opt out of tail recursion optimisation if the owner id has been reassigned - fixes #744

lib/6to5/transformation/transformers/es6/tail-call.js

@@ -64,11 +64,16 @@ TailCallTransformer.prototype.run = function () {
   var scope = this.scope;
   var node  = this.node;
 
-  // only tail recursion can be optimized as for now,
-  // so we can skip anonymous functions entirely
+  // only tail recursion can be optimized as for now, so we can skip anonymous
+  // functions entirely
   var ownerId = this.ownerId;
   if (!ownerId) return;
 
+  // check if the ownerId has been reassigned, if it has then it's not safe to
+  // perform optimisations
+  var ownerIdInfo = this.scope.getBindingInfo(ownerId.name);
+  if (!ownerIdInfo || ownerIdInfo.reassigned) return;
+
   // traverse the function and look for tail recursion
   scope.traverse(node, firstPass, this);
 

lib/6to5/traversal/scope.js

@@ -229,6 +229,14 @@ Scope.prototype.registerDeclaration = function (node) {
   }
 };
 
+Scope.prototype.registerBindingReassignment = function (node) {
+  var ids = t.getBindingIdentifiers(node);
+  for (var name in ids) {
+    var info = this.getBindingInfo(name);
+    if (info) info.reassigned = true;
+  }
+};
+
 Scope.prototype.registerBinding = function (kind, node) {
   if (!kind) throw new ReferenceError("no `kind`");
 
@@ -241,6 +249,7 @@ Scope.prototype.registerBinding = function (kind, node) {
 
     this.bindings[name] = {
       typeAnnotation: this.getTypeAnnotation(name, id, node),
+      reassigned:     false,
       identifier:     id,
       scope:          this,
       kind:           kind
@@ -302,6 +311,8 @@ var programReferenceVisitor = {
       state.addGlobal(node);
     } else if (t.isLabeledStatement(node)) {
       state.addGlobal(node);
+    } else if (t.isAssignmentExpression(node)) {
+      scope.registerBindingReassignment(node);
     }
   }
 };

test/fixtures/transformation/es6-tail-call/ignore-reassigned/actual.js

@@ -0,0 +1,13 @@
+// we need to deopt `test` if it's reassigned as we can't be certain of it's
+// state, ie. it could have been rebound or dereferenced
+
+function test(exit) {
+  if (exit) {
+    return this.x;
+  }
+  return test(true);
+}
+
+test = test.bind({ x: "yay" });
+
+console.log(test());

test/fixtures/transformation/es6-tail-call/ignore-reassigned/expected.js

@@ -0,0 +1,15 @@
+"use strict";
+
+// we need to deopt `test` if it's reassigned as we can't be certain of it's
+// state, ie. it could have been rebound or dereferenced
+
+function test(exit) {
+  if (exit) {
+    return this.x;
+  }
+  return test(true);
+}
+
+test = test.bind({ x: "yay" });
+
+console.log(test());