how to make a polygon from a relation with lines only ?

have an issue to add an OSM relation built from ways only but that I would like to show as a polygon.
relation ID 1234
Line ID 456
Line ID 567
Line ID 678

where line 456 is correctly linked to line 567, itself linked to line 678 and this one linked to line 456.
So the relation defines a polygon but each item from the relation is “only” a line/way.
I want to map this “combination” of ways as a polygon.

I don’t catch how to setup style rules to map such case.

any help welcomed if someone knows.

If I understand your question, I think you want a relation with a type of “multipolygon” with each member way having a “role” of “outer”.

This is not handled by the style, but by osm2pgsql creating the PostGIS database used for rendering (assuming you use the most common osm2pgsql / Mapnik / Carto render pipeline). Alternatively, the ArcGIS Editor for OpenStreetMap can assemble multipolygons as well in File Geodatabase format, or any of the ArcGIS supported enterprise geodatabases (PostGIS, Oracle, SQL Server etc.).

I know there are tools that can render straight from raw XML, but I have never used them, and don’t know if they are capable of assembling multipolygons “on-the-fly”. If they do, the processing cost will be considerable… it may not be viable for big datasets and large number of features.

to N76,
the relation already exists in OSM → my point is that because each of the “components” in the relation is a way, my current style rules don’t allow showing all the ways together in the relation as a polygon.

thanks to MBOERINGA. My set of Tools is very basic : Potlach2, JOSM, Mkgmap, osmconvert and srtm2osm. I didn’t yet “investigate” what can be obtained from the ones you mentioned (osm2pgsql, ArcGis…).
For the time being, I’m still working on defining a convenient set of style rules and an ad-hoc TYP file for my need: it appears to be a longer activity than initially expected and i even realized during some holidays in other countries that there are in such other places, different objects that can be useful to map that I don’t really have in “my” country.
So I guess I’m not at the end of building the TYP & styles that I want.
this is to say that based on your comment, I’ll certainly let this investigation for later.
Many thanks.

If this is for a Garmin you better ask it here:
It will be possible, but can you give an OSM id as example?
That will be easier to explain how you can do it.