cleanup: fix compiler errors with latest version of C# 8 compiler

Author: natemcmaster

src/CommandLineUtils/IO/IConsole.cs

@@ -15,7 +15,7 @@ public interface IConsole
         /// <summary>
         /// Raised when Ctrl+C is pressed.
         /// </summary>
-        event ConsoleCancelEventHandler CancelKeyPress;
+        event ConsoleCancelEventHandler? CancelKeyPress;
 
         /// <summary>
         /// stdout

src/CommandLineUtils/IO/NullConsole.cs

@@ -61,7 +61,7 @@ private NullConsole()
         /// <summary>
         /// This event never fires.
         /// </summary>
-        public event ConsoleCancelEventHandler CancelKeyPress
+        public event ConsoleCancelEventHandler? CancelKeyPress
         {
             add { }
             remove { }

src/CommandLineUtils/IO/PhysicalConsole.cs

@@ -26,7 +26,7 @@ public class PhysicalConsole : IConsole
         /// <summary>
         /// <see cref="Console.CancelKeyPress"/>.
         /// </summary>
-        public event ConsoleCancelEventHandler CancelKeyPress
+        public event ConsoleCancelEventHandler? CancelKeyPress
         {
             add => Console.CancelKeyPress += value;
             remove => Console.CancelKeyPress -= value;

src/CommandLineUtils/McMaster.Extensions.CommandLineUtils.csproj

@@ -25,6 +25,7 @@ McMaster.Extensions.CommandLineUtils.ArgumentEscaper
  A community-maintained fork of Microsoft.Extensions.CommandLineUtils, plus many enhancements.
     </PackageDescription>
     <PackageTags>commandline;parsing</PackageTags>
+    <WarningsNotAsErrors>8602;8603;8604;$(WarningsNotAsErrors)</WarningsNotAsErrors>
   </PropertyGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">

test/CommandLineUtils.Tests/OptionAttributeTests.cs

@@ -203,7 +203,7 @@ public int Value
             public static int StaticNumber { get; private set; }
 
             [Option("--static-string")]
-            public static string StaticString { get; }
+            public static string? StaticString { get; }
 
             [Option("--static-value")]
             public static int StaticValue

test/CommandLineUtils.Tests/Utilities/TestConsole.cs

@@ -30,7 +30,7 @@ public TestConsole(ITestOutputHelper output)
         public ConsoleColor ForegroundColor { get; set; }
         public ConsoleColor BackgroundColor { get; set; }
 
-        public event ConsoleCancelEventHandler CancelKeyPress;
+        public event ConsoleCancelEventHandler? CancelKeyPress;
 
         public void ResetColor()
         {

test/CommandLineUtils.Tests/ValueParserProviderCustomTests.cs

@@ -38,7 +38,7 @@ private class ComplexTupleParser : IValueParser
                     return default(ValueTuple<int, double, string>?);
                 }
 
-                var fragments = value.Split('=');
+                var fragments = value!.Split('=');
 
                 try
                 {

test/Hosting.CommandLine.Tests/Utilities/TestConsole.cs

@@ -31,7 +31,7 @@ public TestConsole(ITestOutputHelper output)
         public ConsoleColor ForegroundColor { get; set; }
         public ConsoleColor BackgroundColor { get; set; }
 
-        public event ConsoleCancelEventHandler CancelKeyPress
+        public event ConsoleCancelEventHandler? CancelKeyPress
         {
             add { }
             remove { }