Unable to use in expression as layer filter

[1ec5]

The in expression operator added in #16162 can only be used with paint and layout properties, not with layer filters. To reproduce, load this style and observe that nothing is visible:

colors.json

{
  "version": 8,
  "zoom": 0,
  "center": [0, 0],
  "sources": {
    "quadrants": {
      "type": "geojson",
      "data": {
        "type": "FeatureCollection",
        "features": [
          {
            "type": "Feature",
            "properties": {
              "name": "ABC",
              "color": "red"
            },
            "geometry": {
              "type": "Polygon",
              "coordinates": [[
                [-180, -90],
                [-180, 0],
                [0, 0],
                [0, -90],
                [-180, -90]
              ]]
            }
          },
          {
            "type": "Feature",
            "properties": {
              "name": "DEF",
              "color": "#00ff00"
            },
            "geometry": {
              "type": "Polygon",
              "coordinates": [[
                [-180, 90],
                [-180, 0],
                [0, 0],
                [0, 90],
                [-180, 90]
              ]]
            }
          },
          {
            "type": "Feature",
            "properties": {
              "name": "GHI",
              "color": "#0000ff"
            },
            "geometry": {
              "type": "Polygon",
              "coordinates": [[
                [180, -90],
                [180, 0],
                [0, 0],
                [0, -90],
                [180, -90]
              ]]
            }
          },
          {
            "type": "Feature",
            "properties": {
              "name": "GHI",
              "color": "#ffff00"
            },
            "geometry": {
              "type": "Polygon",
              "coordinates": [[
                [180, 90],
                [180, 0],
                [0, 0],
                [0, 90],
                [180, 90]
              ]]
            }
          }
        ]
      }
    }
  },
  "glyphs": "local://glyphs/{fontstack}/{range}.pbf",
  "layers": [
    {
      "id": "colors",
      "type": "fill",
      "source": "quadrants",
      "filter": ["in", "#", ["get", "color"]],
      "paint": {
        "fill-color": ["get", "color"]
      }
    }
  ]
}

GL JS v1.8.1	Expected	GL Native v1.3.0

mbgl::style::conversion::isExpression() always returns false for the filter ["in", "#", ["get", "color"]], because in is also a legacy filter operator:

mapbox-gl-native/src/mbgl/style/conversion/filter.cpp

Lines 56 to 57 in 305df25

	} else if (*op == "in" || *op == "!in" || *op == "!has" || *op == "none") {
	return false;

/ref mapbox/mapbox-gl-native-ios#183 (comment)
/cc @jmkiley @Chaoba @chloekraw

[1ec5]

mbgl::style::conversion::isExpression() does have some code to distinguish some expression operators from identically named filters, but in with two parameters might not be possible to resolve through static type checking. For example, ["in", "a", "abc"] has two equally valid interpretations:

• Filter: Is the a property equal to “abc”?
• Expression: Is the string “a” a substring of “abc”?

To avoid breaking backwards compatibility with existing styles, we’d have to interpret it as a filter, but that would prevent us from fixing this issue and fully supporting mapbox/mapbox-gl-js#4113.

This is a contrived case, to be sure: it’s more likely for the filter to be written ["==", "a", "abc"], and there’s no practical reason for someone to test whether a literal string is in a literal string. But nothing would’ve stopped someone from using in instead of == back before Studio launched expression support. And isExpression() relies on static type checking, which means it doesn’t know whether a is a string literal or a reference to a feature property.

[1ec5]

GL JS has code to avoid the original issue above, but it doesn’t quite address the ambiguity: mapbox/mapbox-gl-js#9373.

[kkaefer]

JS PR is in mapbox/mapbox-gl-js#9374