Add extra information to nullable conversion test file

csharp/ql/test/library-tests/conversion/nullable/Nullable.cs

@@ -1,20 +1,29 @@
 using System;
+using System.Runtime.Serialization;
 
 class C
 {
-    sbyte x1;
-    sbyte? x2;
+    int x1;
+    int? x2;
     Nullable<int> x3;
     Nullable<char> x4;
 
     // Verify conversions
     void M()
     {
-        x2 = x1;
-        x3 = x4;
+        x2 = x1;    // T -> T?      conversion: implicit, nullable          -> implicit cast
+        x3 = x4;    // T1? -> T2?   conversion: implicit, nullable          -> implicit cast
+
+        x12 = x1;   // T1 -> T2?    conversion: implicit, nullable          -> implicit cast
+        x12 = null; // null -> T?   conversion: implicit, null literal      -> no cast
+
+        x3 = x2;    // T? -> T?     no conversion
+
+        x14 = x15;  // T1? -> T2?   conversion: implicit, user defined      -> implicit cast
     }
 
     // Cause the following types to exist in the database:
+    sbyte? x0;
     byte? x5;
     short? x6;
     ushort? x7;
@@ -24,4 +33,19 @@ class C
     double? x11;
     long? x12;
     float? x13;
+
+    A? x14;
+    B? x15;
+
+    struct A
+    {
+    }
+
+    struct B
+    {
+        public static implicit operator A(B value)
+        {
+            return new A();
+        }
+    }
 }