preserve natural=coastline on outer ways during Merge operation (Fixes #11816)

modules/actions/merge_polygon.js

@@ -107,8 +107,15 @@ export function actionMergePolygon(ids, newRelationId) {
                 return m.id === way.id && m.role !== 'inner';
             }
             if (members.some(isThisOuter)) {
-                relation = relation.mergeTags(way.tags);
-                graph = graph.replace(way.update({ tags: {} }));
+                //separate area tags from line-only tags (like natural=coastline)
+                var areaTags = Object.assign({}, way.tags);
+                var lineTags = {};
+                if (areaTags.natural === 'coastline') {
+                    delete areaTags.natural;
+                    lineTags.natural = 'coastline';
+                }
+                relation = relation.mergeTags(areaTags);
+                graph = graph.replace(way.update({ tags: lineTags }));
             }
         });
 

test/spec/actions/merge_polygon.js

@@ -162,4 +162,16 @@ describe('iD.actionMergePolygon', function () {
         expect(find(r, 'w3').role).to.equal('inner');
         expect(find(r, 'w4').role).to.equal('inner');
     });
+
+    it('preserves coastline tag on the outer ways when creating a multipolygon', function() {
+        graph = graph.replace(graph.entity('w0').update({ tags: { 'natural': 'coastline', 'place': 'island' }}));
+        graph = iD.actionMergePolygon(['w0', 'w1'], 'r')(graph);
+
+        //coastline should stay on the way
+        expect(graph.entity('w0').tags).to.eql({ natural: 'coastline' });
+
+        //other tags should move to the relation
+        var r = graph.entity('r');
+        expect(r.tags).to.eql({ type: 'multipolygon', place: 'island' });
+    });
 });