Allow DenseSet::iterators to be conveted to and compared with const_iterator

[varno]

Summary

This seemed to be an oversight seeing as DenseMap has these conversions.

This patch does the following:

• Adds a default constructor to the iterators.
• Allows DenseSet::ConstIterators to be copy constructed from DenseSet::Iterators
• Allows mutual comparison between Iterators and ConstIterators.

All of these are available in the DenseMap implementation, so the implementation here is trivial.

[dberris]

include/llvm/ADT/DenseSet.h

107

Spurious ;

135

Spurious ;

[varno]

• address review comments
• add tests

[varno]

• add space

[varno]

I cannot commit myself, could someone do this for me?

include/llvm/ADT/DenseSet.h

public:
   bool erase(const ValueT &V) {
     return TheMap.erase(V);
   }
 
   void swap(DenseSetImpl &RHS) { TheMap.swap(RHS.TheMap); }
 
   // Iterators.
 
+  class ConstIterator;
+
   class Iterator {
     typename MapTy::iterator I;
     friend class DenseSetImpl;
+    friend class ConstIterator;
 
   public:
     typedef typename MapTy::iterator::difference_type difference_type;
     typedef ValueT value_type;
     typedef value_type *pointer;
     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(); }
     const ValueT &operator*() const { return I->getFirst(); }
     ValueT *operator->() { return &I->getFirst(); }
     const ValueT *operator->() const { return &I->getFirst(); }
 
     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;
     typedef ValueT value_type;
     typedef value_type *pointer;
     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(); }
     const ValueT *operator->() const { return &I->getFirst(); }
 
     ConstIterator& operator++() { ++I; return *this; }
     ConstIterator operator++(int) { auto T = *this; ++I; return T; }
     bool operator==(const ConstIterator& X) const { return I == X.I; }

unittests/ADT/DenseSetTest.cpp

TYPED_TEST(DenseSetTest, InitializerList) {
   TypeParam set({1, 2, 1, 4});
   EXPECT_EQ(3u, set.size());
   EXPECT_EQ(1u, set.count(1));
   EXPECT_EQ(1u, set.count(2));
   EXPECT_EQ(1u, set.count(4));
   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());
   EXPECT_EQ(0u, set.count(0));
 }
 
 TYPED_TEST(DenseSetTest, FindAsTest) {
   auto &set = this->Set;