Index: include/llvm/ADT/IntervalMap.h
===================================================================
--- include/llvm/ADT/IntervalMap.h
+++ include/llvm/ADT/IntervalMap.h
@@ -150,6 +150,12 @@
     return a+1 == b;
   }
 
+  /// nonEmpty - Return true if [a;b] is non-empty.
+  /// This is a <= b for a closed interval, a < b for [a;b) half-open intervals.
+  static inline bool nonEmpty(const T &a, const T &b) {
+    return a <= b;
+  }
+
 };
 
 template <typename T>
@@ -170,6 +176,11 @@
     return a == b;
   }
 
+  /// nonEmpty - Return true if [a;b) is non-empty.
+  static inline bool nonEmpty(const T &a, const T &b) {
+    return a < b;
+  }
+
 };
 
 /// IntervalMapImpl - Namespace used for IntervalMap implementation details.
@@ -1669,7 +1680,7 @@
 template <typename KeyT, typename ValT, unsigned N, typename Traits>
 void IntervalMap<KeyT, ValT, N, Traits>::
 iterator::setStart(KeyT a) {
-  assert(Traits::stopLess(a, this->stop()) && "Cannot move start beyond stop");
+  assert(Traits::nonEmpty(a, this->stop()) && "Cannot move start beyond stop");
   KeyT &CurStart = this->unsafeStart();
   if (!Traits::startLess(a, CurStart) || !canCoalesceLeft(a, this->value())) {
     CurStart = a;
@@ -1685,7 +1696,7 @@
 template <typename KeyT, typename ValT, unsigned N, typename Traits>
 void IntervalMap<KeyT, ValT, N, Traits>::
 iterator::setStop(KeyT b) {
-  assert(Traits::stopLess(this->start(), b) && "Cannot move stop beyond start");
+  assert(Traits::nonEmpty(this->start(), b) && "Cannot move stop beyond start");
   if (Traits::startLess(b, this->stop()) ||
       !canCoalesceRight(b, this->value())) {
     setStopUnchecked(b);
Index: unittests/ADT/IntervalMapTest.cpp
===================================================================
--- unittests/ADT/IntervalMapTest.cpp
+++ unittests/ADT/IntervalMapTest.cpp
@@ -15,6 +15,8 @@
 namespace {
 
 typedef IntervalMap<unsigned, unsigned, 4> UUMap;
+typedef IntervalMap<unsigned, unsigned, 4,
+                    IntervalMapHalfOpenInfo<unsigned>> UUHalfOpenMap;
 
 // Empty map tests
 TEST(IntervalMapTest, EmptyMap) {
@@ -125,18 +127,63 @@
   EXPECT_EQ(200u, I.stop());
   EXPECT_EQ(2u, I.value());
 
+  // Shrink the interval to have a length of 1
+  I.setStop(150);
+  ASSERT_TRUE(I.valid());
+  EXPECT_EQ(150u, I.start());
+  EXPECT_EQ(150u, I.stop());
+  EXPECT_EQ(2u, I.value());
+
   I.setStop(160);
   ASSERT_TRUE(I.valid());
   EXPECT_EQ(150u, I.start());
   EXPECT_EQ(160u, I.stop());
   EXPECT_EQ(2u, I.value());
 
+  // Shrink the interval to have a length of 1
+  I.setStart(160);
+  ASSERT_TRUE(I.valid());
+  EXPECT_EQ(160u, I.start());
+  EXPECT_EQ(160u, I.stop());
+  EXPECT_EQ(2u, I.value());
+
   // Erase last elem.
   I.erase();
   EXPECT_TRUE(map.empty());
   EXPECT_EQ(0, std::distance(map.begin(), map.end()));
 }
 
+// Single entry half-open map tests
+TEST(IntervalMapTest, SingleEntryHalfOpenMap) {
+  UUHalfOpenMap::Allocator allocator;
+  UUHalfOpenMap map(allocator);
+  map.insert(100, 150, 1);
+  EXPECT_FALSE(map.empty());
+
+  UUHalfOpenMap::iterator I = map.begin();
+  ASSERT_TRUE(I.valid());
+
+  // Shrink the interval to have a length of 1
+  I.setStart(149);
+  ASSERT_TRUE(I.valid());
+  EXPECT_EQ(149u, I.start());
+  EXPECT_EQ(150u, I.stop());
+  EXPECT_EQ(1u, I.value());
+
+  I.setStop(160);
+  ASSERT_TRUE(I.valid());
+  EXPECT_EQ(149u, I.start());
+  EXPECT_EQ(160u, I.stop());
+  EXPECT_EQ(1u, I.value());
+
+  // Shrink the interval to have a length of 1
+  I.setStop(150);
+  ASSERT_TRUE(I.valid());
+  EXPECT_EQ(149u, I.start());
+  EXPECT_EQ(150u, I.stop());
+  EXPECT_EQ(1u, I.value());
+}
+
 // Flat coalescing tests.
 TEST(IntervalMapTest, RootCoalescing) {
   UUMap::Allocator allocator;