make let scoping collision detection much more reliable

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

@@ -162,21 +162,32 @@ LetScoping.prototype.noClosure = function () {
  */
 
 LetScoping.prototype.remap = function () {
-  var replacements = this.info.duplicates;
-  var block        = this.block;
+  var duplicates = this.info.duplicates;
+  var block      = this.block;
+  var scope      = this.scope;
 
   if (!this.info.hasDuplicates) return;
 
-  var replace = function (node, parent, scope, context, replacements) {
+  var replace = function (node, parent, scope, context, duplicates) {
     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;
+
+    var duplicate = duplicates[node.name];
+    if (!duplicate) return;
+
+    var own = scope.get(node.name, true);
+    if (own === duplicate.node) {
+      node.name = duplicate.uid;
+    } else {
+      // scope already has it's own declaration that doesn't
+      // match the one we have a stored replacement for
+      context.skip();
+    }
   };
 
   var traverseReplace = function (node, parent) {
     replace(node, parent);
-    traverse(node, { enter: replace }, null, replacements);
+    traverse(node, { enter: replace }, scope, duplicates);
   };
 
   var loopParent = this.loopParent;
@@ -186,7 +197,7 @@ LetScoping.prototype.remap = function () {
     traverseReplace(loopParent.update, loopParent);
   }
 
-  traverse(block, { enter: replace }, null, replacements);
+  traverse(block, { enter: replace }, scope, duplicates);
 };
 
 /**
@@ -221,12 +232,14 @@ LetScoping.prototype.getInfo = function () {
   };
 
   var duplicates = function (id, key) {
-    var has = scope.parentGet(key);
-
-    if (has && has !== id) {
-      // there's a variable with this exact name in an upper scope so we need
-      // to generate a new name
-      opts.duplicates[key] = id.name = file.generateUidIdentifier(key, scope).name;
+    // there's a variable with this exact name in an upper scope so we need
+    // to generate a new name
+    if (scope.parentHas(key, true)) {
+      var duplicate = opts.duplicates[key] = {
+        uid: file.generateUidIdentifier(key, scope).name,
+        node: id
+      };
+      id.name = duplicate.uid;
       opts.hasDuplicates = true;
     }
   };
@@ -305,7 +318,7 @@ LetScoping.prototype.checkLoop = function () {
 
       if (replace) return t.inherits(replace, node);
     }
-  }, null, has);
+  }, this.scope, has);
 
   return has;
 };
@@ -332,7 +345,7 @@ LetScoping.prototype.hoistVarDeclarations = function () {
         return context.skip();
       }
     }
-  }, null, this);
+  }, this.scope, this);
 };
 
 /**
@@ -347,7 +360,8 @@ LetScoping.prototype.getParams = function (params) {
   var info = this.info;
   params = _.cloneDeep(params);
   _.each(params, function (param) {
-    param.name = info.duplicates[param.name] || param.name;
+    var duplicate = info.duplicates[param.name];
+    if (duplicate) param.name = duplicate.uid;
   });
   return params;
 };
@@ -388,12 +402,12 @@ LetScoping.prototype.getLetReferences = function () {
             // push this badboy
             state.self.letReferences[node.name] = node;
           }
-        }, null, state);
+        }, scope, state);
 
         return context.skip();
       }
     }
-  }, null, state);
+  }, this.scope, state);
 
   return state.closurify;
 };