Index: llvm/trunk/include/llvm/ADT/DenseSet.h
===================================================================
--- llvm/trunk/include/llvm/ADT/DenseSet.h
+++ llvm/trunk/include/llvm/ADT/DenseSet.h
@@ -90,9 +90,12 @@
 
   // Iterators.
 
+  class ConstIterator;
+
   class Iterator {
     typename MapTy::iterator I;
     friend class DenseSetImpl;
+    friend class ConstIterator;
 
   public:
     typedef typename MapTy::iterator::difference_type difference_type;
@@ -101,6 +104,7 @@
     typedef value_type &reference;
     typedef std::forward_iterator_tag iterator_category;
 
+    Iterator() = default;
     Iterator(const typename MapTy::iterator &i) : I(i) {}
 
     ValueT &operator*() { return I->getFirst(); }
@@ -110,13 +114,14 @@
 
     Iterator& operator++() { ++I; return *this; }
     Iterator operator++(int) { auto T = *this; ++I; return T; }
-    bool operator==(const Iterator& X) const { return I == X.I; }
-    bool operator!=(const Iterator& X) const { return I != X.I; }
+    bool operator==(const ConstIterator& X) const { return I == X.I; }
+    bool operator!=(const ConstIterator& X) const { return I != X.I; }
   };
 
   class ConstIterator {
     typename MapTy::const_iterator I;
     friend class DenseSet;
+    friend class Iterator;
 
   public:
     typedef typename MapTy::const_iterator::difference_type difference_type;
@@ -125,6 +130,10 @@
     typedef value_type &reference;
     typedef std::forward_iterator_tag iterator_category;
 
+    ConstIterator(const Iterator &B) : I(B.I) {}
+
+    ConstIterator() = default;
+
     ConstIterator(const typename MapTy::const_iterator &i) : I(i) {}
 
     const ValueT &operator*() const { return I->getFirst(); }
Index: llvm/trunk/unittests/ADT/DenseSetTest.cpp
===================================================================
--- llvm/trunk/unittests/ADT/DenseSetTest.cpp
+++ llvm/trunk/unittests/ADT/DenseSetTest.cpp
@@ -73,6 +73,15 @@
   EXPECT_EQ(0u, set.count(3));
 }
 
+TYPED_TEST(DenseSetTest, ConstIteratorComparison){
+  TypeParam set({1});
+  const TypeParam &cset = set;
+  EXPECT_EQ(set.begin(), cset.begin());
+  EXPECT_EQ(set.end(), cset.end());
+  EXPECT_NE(set.end(), cset.begin());
+  EXPECT_NE(set.begin(), cset.end());
+}
+
 TYPED_TEST(DenseSetTest, EmptyInitializerList) {
   TypeParam set({});
   EXPECT_EQ(0u, set.size());