@@ -69,6 +69,41 @@ class State {
 }
 let state = new State();
 
+/**
+ * Debugging method for finding cycles in the TypeScript AST. Should not be used in production.
+ *
+ * If cycles are found, the whitelist in `astProperties` is too permissive.
+ */
+// tslint:disable-next-line:no-unused-variable
+function checkCycle(root: any) {
+    let path: string[] = [];
+    function visit(obj: any): boolean {
+        if (obj == null || typeof obj !== "object") return false;
+        if (obj.$cycle_visiting) {
+            return true;
+        }
+        obj.$cycle_visiting = true;
+        for (let k in obj) {
+            if (!obj.hasOwnProperty(k)) continue;
+            // Ignore numeric and whitelisted properties.
+            if (+k !== +k && !astPropertySet.has(k)) continue;
+            if (k === "$cycle_visiting") continue;
+            let cycle = visit(obj[k]);
+            if (cycle) {
+                path.push(k);
+                return true;
+            }
+        }
+        obj.$cycle_visiting = undefined;
+        return false;
+    }
+    visit(root);
+    if (path.length > 0) {
+        path.reverse();
+        console.log(JSON.stringify({ type: "error", message: "Cycle = " + path.join(".") }));
+    }
+}
+
 /** Property names to extract from the TypeScript AST. */
 const astProperties: string[] = [
     "$declarationKind",