Fix #2047: Infinite loop possible when for loop with range uses variables (#4853)

* fix #2047

* Additional check for 'step'; tests

* Fix #4105 (#4855)

* Update output

* Throw warning for unsupported runtimes, e.g. Node < 6 (#4839)

* fix #1403 (#4854)

* Update output

* [Change]: Destructuring with non-final spread should still use rest syntax (#4517) (#4825)

* destructuring optimization

* refactor

* minor improvement, fix errors

* minor refactoring

* improvements

* Update output

* Update output

* Fix #4843: bad output when assigning to @prop in destructuring assignment with defaults (#4848)

* fix #4843

* improvements

* typo

* small fix

* Fix #3441: parentheses wrapping expression throw invalid error  (#4849)

* fix #3441

* improvements

* refactor

* Fix #1726: expression in property access causes unexpected results (#4851)

* fix #1726

* Explain what's happening, rather than just linking to an issue

* Updated output

* Optimization

* Update output

* remove casting to number

* cleanup tests

Author: zdenko

lib/coffeescript/nodes.js

@@ -1359,14 +1359,27 @@ exports.Range = class Range extends Base
     [lt, gt] = ["#{idx} <#{@equals}", "#{idx} >#{@equals}"]
 
     # Generate the condition.
-    condPart = if @stepNum?
-      if @stepNum > 0 then "#{lt} #{@toVar}" else "#{gt} #{@toVar}"
-    else if known
-      [from, to] = [@fromNum, @toNum]
-      if from <= to then "#{lt} #{to}" else "#{gt} #{to}"
-    else
-      cond = if @stepVar then "#{@stepVar} > 0" else "#{@fromVar} <= #{@toVar}"
-      "#{cond} ? #{lt} #{@toVar} : #{gt} #{@toVar}"
+    [from, to] = [@fromNum, @toNum]
+    # Always check if the `step` isn't zero to avoid the infinite loop.
+    stepCond = if @stepNum then "#{@stepNum} !== 0" else "#{@stepVar} !== 0"
+    condPart =
+      if known
+        unless @step?
+          if from <= to then "#{lt} #{to}" else "#{gt} #{to}"
+        else
+          # from < to
+          lowerBound = "#{from} <= #{idx} && #{lt} #{to}"
+          # from > to
+          upperBound = "#{from} >= #{idx} && #{gt} #{to}"
+          if from <= to then "#{stepCond} && #{lowerBound}" else "#{stepCond} && #{upperBound}"
+      else
+        # from < to
+        lowerBound = "#{@fromVar} <= #{idx} && #{lt} #{@toVar}"
+        # from > to
+        upperBound = "#{@fromVar} >= #{idx} && #{gt} #{@toVar}"
+        "#{stepCond} && (#{@fromVar} <= #{@toVar} ? #{lowerBound} : #{upperBound})"
+
+    cond = if @stepVar then "#{@stepVar} > 0" else "#{@fromVar} <= #{@toVar}"
 
     # Generate the step.
     stepPart = if @stepVar

lib/coffeescript/rewriter.js

@@ -116,3 +116,82 @@ test "#1012 slices with arguments object", ->
 
 test "#1409: creating large ranges outside of a function body", ->
   CoffeeScript.eval '[0..100]'
+
+test "#2047: Infinite loop possible when `for` loop with `range` uses variables", ->
+  up = 1
+  down = -1
+  a = 1
+  b = 5
+
+  testRange = (arg) ->
+    [from, to, step, expectedResult] = arg
+    r = (x for x in [from..to] by step)
+    arrayEq r, expectedResult
+
+  testData = [
+    [1, 5, 1, [1..5]]
+    [1, 5, -1, [1]]
+    [1, 5, up, [1..5]]
+    [1, 5, down, [1]]
+
+    [a, 5, 1, [1..5]]
+    [a, 5, -1, [1]]
+    [a, 5, up, [1..5]]
+    [a, 5, down, [1]]
+
+    [1, b, 1, [1..5]]
+    [1, b, -1, [1]]
+    [1, b, up, [1..5]]
+    [1, b, down, [1]]
+
+    [a, b, 1, [1..5]]
+    [a, b, -1, [1]]
+    [a, b, up, [1..5]]
+    [a, b, down, [1]]
+
+    [5, 1, 1, [5]]
+    [5, 1, -1, [5..1]]
+    [5, 1, up, [5]]
+    [5, 1, down,  [5..1]]
+
+    [5, a, 1, [5]]
+    [5, a, -1, [5..1]]
+    [5, a, up, [5]]
+    [5, a, down, [5..1]]
+
+    [b, 1, 1, [5]]
+    [b, 1, -1, [5..1]]
+    [b, 1, up, [5]]
+    [b, 1, down, [5..1]]
+
+    [b, a, 1, [5]]
+    [b, a, -1, [5..1]]
+    [b, a, up, [5]]
+    [b, a, down, [5..1]]
+  ]
+
+  testRange d for d in testData
+
+test "#2047: from, to and step as variables", ->
+  up = 1
+  down = -1
+  a = 1
+  b = 5
+
+  r = (x for x in [a..b] by up)
+  arrayEq r, [1..5]
+
+  r = (x for x in [a..b] by down)
+  arrayEq r, [1]
+
+  r = (x for x in [b..a] by up)
+  arrayEq r, [5]
+
+  r = (x for x in [b..a] by down)
+  arrayEq r, [5..1]
+
+  a = 1
+  b = -1
+  step = 0
+  r = (x for x in [b..a] by step)
+  arrayEq r, []