NotNull Annotation on implicit cast operators Follow

Hi there

Is it possible to write an implicit cast operator in C# using the [NotNull] Attribute?

My attempt so far was:

public struct NotNullable<T> where T : class
{
    public NotNullable([NotNull]T value)
    {
        if (value == null)
            throw new ArgumentNullException(nameof(value), @"A NotNullable type cannot be null");
        Value = value;
    }

    [ContractAnnotation("notnull => notnull")] // Tried contract annotations to check if they are more strict, but also didn't work
    public static implicit operator NotNullable<T>([NotNull]T value)
    {
        return new NotNullable<T>(value);
    }
    ...
}

It works for the constructor parameter but the implicit cast does not show a warning:

object test = null;
NotNullable<object> notNullableTest = new NotNullable<object>(test); // Parameter test is underlined
notNullableTest = test; // but no warnings are shown here

Thought someone got an idea here or if someone can tell me that it is not possible to do, would help too.

Cheers,
Philipp