
             {
                 new Command("one")
                 {
-                    new Argument<string>().AcceptOnlyFromAmong("one-a", "one-b", "one-c")
+                    CreateArgumentWithAcceptOnlyFromAmong("one-a", "one-b", "one-c")
                 },
                 new Command("two")
                 {
-                    new Argument<string>().AcceptOnlyFromAmong("two-a", "two-b", "two-c")
+                    CreateArgumentWithAcceptOnlyFromAmong("two-a", "two-b", "two-c")
                 },
                 new Command("three")
                 {
-                    new Argument<string>().AcceptOnlyFromAmong("three-a", "three-b", "three-c")
+                    CreateArgumentWithAcceptOnlyFromAmong("three-a", "three-b", "three-c")
                 }
             };
 
@@ -968,5 +968,12 @@ public void When_option_completions_are_available_then_they_are_suggested_when_a
                   .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");
         }
+
+        private Argument<string> CreateArgumentWithAcceptOnlyFromAmong(params string[] values)
+        {
+            Argument<string> argument = new();
+            argument.AcceptOnlyFromAmong(values);
+            return argument;
+        }
     }
 }

src/System.CommandLine.Tests/ParsingValidationTests.cs

@@ -71,12 +71,14 @@ public void When_FromAmong_is_used_then_the_OptionResult_ErrorMessage_is_set()
         [Fact] // https://github.com/dotnet/command-line-api/issues/1475
         public void When_FromAmong_is_used_then_the_ArgumentResult_ErrorMessage_is_set()
         {
-            var option = new Argument<string>().AcceptOnlyFromAmong("a", "b");
-            var command = new Command("test") { option };
+            var argument = new Argument<string>();
+            argument.AcceptOnlyFromAmong("a", "b");
+
+            var command = new Command("test") { argument };
 
             var parseResult = command.Parse("test c");
 
-            parseResult.FindResultFor(option)
+            parseResult.FindResultFor(argument)
                        .ErrorMessage
                        .Should()
                        .Be(parseResult.Errors.Single().Message)
@@ -90,8 +92,8 @@ public void When_FromAmong_is_used_for_multiple_arguments_and_valid_input_is_pro
         {
             var command = new Command("set")
             {
-                new Argument<string>("key").AcceptOnlyFromAmong("key1", "key2"),
-                new Argument<string>("value").AcceptOnlyFromAmong("value1", "value2")
+                CreateArgumentWithAcceptOnlyFromAmong(name: "key", "key1", "key2"),
+                CreateArgumentWithAcceptOnlyFromAmong(name : "value", "value1", "value2")
             };
 
             var result = command.Parse("set key1 value1");
@@ -104,8 +106,8 @@ public void When_FromAmong_is_used_for_multiple_arguments_and_invalid_input_is_p
         {
             var command = new Command("set")
             {
-                new Argument<string>("key").AcceptOnlyFromAmong("key1", "key2"),
-                new Argument<string>("value").AcceptOnlyFromAmong("value1", "value2")
+                CreateArgumentWithAcceptOnlyFromAmong(name : "key", "key1", "key2"),
+                CreateArgumentWithAcceptOnlyFromAmong(name : "value", "value1", "value2")
             };
 
             var result = command.Parse("set not-key1 value1");
@@ -152,8 +154,8 @@ public void When_FromAmong_is_used_for_multiple_arguments_and_invalid_input_is_p
         {
             var command = new Command("set")
             {
-                new Argument<string>("key").AcceptOnlyFromAmong("key1", "key2"),
-                new Argument<string>("value").AcceptOnlyFromAmong("value1", "value2")
+                CreateArgumentWithAcceptOnlyFromAmong(name : "key", "key1", "key2"),
+                CreateArgumentWithAcceptOnlyFromAmong(name : "value", "value1", "value2")
             };
 
             var result = command.Parse("set key1 not-value1");
@@ -1233,5 +1235,12 @@ internal void When_there_is_an_arity_error_then_further_errors_are_not_reported(
                        .Should()
                        .Be("Required argument missing for option: '-o'.");
         }
+
+        private Argument<string> CreateArgumentWithAcceptOnlyFromAmong(string name, params string[] values)
+        {
+            Argument<string> argument = new(name);
+            argument.AcceptOnlyFromAmong(values);
+            return argument;
+        }
     }
 }

src/System.CommandLine/Argument{T}.cs

@@ -174,8 +174,7 @@ public void SetDefaultValueFactory(Func<ArgumentResult, T> defaultValueFactory)
         /// Configures the argument to accept only the specified values, and to suggest them as command line completions.
         /// </summary>
         /// <param name="values">The values that are allowed for the argument.</param>
-        /// <returns>The configured argument.</returns>
-        public Argument<T> AcceptOnlyFromAmong(params string[] values)
+        public void AcceptOnlyFromAmong(params string[] values)
         {
             if (values is not null && values.Length > 0)
             {
@@ -185,8 +184,6 @@ public Argument<T> AcceptOnlyFromAmong(params string[] values)
                 CompletionSources.Add(values);
             }
 
-            return this;
-
             void UnrecognizedArgumentError(ArgumentResult argumentResult)
             {
                 for (var i = 0; i < argumentResult.Tokens.Count; i++)