Fix a bug where lines with duplicate endpoint disappear on z18+ (#9218)

* fix a bug with vanishing line with duplicate end points

* fix flow :(

src/data/bucket/line_bucket.js

@@ -274,8 +274,8 @@ class LineBucket implements Bucket {
 
         for (let i = first; i < len; i++) {
 
-            nextVertex = isPolygon && i === len - 1 ?
-                vertices[first + 1] : // if the line is closed, we treat the last vertex like the first
+            nextVertex = i === len - 1 ?
+                (isPolygon ? vertices[first + 1] : (undefined: any)) : // if it's a polygon, treat the last vertex like the first
                 vertices[i + 1]; // just the next vertex
 
             // if two consecutive vertices exist, skip the current one

src/style-spec/types.js

@@ -88,7 +88,8 @@ export type VectorSourceSpecification = {
     "scheme"?: "xyz" | "tms",
     "minzoom"?: number,
     "maxzoom"?: number,
-    "attribution"?: string
+    "attribution"?: string,
+    "promoteId"?: PromoteIdSpecification
 }
 
 export type RasterSourceSpecification = {
@@ -112,8 +113,7 @@ export type RasterDEMSourceSpecification = {
     "maxzoom"?: number,
     "tileSize"?: number,
     "attribution"?: string,
-    "encoding"?: "terrarium" | "mapbox",
-    "promoteId"?: PromoteIdSpecification
+    "encoding"?: "terrarium" | "mapbox"
 }
 
 export type GeoJSONSourceSpecification = {|

test/integration/render-tests/regressions/mapbox-gl-js#5171/style.json

@@ -0,0 +1,35 @@
+{
+  "version": 8,
+  "metadata": {
+    "test": {
+      "width": 64,
+      "height": 32
+    }
+  },
+  "center": [-0.63938, 44.77458],
+  "zoom": 18,
+  "sources": {
+    "geojson": {
+      "type": "geojson",
+      "data": {
+        "type": "LineString",
+        "coordinates": [
+          [-0.639208, 44.774651],
+          [-0.639605, 44.774487],
+          [-0.639605, 44.774487]
+        ]
+      }
+    }
+  },
+  "layers": [
+    {
+      "id": "line",
+      "type": "line",
+      "source": "geojson",
+      "paint": {
+        "line-color": "#f44336",
+        "line-width": 10
+      }
+    }
+  ]
+}