Sort suggestions by match index, then alphabetically

src/System.CommandLine.Tests/SuggestionTests.cs

@@ -5,6 +5,7 @@
 using System.CommandLine.Builder;
 using System.CommandLine.Invocation;
 using System.CommandLine.Parsing;
+using System.CommandLine.Tests.Utility;
 using System.IO;
 using FluentAssertions;
 using Xunit;
@@ -112,6 +113,36 @@ public void Command_Suggest_returns_available_subcommands_and_option_aliases_and
                        .BeEquivalentTo("subcommand", "--option", "command-argument");
         }
 
+        [Fact]
+        public void Command_Suggest_without_text_to_match_orders_alphabetically()
+        {
+            var command = new Command("command")
+            {
+                new Command("andmythirdsubcommand"),
+                new Command("mysubcommand"),
+                new Command("andmyothersubcommand"),
+            };
+
+            var suggestions = command.GetSuggestions();
+
+            suggestions.Should().BeEquivalentSequenceTo("andmyothersubcommand", "andmythirdsubcommand", "mysubcommand");
+        }
+
+        [Fact]
+        public void Command_Suggest_with_text_to_match_orders_by_match_position_then_alphabetically()
+        {
+            var command = new Command("command")
+            {
+                new Command("andmythirdsubcommand"),
+                new Command("mysubcommand"),
+                new Command("andmyothersubcommand"),
+            };
+
+            var suggestions = command.GetSuggestions("my");
+
+            suggestions.Should().BeEquivalentSequenceTo("mysubcommand", "andmyothersubcommand", "andmythirdsubcommand");
+        }
+
         [Fact]
         public void When_an_option_has_a_default_value_it_will_still_be_suggested()
         {

src/System.CommandLine/Parsing/StringExtensions.cs

@@ -15,13 +15,18 @@ public static class StringExtensions
         private static readonly string[] _optionPrefixStrings = { "--", "-", "/" };
 
         internal static bool ContainsCaseInsensitive(
+            this string source,
+            string value) =>
+            source.IndexOfCaseInsensitive(value) >= 0;
+
+        internal static int IndexOfCaseInsensitive(
             this string source,
             string value) =>
             CultureInfo.InvariantCulture
                        .CompareInfo
                        .IndexOf(source,
                                 value ?? "",
-                                CompareOptions.OrdinalIgnoreCase) >= 0;
+                                CompareOptions.OrdinalIgnoreCase);
 
         internal static string RemovePrefix(this string rawAlias)
         {

src/System.CommandLine/Symbol.cs

@@ -146,8 +146,9 @@ public virtual IEnumerable<string> GetSuggestions(string textToMatch = null)
             return this.ChildSymbolAliases()
                        .Concat(argumentSuggestions)
                        .Distinct()
-                       .OrderBy(symbol => symbol, StringComparer.OrdinalIgnoreCase)
-                       .Containing(textToMatch);
+                       .Containing(textToMatch)
+                       .OrderBy(symbol => symbol.IndexOfCaseInsensitive(textToMatch))
+                       .ThenBy(symbol => symbol, StringComparer.OrdinalIgnoreCase);
         }
 
         public override string ToString() => $"{GetType().Name}: {Name}";