Index: lib/Format/UsingDeclarationsSorter.cpp
===================================================================
--- lib/Format/UsingDeclarationsSorter.cpp
+++ lib/Format/UsingDeclarationsSorter.cpp
@@ -34,7 +34,7 @@
       : Line(Line), Label(Label) {}
 
   bool operator<(const UsingDeclaration &Other) const {
-    return Label < Other.Label;
+    return StringRef(Label).compare_lower(Other.Label) < 0;
   }
 };
 
@@ -78,7 +78,8 @@
     const SourceManager &SourceMgr, tooling::Replacements *Fixes) {
   SmallVector<UsingDeclaration, 4> SortedUsingDeclarations(
       UsingDeclarations->begin(), UsingDeclarations->end());
-  std::sort(SortedUsingDeclarations.begin(), SortedUsingDeclarations.end());
+  std::stable_sort(SortedUsingDeclarations.begin(),
+                   SortedUsingDeclarations.end());
   for (size_t I = 0, E = UsingDeclarations->size(); I < E; ++I) {
     if ((*UsingDeclarations)[I].Line == SortedUsingDeclarations[I].Line)
       continue;
Index: unittests/Format/UsingDeclarationsSorterTest.cpp
===================================================================
--- unittests/Format/UsingDeclarationsSorterTest.cpp
+++ unittests/Format/UsingDeclarationsSorterTest.cpp
@@ -86,6 +86,77 @@
                                   "using a, b;"));
 }
 
+TEST_F(UsingDeclarationsSorterTest, SortsCaseInsensitively) {
+  EXPECT_EQ("using A;\n"
+            "using a;",
+            sortUsingDeclarations("using A;\n"
+                                  "using a;"));
+  EXPECT_EQ("using a;\n"
+            "using A;",
+            sortUsingDeclarations("using a;\n"
+                                  "using A;"));
+  EXPECT_EQ("using a;\n"
+            "using B;",
+            sortUsingDeclarations("using B;\n"
+                                  "using a;"));
+  EXPECT_EQ("using _;\n"
+            "using a;",
+            sortUsingDeclarations("using a;\n"
+                                  "using _;"));
+  EXPECT_EQ("using a::_;\n"
+            "using a::a;",
+            sortUsingDeclarations("using a::a;\n"
+                                  "using a::_;"));
+
+  EXPECT_EQ("using ::testing::_;\n"
+            "using ::testing::Aardvark;\n"
+            "using ::testing::apple::Honeycrisp;\n"
+            "using ::testing::Xylophone;\n"
+            "using ::testing::zebra::Stripes;",
+            sortUsingDeclarations("using ::testing::Aardvark;\n"
+                                  "using ::testing::Xylophone;\n"
+                                  "using ::testing::_;\n"
+                                  "using ::testing::apple::Honeycrisp;\n"
+                                  "using ::testing::zebra::Stripes;"));
+}
+
+TEST_F(UsingDeclarationsSorterTest, SortsStably) {
+  EXPECT_EQ("using a;\n"
+            "using a;\n"
+            "using A;\n"
+            "using a;\n"
+            "using A;\n"
+            "using a;\n"
+            "using A;\n"
+            "using a;\n"
+            "using B;\n"
+            "using b;\n"
+            "using b;\n"
+            "using B;\n"
+            "using b;\n"
+            "using b;\n"
+            "using b;\n"
+            "using B;\n"
+            "using b;",
+            sortUsingDeclarations("using a;\n"
+                                  "using B;\n"
+                                  "using a;\n"
+                                  "using b;\n"
+                                  "using A;\n"
+                                  "using a;\n"
+                                  "using b;\n"
+                                  "using B;\n"
+                                  "using b;\n"
+                                  "using A;\n"
+                                  "using a;\n"
+                                  "using b;\n"
+                                  "using b;\n"
+                                  "using B;\n"
+                                  "using b;\n"
+                                  "using A;\n"
+                                  "using a;"));
+}
+
 TEST_F(UsingDeclarationsSorterTest, SortsMultipleTopLevelDeclarations) {
   EXPECT_EQ("using a;\n"
             "using b;\n"