From 7fc2fe41af2b25c1c04ab01f41cb7171534f624b Mon Sep 17 00:00:00 2001
From: Sebastian McKenzie <sebmck@gmail.com>
Date: Wed, 19 Nov 2014 13:46:00 +1100
Subject: [PATCH] fix bug in let scoping resulting in unneccesary replacement -
 closes #193, closes #185

---
 .../transformers/let-scoping.js               | 25 +++++++++++++------
 .../let-scoping/exec-block-scoped/exec.js     |  7 ++++++
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/lib/6to5/transformation/transformers/let-scoping.js b/lib/6to5/transformation/transformers/let-scoping.js
index 6741207e7e..a56d6af6b7 100644
--- a/lib/6to5/transformation/transformers/let-scoping.js
+++ b/lib/6to5/transformation/transformers/let-scoping.js
@@ -88,6 +88,9 @@ LetScoping.prototype.run = function () {
 
   this.info = this.getInfo();
 
+  // remap all let references that exist in upper scopes to their uid
+  this.remap();
+
   // this is a block within a `Function` so we can safely leave it be
   if (t.isFunction(this.parent)) return this.noClosure();
 
@@ -138,23 +141,29 @@ LetScoping.prototype.run = function () {
 };
 
 /**
- * There are no let references accessed within a closure so we can just traverse
- * through this block and replace all references that exist in a higher scope to
- * their uids.
+ * There are no let references accessed within a closure so we can just turn the
+ * lets into vars.
  */
 
 LetScoping.prototype.noClosure = function () {
-  var replacements = this.info.duplicates;
-  var declarators  = this.info.declarators;
-  var block        = this.block;
+  standardiseLets(this.info.declarators);
+};
 
-  standardiseLets(declarators);
+/**
+ * Traverse through block and replace all references that exist in a higher
+ * scope to their uids.
+ */
+
+LetScoping.prototype.remap = function () {
+  var replacements = this.info.duplicates;
+  var block        = this.block;
 
   if (_.isEmpty(replacements)) return;
 
-  var replace = function (node, parent) {
+  var replace = function (node, parent, scope) {
     if (!t.isIdentifier(node)) return;
     if (!t.isReferenced(node, parent)) return;
+    if (scope && scope.hasOwn(node.name)) return;
     node.name = replacements[node.name] || node.name;
   };
 
diff --git a/test/fixtures/transformation/let-scoping/exec-block-scoped/exec.js b/test/fixtures/transformation/let-scoping/exec-block-scoped/exec.js
index 8b1fd6db2b..d90256cbb2 100644
--- a/test/fixtures/transformation/let-scoping/exec-block-scoped/exec.js
+++ b/test/fixtures/transformation/let-scoping/exec-block-scoped/exec.js
@@ -2,5 +2,12 @@ let x = 1;
 {
   let x = 2;
   assert.equal(x, 2);
+  {
+    let x = 3;
+    assert.equal(x, 3);
+
+    x++;
+    assert.equal(x, 4);
+  }
 }
 assert.equal(x, 1);