Lineal TopoGeometry objects loose their direction upon casting to Geometry

[strk]

Simple testcase:

SELECT CreateTopology('t');
CREATE TABLE t.f(i serial primary key);
SELECT AddTopoGeometryColumn('t','t','f','g','LINE');
SELECT ST_AsText(toTopoGeom('LINESTRING(0 0, 10 0)', 't', 1, 0)::geometry);
select ST_AsText(toTopoGeom(ST_Reverse('LINESTRING(0 0, 10 0)'), 't', 1, 0)::geometry);

The last two queries are creating a TopoGeometry with two LINESTRING objects in opposite direction. When casting these two objects back to a GEOMETRY object the _same_ LINESTRING comes back, regardless of the original direction.

I verified this is NOT a problem with toTopoGeom but with the cast to geometry, because changing the sign of the element_id in the relation table does NOT change the output of the ::geometry cast.

This limitation is probably as old as the Topology support in PostGIS.

MULTILINESTRING((0 0,10 0))

[strk]

Work in progress: ​https://gitlab.com/postgis/postgis/-/merge_requests/23

[strk]

A related ticket is about GetTopoGeomElements NOT returning the sign of elements: #4882

[strk]

I spoke too early: while the TopoGeometry::Geometry cast IS BOGUS as verified by manually changing sign of the element_id fields in the relation table, the toTopoGeom function is ALSO BOGUS in that it doesn't correctly set those element_id field signs, see #4884

[Sandro Santilli <strk@…>]

In fea3aac/git:

Rewrite TopoGeometry::geometry cast for lineals to retain direction

Includes test

Closes #4881 in main branch (3.2.0dev)