Why does the first one return `EmptyRegion[2]` while the second one works?
I expected the result is `Line[{{0, 0}, {1, 1}}]` as the second one.  <br/>

    RegionIntersection[
     Polygon[{{-1, 1}, {1, 1}, {1, 0}, {-1, 0}, {-1, 1}}], 
     Line[{{0, 0}, {1, 1}}]]

[![enter image description here][1]][1]


    RegionIntersection[Polygon[{{-1, 1}, {1, 1}, {1, 0}, {-1, 0}}], 
     Line[{{0, 0}, {1, 1}}]]

[![enter image description here][2]][2]

Aslo how does EmptyRegion is defined?
I read the docs but still not quite clear.
It seems like a point is not empty in R but empty in R2. 

How would I define a polygon region like above but without the boundary?

  [1]: https://i.sstatic.net/KvJN3.png
  [2]: https://i.sstatic.net/fU1vx.png