add error for dagre container-child edge

d2graph/d2graph.go

@@ -1586,3 +1586,13 @@ func (g *Graph) SortEdgesByAST() {
 	})
 	g.Edges = edges
 }
+
+func (obj *Object) IsDescendantOf(ancestor *Object) bool {
+	if obj == ancestor {
+		return true
+	}
+	if obj.Parent == nil {
+		return false
+	}
+	return obj.Parent.IsDescendantOf(ancestor)
+}

d2plugin/plugin_elk.go

@@ -89,6 +89,7 @@ func (p elkPlugin) Info(ctx context.Context) (*PluginInfo, error) {
 		Type: "bundled",
 		Features: []PluginFeature{
 			CONTAINER_DIMENSIONS,
+			DESCENDANT_EDGES,
 		},
 		ShortHelp: "Eclipse Layout Kernel (ELK) with the Layered algorithm.",
 		LongHelp: fmt.Sprintf(`ELK is a layout engine offered by Eclipse.

d2plugin/plugin_features.go

@@ -18,6 +18,9 @@ const CONTAINER_DIMENSIONS PluginFeature = "container_dimensions"
 // When this is true, objects can specify their `top` and `left` keywords
 const TOP_LEFT PluginFeature = "top_left"
 
+// When this is true, containers can have connections to descendants
+const DESCENDANT_EDGES PluginFeature = "descendant_edges"
+
 func FeatureSupportCheck(info *PluginInfo, g *d2graph.Graph) error {
 	// Older version of plugin. Skip checking.
 	if info.Features == nil {
@@ -50,5 +53,12 @@ func FeatureSupportCheck(info *PluginInfo, g *d2graph.Graph) error {
 			}
 		}
 	}
+	if _, ok := featureMap[DESCENDANT_EDGES]; !ok {
+		for _, e := range g.Edges {
+			if e.Src.IsDescendantOf(e.Dst) || e.Dst.IsDescendantOf(e.Src) {
+				return fmt.Errorf(`Connection "%s" goes from a container to a descendant, but layout engine "%s" does not support this.`, e.AbsID(), info.Name)
+			}
+		}
+	}
 	return nil
 }