Nullability of generic type in Action<T>

Question

I have the following code:

public class Maybe<T>
{
    private readonly T? _value;

    public Maybe(T? value) => _value = value;

    public void WhenNotNull(Action<T> func)
    {
        if (_value != null)
            func(_value);
    }
}

public class Test
{
    void DoSomething<T>(Maybe<T> s)
    {
        s.WhenNotNull(TestMethod);
    }

    private void TestMethod<T>([DisallowNull] T x) => Console.WriteLine(x.ToString());
}

Now s.WhenNotNull(TestMethod); generates

warning CS8622: Nullability of reference types in type of parameter 'x' of 'void Test.TestMethod<T>(T x)' doesn't match the target delegate 'Action<T>' (possibly because of nullability attributes)

There is no way for the parameter to be null, is there any way to signal this to the compiler?
NB: If the Maybe object is concrete (eg. Maybe<string>), this warning is not generated.

Answer 1

The Action<T> delegate is defined as follow:

public delegate void Action<in T>(T obj);

You can define your own delegate which includes the DisallowNull attribute:

public delegate void MyAction<T>([DisallowNull] T x);

If you now change WhenNotNull to use MyAction<T> instead of Action<T>, the warning will be gone.

Answer 2

Changed the signature of DoSomething to include where T: notnull

public void DoSomething<T>(Maybe<T> s) where T: notnull

Credit to @nbokmans and @Matthew Watson.