public interface IDo<T>
{
    IResult<TReturn> Do<TReturn>(Func<T, TReturn> fn);
}

public interface IResult<TReturn>
{
}

public class Class1
{
    public void Test1(IDo<(int, int)> impl)
    {
        var case1 = impl.Do<int>(((int x, int y) a) => a.x * a.y); // compiles
        var case2 = impl.Do(a =>
        {
            (var x, var y) = a;
            return x * y;
        }); // compiles

        var case3 = impl.Do(((int x, int y) a) => a.x * a.y); // <-- does not compile
    }

    public void Test2(IDo<(int x, int y)> impl)
    {
        var case4 = impl.Do(a => a.x * a.y); // compiles
    }
}