remap head variables when a closure is required in blockScoping transformer - fixes #763

lib/6to5/transformation/transformers/es6/block-scoping.js

@@ -124,7 +124,7 @@ BlockScoping.prototype.run = function () {
   if (!this.hasLetReferences) return;
 
   if (needsClosure) {
-    this.needsClosure();
+    this.wrapClosure();
   } else {
     this.remap();
   }
@@ -205,9 +205,28 @@ BlockScoping.prototype.remap = function () {
  * Description
  */
 
-BlockScoping.prototype.needsClosure = function () {
+BlockScoping.prototype.wrapClosure = function () {
   var block = this.block;
 
+  var outsideRefs = this.outsideLetReferences;
+
+  // remap loop heads with colliding variables
+  if (this.loopParent) {
+    for (var name in outsideRefs) {
+      var id = outsideRefs[name];
+
+      if (this.scope.hasGlobal(id.name)) {
+        delete outsideRefs[id.name];
+        delete this.letReferences[id.name];
+
+        this.scope.rename(id.name);
+
+        this.letReferences[id.name] = id;
+        outsideRefs[id.name] = id;
+      }
+    }
+  }
+
   // if we're inside of a for loop then we search to see if there are any
   // `break`s, `continue`s, `return`s etc
   this.has = this.checkLoop();
@@ -216,7 +235,7 @@ BlockScoping.prototype.needsClosure = function () {
   this.hoistVarDeclarations();
 
   // turn outsideLetReferences into an array
-  var params = values(this.outsideLetReferences);
+  var params = values(outsideRefs);
 
   // build the closure that we're going to wrap the block with
   var fn = t.functionExpression(null, params, t.blockStatement(block.body));

test/fixtures/transformation/es6-block-scoping-exec/closure-wrap-collision.js

@@ -0,0 +1,9 @@
+for (let i = 1; i < 3; i += 1) {
+  (function () {
+    i;
+  })();
+}
+
+assert.throws(function () {
+  i;
+}, ReferenceError);