remove CompletionSourceList, expose a ICollection<ICompletionSource> …

…instead:

- it has Add and Clear
- it has no indexer, but we don't need it

fixes #1911

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

@@ -1,7 +1,7 @@
 System.CommandLine
   public abstract class Argument : Symbol, System.CommandLine.Binding.IValueDescriptor, System.CommandLine.Completions.ICompletionSource
     public ArgumentArity Arity { get; set; }
-    public CompletionSourceList Completions { get; }
+    public System.Collections.Generic.ICollection<System.CommandLine.Completions.ICompletionSource> Completions { get; }
     public System.Boolean HasDefaultValue { get; }
     public System.String HelpName { get; set; }
     public System.Type ValueType { get; }
@@ -109,16 +109,9 @@ System.CommandLine
     .ctor()
     .ctor(System.String message, System.Exception innerException)
   public static class CompletionSourceExtensions
-    public static System.Void Add(this CompletionSourceList completionSources, System.Func<System.CommandLine.Completions.CompletionContext,System.Collections.Generic.IEnumerable<System.String>> complete)
-    public static System.Void Add(this CompletionSourceList completionSources, System.CommandLine.Completions.CompletionDelegate complete)
-    public static System.Void Add(this CompletionSourceList completionSources, System.String[] completions)
-  public class CompletionSourceList, System.Collections.Generic.IEnumerable<System.CommandLine.Completions.ICompletionSource>, System.Collections.Generic.IReadOnlyCollection<System.CommandLine.Completions.ICompletionSource>, System.Collections.Generic.IReadOnlyList<System.CommandLine.Completions.ICompletionSource>, System.Collections.IEnumerable
-    .ctor()
-    public System.Int32 Count { get; }
-    public System.CommandLine.Completions.ICompletionSource Item { get; }
-    public System.Void Add(System.CommandLine.Completions.ICompletionSource source)
-    public System.Void Clear()
-    public System.Collections.Generic.IEnumerator<System.CommandLine.Completions.ICompletionSource> GetEnumerator()
+    public static System.Void Add(this System.Collections.Generic.ICollection<System.CommandLine.Completions.ICompletionSource> completionSources, System.Func<System.CommandLine.Completions.CompletionContext,System.Collections.Generic.IEnumerable<System.String>> complete)
+    public static System.Void Add(this System.Collections.Generic.ICollection<System.CommandLine.Completions.ICompletionSource> completionSources, System.CommandLine.Completions.CompletionDelegate complete)
+    public static System.Void Add(this System.Collections.Generic.ICollection<System.CommandLine.Completions.ICompletionSource> completionSources, System.String[] completions)
   public static class ConsoleExtensions
     public static System.Void Write(this IConsole console, System.String value)
     public static System.Void WriteLine(this IConsole console, System.String value)

src/System.CommandLine/Argument.cs

@@ -17,7 +17,7 @@ public abstract class Argument : Symbol, IValueDescriptor
         private Func<ArgumentResult, object?>? _defaultValueFactory;
         private ArgumentArity _arity;
         private TryConvertArgument? _convertArguments;
-        private CompletionSourceList? _completions = null;
+        private List<ICompletionSource>? _completions = null;
         private List<ValidateSymbolResult<ArgumentResult>>? _validators = null;
 
         /// <summary>
@@ -72,10 +72,10 @@ internal TryConvertArgument? ConvertArguments
         }
 
         /// <summary>
-        /// Gets the list of completion sources for the argument.
+        /// Gets the collection of completion sources for the argument.
         /// </summary>
-        public CompletionSourceList Completions =>
-            _completions ??= new CompletionSourceList
+        public ICollection<ICompletionSource> Completions =>
+            _completions ??= new ()
             {
                 CompletionSource.ForType(ValueType)
             };

src/System.CommandLine/CompletionSourceExtensions.cs

@@ -18,7 +18,7 @@ public static class CompletionSourceExtensions
         /// <param name="completionSources">The list of completion sources to add to.</param>
         /// <param name="complete">The delegate to be called when calculating completions.</param>
         public static void Add(
-            this CompletionSourceList completionSources,
+            this ICollection<ICompletionSource> completionSources,
             Func<CompletionContext, IEnumerable<string>> complete)
         {
             if (completionSources is null)
@@ -40,7 +40,7 @@ public static void Add(
         /// <param name="completionSources">The list of completion sources to add to.</param>
         /// <param name="complete">The delegate to be called when calculating completions.</param>
         public static void Add(
-            this CompletionSourceList completionSources,
+            this ICollection<ICompletionSource> completionSources,
             CompletionDelegate complete)
         {
             if (completionSources is null)
@@ -62,7 +62,7 @@ public static void Add(
         /// <param name="completionSources">The list of completion sources to add to.</param>
         /// <param name="completions">A list of strings to be suggested for command line completions.</param>
         public static void Add(
-            this CompletionSourceList completionSources,
+            this ICollection<ICompletionSource> completionSources,
             params string[] completions)
         {
             if (completionSources is null)