Port the fix as it was lost during the merge and fixup the tests.

Author: AlexGhiondea

src/Parser.HelpColors.cs

@@ -19,7 +19,6 @@ public static class Colors
             private static IColors s_helpColors;
             private static object s_lockObj = new object();
 
-
             public static IColors Get(ConsoleColor backgroundColor = (ConsoleColor)(-1))
             {
                 if (s_helpColors != null)

src/Parser.Internal.cs

@@ -192,7 +192,7 @@ private static void SplitCommandLineIntoSegments(string line, ref List<string> n
                 var optionalParamName = args[offsetInArray + currentLogicalPosition].Substring(1);
                 if (!TypeArgumentInfo.OptionalArguments.TryGetValue(optionalParamName, out optionalProp))
                 {
-                    throw new ArgumentException($"Could not find argument {args[currentLogicalPosition]}");
+                    throw new ArgumentException($"Could not find argument {args[offsetInArray + currentLogicalPosition]}");
                 }
 
                 // skip over the parameter name

test/CommandLineTests.Help.cs

@@ -801,11 +801,12 @@ public void EnsureThatRightParameterIsReportedForGroups()
 
             Validate(_printer,
                 new TextAndColor(ConsoleColor.Red, "Error"),
-                new TextAndColor(ConsoleColor.Black, $": Could not find argument -opt1=value {Environment.NewLine}"),
+                new TextAndColor(ConsoleColor.Black, $": Could not find argument -opt1=value {Environment.NewLine}"), 
                 new TextAndColor(ConsoleColor.Black, "Usage: "),
                 new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.White, "testhost.exe"),
                 new TextAndColor(ConsoleColor.Black, " "),
+                new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.Green, "Command1"),
                 new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.Cyan, "p1"),
@@ -816,6 +817,7 @@ public void EnsureThatRightParameterIsReportedForGroups()
                 new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.White, "testhost.exe"),
                 new TextAndColor(ConsoleColor.Black, " "),
+                new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.Green, "Command2"),
                 new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.Cyan, "opt1"),
@@ -846,6 +848,7 @@ public void ErrorWhenNoRequiredParametersInGroupSpecified()
                 new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.White, "testhost.exe"),
                 new TextAndColor(ConsoleColor.Black, " "),
+                new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.Green, "Command1"),
                 new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.Cyan, "p1"),
@@ -856,6 +859,7 @@ public void ErrorWhenNoRequiredParametersInGroupSpecified()
                 new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.White, "testhost.exe"),
                 new TextAndColor(ConsoleColor.Black, " "),
+                new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.Green, "Command2"),
                 new TextAndColor(ConsoleColor.Black, " "),
                 new TextAndColor(ConsoleColor.Cyan, "opt1"),