Refuse an option alias that a standard switch or another option already owns

TrayApp.Test/TrayAppBuilderTests.cs

@@ -31,6 +31,47 @@
 		Assert.AreEqual("demo", app.DisplayName);
 	}
 
+	[TestMethod]
+	[DataRow("-s")]
+	[DataRow("--status")]
+	[DataRow("-f")]
+	[DataRow("--for")]
+	[DataRow("-t")]
+	[DataRow("--no-tray")]
+	[DataRow("-h")]
+	[DataRow("-?")]
+	[DataRow("-v")]
+	public void Flag_WithAStandardSwitchAlias_Throws(string alias)
+	{
+		// The standard switches are read first, so an option spelled the same way could never be reached.
+		ArgumentException ex = Assert.ThrowsExactly<ArgumentException>(
+			() => TrayAppBuilder.Create("demo").Flag(["--quiet", alias], "Quiet.", () => { }));
+
+		Assert.Contains($"'{alias}'", ex.Message, StringComparison.Ordinal);
+		Assert.AreEqual("aliases", ex.ParamName);
+	}
+
+	[TestMethod]
+	public void Option_WithAStandardSwitchAlias_Throws()
+	{
+		ArgumentException ex = Assert.ThrowsExactly<ArgumentException>(
+			() => TrayAppBuilder.Create("demo").Option(["-f", "--file"], "path", "A file.", _ => { }));
+
+		Assert.Contains("'-f'", ex.Message, StringComparison.Ordinal);
+	}
+
+	[TestMethod]
+	public void Flag_WithAnAliasAnotherOptionTook_Throws()
+	{
+		TrayAppBuilder builder = TrayAppBuilder.Create("demo").Option(["-d", "--delay"], "time", "Delay.", _ => { });
+
+		ArgumentException ex = Assert.ThrowsExactly<ArgumentException>(
+			() => builder.Flag(["-d", "--display"], "Display.", () => { }));
+
+		Assert.Contains("'-d'", ex.Message, StringComparison.Ordinal);
+		Assert.AreEqual(1, builder.Build().Options.Count);
+	}
+
 	[TestMethod]
 	public void Build_CollectsItemsTogglesAndOptions()
 	{

TrayApp/Cli/CommandLineParser.cs

@@ -31,6 +31,13 @@ private enum ReadResult
 		Failed,
 	}
 
+	/// <summary>
+	/// Gets the spellings of the standard switches, which an application option may not reuse: they are read
+	/// first, so an application option spelled the same way could never be reached.
+	/// </summary>
+	internal static IReadOnlyList<string> StandardAliases { get; } =
+		["-t", "--tray", "--no-tray", "-s", "--status", "-h", "--help", "-?", "-v", "--version", "-f", "--for"];
+
 	/// <summary>
 	/// Parses the arguments.
 	/// </summary>

TrayApp/TrayAppBuilder.cs

@@ -5,6 +5,7 @@ namespace ktsu.TrayApp;
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
+using System.Linq;
 using System.Reflection;
 using System.Threading.Tasks;
 using Avalonia;
@@ -247,9 +248,12 @@ public TrayAppBuilder Separator()
 	/// <param name="onSet">Called once when the switch is present.</param>
 	/// <returns>The builder, for chaining.</returns>
 	/// <exception cref="ArgumentNullException">Any argument is <see langword="null"/>.</exception>
+	/// <exception cref="ArgumentException">
+	/// An alias is one of the standard switches, or is already taken by another option.
+	/// </exception>
 	public TrayAppBuilder Flag(IReadOnlyList<string> aliases, string description, Action onSet)
 	{
-		appOptions.Add(AppOption.Flag(aliases, description, onSet));
+		AddOption(AppOption.Flag(aliases, description, onSet), nameof(aliases));
 		return this;
 	}
 
@@ -265,9 +269,12 @@ public TrayAppBuilder Flag(IReadOnlyList<string> aliases, string description, Ac
 	/// </param>
 	/// <returns>The builder, for chaining.</returns>
 	/// <exception cref="ArgumentNullException">Any argument is <see langword="null"/>.</exception>
+	/// <exception cref="ArgumentException">
+	/// An alias is one of the standard switches, or is already taken by another option.
+	/// </exception>
 	public TrayAppBuilder Option(IReadOnlyList<string> aliases, string valueName, string description, Action<string> onValue)
 	{
-		appOptions.Add(AppOption.Value(aliases, valueName, description, onValue));
+		AddOption(AppOption.Value(aliases, valueName, description, onValue), nameof(aliases));
 		return this;
 	}
 
@@ -660,4 +667,26 @@ internal static int RecoverFromTrayFailure(
 			return first.IsChecked;
 		};
 	}
+
+	/// <summary>
+	/// Registers an application option, refusing an alias that could never be reached: the standard switches
+	/// are read before any application option, and the first option to claim an alias wins it.
+	/// </summary>
+	private void AddOption(AppOption option, string paramName)
+	{
+		foreach (string alias in option.Aliases)
+		{
+			if (CommandLineParser.StandardAliases.Contains(alias, StringComparer.Ordinal))
+			{
+				throw new ArgumentException($"'{alias}' is a standard switch and cannot be reused by an application option.", paramName);
+			}
+
+			if (appOptions.Any(existing => existing.Matches(alias)))
+			{
+				throw new ArgumentException($"'{alias}' is already taken by another option.", paramName);
+			}
+		}
+
+		appOptions.Add(option);
+	}
 }