Index: lib/Format/UsingDeclarationsSorter.cpp
===================================================================
--- lib/Format/UsingDeclarationsSorter.cpp
+++ lib/Format/UsingDeclarationsSorter.cpp
@@ -33,8 +33,27 @@
   UsingDeclaration(const AnnotatedLine *Line, const std::string &Label)
       : Line(Line), Label(Label) {}
 
+  // Compares lexicographically with the exception that '_' is just before 'A'.
   bool operator<(const UsingDeclaration &Other) const {
-    return StringRef(Label).compare_lower(Other.Label) < 0;
+    return computeSortLabel(Label) < computeSortLabel(Other.Label);
+  }
+
+  // Returns the position of c in a lexicographic ordering with the exception
+  // that '_' is just before 'A'.
+  static char rank(char c) {
+    if (c == '_')
+      return 'A';
+    if ('A' <= c && c < '_')
+      return c + 1;
+    return c;
+  }
+
+  static std::string computeSortLabel(const std::string &Label) {
+    std::string SortLabel;
+    SortLabel.reserve(Label.size());
+    for (char C : Label)
+      SortLabel += rank(C);
+    return SortLabel;
   }
 };
 
@@ -77,7 +96,7 @@
     SmallVectorImpl<UsingDeclaration> *UsingDeclarations,
     const SourceManager &SourceMgr, tooling::Replacements *Fixes) {
   bool BlockAffected = false;
-  for (const UsingDeclaration& Declaration : *UsingDeclarations) {
+  for (const UsingDeclaration &Declaration : *UsingDeclarations) {
     if (Declaration.Line->Affected) {
       BlockAffected = true;
       break;
Index: unittests/Format/UsingDeclarationsSorterTest.cpp
===================================================================
--- unittests/Format/UsingDeclarationsSorterTest.cpp
+++ unittests/Format/UsingDeclarationsSorterTest.cpp
@@ -86,19 +86,25 @@
                                   "using a, b;"));
 }
 
-TEST_F(UsingDeclarationsSorterTest, SortsCaseInsensitively) {
+TEST_F(UsingDeclarationsSorterTest, SortsCaseSensitively) {
   EXPECT_EQ("using A;\n"
             "using a;",
             sortUsingDeclarations("using A;\n"
                                   "using a;"));
-  EXPECT_EQ("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;",
+  EXPECT_EQ("using B;\n"
+            "using a;",
             sortUsingDeclarations("using B;\n"
                                   "using a;"));
+
+  // Sorts '_' right before 'A'.
+  EXPECT_EQ("using _;\n"
+            "using A;",
+            sortUsingDeclarations("using A;\n"
+                                  "using _;"));
   EXPECT_EQ("using _;\n"
             "using a;",
             sortUsingDeclarations("using a;\n"
@@ -110,8 +116,8 @@
 
   EXPECT_EQ("using ::testing::_;\n"
             "using ::testing::Aardvark;\n"
-            "using ::testing::apple::Honeycrisp;\n"
             "using ::testing::Xylophone;\n"
+            "using ::testing::apple::Honeycrisp;\n"
             "using ::testing::zebra::Stripes;",
             sortUsingDeclarations("using ::testing::Aardvark;\n"
                                   "using ::testing::Xylophone;\n"
@@ -120,43 +126,6 @@
                                   "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"