remove TokenCompletionContext, make CompletionContext non-abstract (#2126)

Author: adamsitnik

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

@@ -158,7 +158,7 @@ System.CommandLine
     public System.Boolean Equals(System.Object obj)
     public System.Int32 GetHashCode()
 System.CommandLine.Completions
-  public abstract class CompletionContext
+  public class CompletionContext
     public static CompletionContext Empty { get; }
     public System.CommandLine.ParseResult ParseResult { get; }
     public System.String WordToComplete { get; }
@@ -178,7 +178,6 @@ System.CommandLine.Completions
     public System.String CommandLineText { get; }
     public System.Int32 CursorPosition { get; }
     public TextCompletionContext AtCursorPosition(System.Int32 position)
-  public class TokenCompletionContext : CompletionContext
 System.CommandLine.Help
   public class HelpAction : System.CommandLine.CliAction
     .ctor()

src/System.CommandLine.Tests/CompletionContextTests.cs

@@ -68,7 +68,7 @@ public void CommandLineText_is_unavailable_when_string_array_is_parsed()
 
             parseResult.GetCompletionContext()
                        .Should()
-                       .BeOfType<TokenCompletionContext>();
+                       .BeOfType<CompletionContext>();
         }
 
         [Fact]

src/System.CommandLine/Completions/CompletionContext.cs

@@ -9,10 +9,14 @@ namespace System.CommandLine.Completions
     /// <summary>
     /// Supports command line completion operations.
     /// </summary>
-    public abstract class CompletionContext
+    public class CompletionContext
     {
         private static CompletionContext? _empty;
 
+        internal CompletionContext(ParseResult parseResult) : this(parseResult, GetWordToComplete(parseResult))
+        {
+        }
+
         internal CompletionContext(ParseResult parseResult, string wordToComplete)
         {
             ParseResult = parseResult;
@@ -29,7 +33,7 @@ internal CompletionContext(ParseResult parseResult, string wordToComplete)
         /// Gets an empty CompletionContext.
         /// </summary>
         /// <remarks>Can be used for testing purposes.</remarks>
-        public static CompletionContext Empty => _empty ??= new TokenCompletionContext(ParseResult.Empty());
+        public static CompletionContext Empty => _empty ??= new CompletionContext(ParseResult.Empty());
 
         internal bool IsEmpty => ReferenceEquals(this, _empty);
 

src/System.CommandLine/Completions/TokenCompletionContext.cs

@@ -103,7 +103,7 @@ public string[] UnmatchedTokens
         public CompletionContext GetCompletionContext() =>
             _completionContext ??=
                 CommandLineText is null
-                    ? new TokenCompletionContext(this)
+                    ? new CompletionContext(this)
                     : new TextCompletionContext(this, CommandLineText);
 
         /// <summary>