Turn on WarningsAsErrors for nullable warnings in ICSharpCode.Decompiler

ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj

@@ -32,6 +32,7 @@
     <SignAssembly>True</SignAssembly>
     <AssemblyOriginatorKeyFile>ICSharpCode.Decompiler.snk</AssemblyOriginatorKeyFile>
     <NoWarn>1701;1702;1591;1573</NoWarn>
+    <WarningsAsErrors>nullable</WarningsAsErrors>
 
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
     <DebugType>embedded</DebugType>

ICSharpCode.Decompiler/Util/KeyComparer.cs

@@ -71,12 +71,12 @@ namespace ICSharpCode.Decompiler.Util
 
 		public int Compare(TElement? x, TElement? y)
 		{
-			return keyComparer.Compare(keySelector(x), keySelector(y));
+			return keyComparer.Compare(keySelector(x!), keySelector(y!));
 		}
 
 		public bool Equals(TElement? x, TElement? y)
 		{
-			return keyEqualityComparer.Equals(keySelector(x), keySelector(y));
+			return keyEqualityComparer.Equals(keySelector(x!), keySelector(y!));
 		}
 
 		public int GetHashCode(TElement obj)