Copy into a C# 8.0 app:

#nullable enable

namespace ConsoleApp65
{
    class C
    {
        void M()
        {
            object? o = null;
            var s = (string?)o;
            Console.WriteLine(s);
        }
    }
}

Invoke extract method on the "o" in the "var s..." line. Do not select the cast, just the o.

Expected: the generated method returns string? because that's the explicit cast before it.
Actual: it returns just string which breaks code.