Add tests for generic types for Marshal.OffsetOf

src/libraries/System.Runtime.InteropServices/tests/System/Runtime/InteropServices/Marshal/OffsetOfTests.cs

@@ -224,6 +224,21 @@ public void OffsetOf_NoLayoutPoint_ThrowsArgumentException()
             AssertExtensions.Throws<ArgumentException>(null, () => Marshal.OffsetOf<NoLayoutPoint>(nameof(NoLayoutPoint.x)));
         }
 
+        [Fact]
+        public void OffsetOf_Field_BlittableGeneric()
+        {
+            Assert.Equal((IntPtr)4, Marshal.OffsetOf<Point2<int>>(nameof(Point2<int>.y)));
+            Assert.Equal((IntPtr)8, Marshal.OffsetOf<Point2<ulong>>(nameof(Point2<int>.y)));
+            Assert.Equal((IntPtr)4, Marshal.OffsetOf<Point2<float>>(nameof(Point2<int>.y)));
+            Assert.Equal((IntPtr)8, Marshal.OffsetOf<Point2<double>>(nameof(Point2<int>.y)));
+
+            // [COMPAT] Non-blittable generic types are supported in OffsetOf since they've always been allowed
+            // and it likely doesn't meet the bar to break back-compat.
+            Assert.Equal((IntPtr)1, Marshal.OffsetOf<Point2<char>>(nameof(Point2<int>.y)));
+            Assert.Equal((IntPtr)1, Marshal.OffsetOf<Point2<byte>>(nameof(Point2<int>.y)));
+            Assert.Equal((IntPtr)IntPtr.Size, Marshal.OffsetOf<Point2<string>>(nameof(Point2<int>.y)));
+        }
+
         public class NonRuntimeType : Type
         {
             public override FieldInfo GetField(string name, BindingFlags bindingAttr)
@@ -496,5 +511,11 @@ struct FieldAlignmentTest_Variant
         public short s; // 2 bytes
                         // 6 bytes of padding
     };
+
+    struct Point2<T>
+    {
+        public T x;
+        public T y;
+    }
 #pragma warning restore 169, 649, 618
 }