clarify geometry-type expression behavior for vector tile sources

[samanpwbb]

Problem

The documentation for geometry-type in the spec (

mapbox-gl-js/src/style-spec/reference/v8.json

Line 2738 in 64ebdda

"doc": "Gets the feature's geometry type: Point, MultiPoint, LineString, MultiLineString, Polygon, MultiPolygon.",

) could clarify which property types will be available for which source types. Current copy is:

Gets the feature's geometry type: Point, MultiPoint, LineString, MultiLineString, Polygon, MultiPolygon.

Based on a quick experiment, it doesn't appear like the geometry-type expression ever returns a multi* feature when applied to a vector tile source.

Suggestion

There's likely more nuance going on here than I'm aware of – in which case, the appropriate explanation in the appropriate place would solve this issue better than anything I can come up with. Here are some suggestions based on what I know:

• Add a sentence to the spec documentation, something like: "Multi* feature types will only be detectable in GeoJSON sources, use the singular forms when working with vector tile sources".
• Change the behavior of geometry-type to return true for multi-features in both vector tile and GeoJSON source types.

[mourner]

I wonder whether it makes sense to differentiate between Multi and Single geometries in the context of a Mapbox style — perhaps we should do the other way around and fix GeoJSON to return Polygon/Line/Point for multi geometries as well?

It's definitely a design omission — we should have thought this out to be consistent. The only problem with fixing this is that it's a breaking change, so perhaps updating the docs would be an easier first step, assuming a follow-up.

[chloekraw]

cc @zmiao @alexshalamov @tmpsantos @pozdnyakov

[SnailBones]

@HeyStenson's commit provides the documentation fix.

Added to #6584 so we can revisit the breaking change in the next style spec version.