`Interval.difference/2` can produce intervals not creatable by `Interval.new`

[tisdall]

It seems like it's not possible to create a zero-length interval:

iex(1)> Interval.new(from: ~N[2025-01-01 13:00:00], until: ~N[2025-01-01 13:00:00], left_open: false, right_open: false)
{:error, :invalid_until}

If both left and right ends are not open, it seems technically it should be allowed. However, a zero length interval isn't really useful, so I'm fine with this limitation.

So, I was surprised to find that Interval.difference/2 will create zero-length intervals:

iex(2)> (
             Interval.difference(
                 Interval.new(from: ~N[2025-01-01 12:00:00], until: ~N[2025-01-01 13:00:00], right_open: true), 
                 Interval.new(from: ~N[2025-01-01 12:00:00], until: ~N[2025-01-01 13:00:00], right_open: true)
             )
         )
[]
iex(3)> (
             Interval.difference(
                 Interval.new(from: ~N[2025-01-01 12:00:00], until: ~N[2025-01-01 13:00:00], right_open: false), 
                 Interval.new(from: ~N[2025-01-01 12:00:00], until: ~N[2025-01-01 13:00:00], right_open: true)
             )
         )
[
  %Timex.Interval{
    from: ~N[2025-01-01 13:00:00],
    until: ~N[2025-01-01 13:00:00],
    left_open: false,
    right_open: false,
    step: [days: 1]
  }
]

If I pass the zero-length interval into |> Interval.duration(:duration) I get back #<Duration(PT0S)>.

[tisdall]

I found a similar issue with Date intervals. I wanted to create an enumerable that outputs all the dates from a starting date to an ending date (including the ending date). If it happens to have the same date for both, I expected it to output just that day, but instead it just renders an error.

iex(1)> Timex.Interval.new(from: ~D[2025-01-01], until: ~D[2025-01-02], right_open: false) |> Enum.to_list()
[~N[2025-01-01 00:00:00], ~N[2025-01-02 00:00:00]]
iex(2)> Timex.Interval.new(from: ~D[2025-01-01], until: ~D[2025-01-01], right_open: false)
{:error, :invalid_until}

If I bypass new/1 then it works as I expect:

iex(3)> %Timex.Interval{from: ~N[2025-01-01 00:00:00], until: ~N[2025-01-01 00:00:00], 
        left_open: false, right_open: false, step: [days: 1]} |> Enum.to_list()
[~N[2025-01-01 00:00:00]]