ST_CollectionExtract forms invalid polygons

[robe]

ST_CollectionExtract is similarly flawed as noted in #2377 for ST_CollectionHomogenize

SELECT       
    ST_AsText(ST_CollectionExtract(ST_Split(g1.geom1, g1.geom2),3))
    As geom
FROM (SELECT 
  ST_GeomFromText('POLYGON((2 4.5,3 2.6,3 1.8,2 0,-1.5 2.2,
   0.056 3.222,-1.5 4.2,2 6.5,2 4.5))') As geom1, 
ST_GeomFromText('LINESTRING(-0.62 5.84,-0.8 0.59)') As geom2) AS g1;

return invalid multipolygon:

MULTIPOLYGON(((2 4.5,3 2.6,3 1.8,2 0,-0.760731980182196 1.73531724468595,-0.727401641068799 2.70745213549337,0.056 3.222,-0.693606166057922 3.69315348997728,-0.657240688157675 4.75381326206781,2 6.5,2 4.5)),((-0.760731980182196 1.73531724468595,-1.5 2.2,-0.727401641068799 2.70745213549337,-0.760731980182196 1.73531724468595)),((-0.693606166057922 3.69315348997728,-1.5 4.2,-0.657240688157675 4.75381326206781,-0.693606166057922 3.69315348997728)))

which is invalid because edges are shared.

[pramsey]

Save comment as homogenize. We're not going to rework the inputs to make them valid automagically, that's just too scary. We have an isvalid function so people can filter things as necessary.

[robe]

I guess we can at least put a notice in the docs — This will form invalid multipolygons.

[robe]

documented that flawed behavior at r11646 for 2.0, r11647 at 2.1