Fixed type safety

Author: sakno

src/DotNext/Sentinel.cs

@@ -1,7 +1,13 @@
+using System.Runtime.InteropServices;
+
 namespace DotNext;
 
 internal static class Sentinel
 {
     // instance has fixed address in memory which is critical to ValueReference implementation
-    internal static readonly object Instance = GC.AllocateUninitializedArray<byte>(length: 0, pinned: true);
+    internal static readonly object Instance = GC.AllocateUninitializedArray<Dummy>(length: 0, pinned: true);
+
+    // The struct is needed to avoid false positives caused by the type check on the variable that stores singleton
+    [StructLayout(LayoutKind.Auto)]
+    private struct Dummy;
 }