Warning inferring nullability of lambda return type tuple

[cston]

using System;

class Program
{
    static T F<T>(Func<bool, T> f)
    {
        return f(true);
    }

    static void G<T>(object x)
    {
        F(b =>
        {
            if (b) return (x, null); // warning
            return (x, x);
        });
    }
}

(14,27): warning CS8619: Nullability of reference types in value of type '(object x, object?)'
    doesn't match target type '(object, object)'.