Fixing Issue #643. Be a little bit safer about declaring block variables as close to the block scope as possible.

lib/nodes.js

@@ -1598,10 +1598,14 @@
       name = (this.name && this.name.compile(o)) || scope.freeVariable();
       index = this.index && this.index.compile(o);
       if (name && !this.pattern && (range || !codeInBody)) {
-        scope.find(name);
+        scope.find(name, {
+          immediate: true
+        });
       }
       if (index) {
-        scope.find(index);
+        scope.find(index, {
+          immediate: true
+        });
       }
       if (!(topLevel)) {
         rvar = scope.freeVariable();

lib/scope.js

@@ -21,8 +21,8 @@
       return this;
     };
     Scope.root = null;
-    Scope.prototype.find = function(name) {
-      if (this.check(name)) {
+    Scope.prototype.find = function(name, options) {
+      if (this.check(name, options)) {
         return true;
       }
       this.variables[name] = 'var';
@@ -43,9 +43,11 @@
     Scope.prototype.parameter = function(name) {
       return (this.variables[name] = 'param');
     };
-    Scope.prototype.check = function(name) {
-      if (Object.prototype.hasOwnProperty.call(this.variables, name)) {
-        return true;
+    Scope.prototype.check = function(name, options) {
+      var immediate;
+      immediate = Object.prototype.hasOwnProperty.call(this.variables, name);
+      if (immediate || (options && options.immediate)) {
+        return immediate;
       }
       return !!(this.parent && this.parent.check(name));
     };

src/nodes.coffee

@@ -1357,8 +1357,8 @@ exports.ForNode = class ForNode extends BaseNode
     scope         = o.scope
     name          = (@name and @name.compile(o)) or scope.freeVariable()
     index         = @index and @index.compile o
-    scope.find name  if name and not @pattern and (range or not codeInBody)
-    scope.find index if index
+    scope.find(name,  immediate: yes) if name and not @pattern and (range or not codeInBody)
+    scope.find(index, immediate: yes) if index
     rvar          = scope.freeVariable() unless topLevel
     ivar          = if codeInBody then scope.freeVariable() else if range then name else index or scope.freeVariable()
     varPart       = ''

src/scope.coffee

@@ -28,8 +28,8 @@ exports.Scope = class Scope
 
   # Look up a variable name in lexical scope, and declare it if it does not
   # already exist.
-  find: (name) ->
-    return true if @check name
+  find: (name, options) ->
+    return true if @check name, options
     @variables[name] = 'var'
     false
 
@@ -44,9 +44,11 @@ exports.Scope = class Scope
   parameter: (name) ->
     @variables[name] = 'param'
 
-  # Just check to see if a variable has already been declared, without reserving.
-  check: (name) ->
-    return true if Object::hasOwnProperty.call @variables, name
+  # Just check to see if a variable has already been declared, without reserving,
+  # walks up to the root scope.
+  check: (name, options) ->
+    immediate = Object::hasOwnProperty.call @variables, name
+    return immediate if immediate or (options and options.immediate)
     !!(@parent and @parent.check(name))
 
   # If we need to store an intermediate result, find an available name for a

test/test_comprehensions.coffee

@@ -141,3 +141,14 @@ ok all.sort().join(' ') is 'Whiskers cream tabby'
 exxes = 'x' for [0...10]
 ok exxes.join(' ') is 'x x x x x x x x x x'
 
+
+# Comprehensions safely redeclare parameters if they're not present in closest
+# scope.
+rule = (x) -> x
+
+learn = ->
+  rule for rule in [1, 2, 3]
+
+ok learn().join(' ') is '1 2 3'
+
+ok rule(101) is 101