Index: clang-tools-extra/trunk/clang-tidy/google/CMakeLists.txt
===================================================================
--- clang-tools-extra/trunk/clang-tidy/google/CMakeLists.txt
+++ clang-tools-extra/trunk/clang-tidy/google/CMakeLists.txt
@@ -4,6 +4,7 @@
   AvoidCStyleCastsCheck.cpp
   ExplicitConstructorCheck.cpp
   ExplicitMakePairCheck.cpp
+  GlobalNamesInHeadersCheck.cpp
   GoogleTidyModule.cpp
   IntegerTypesCheck.cpp
   MemsetZeroLengthCheck.cpp
Index: clang-tools-extra/trunk/clang-tidy/google/GlobalNamesInHeadersCheck.h
===================================================================
--- clang-tools-extra/trunk/clang-tidy/google/GlobalNamesInHeadersCheck.h
+++ clang-tools-extra/trunk/clang-tidy/google/GlobalNamesInHeadersCheck.h
@@ -0,0 +1,34 @@
+//===--- GlobalNamesInHeadersCheck.h - clang-tidy ---------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_GOOGLE_GLOBAL_NAMES_IN_HEADERS_CHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_GOOGLE_GLOBAL_NAMES_IN_HEADERS_CHECK_H
+
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+namespace readability {
+
+// Flag global namespace pollution in header files.
+// Right now it only triggers on using declarations and directives.
+class GlobalNamesInHeadersCheck : public ClangTidyCheck {
+public:
+  GlobalNamesInHeadersCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace readability
+} // namespace tidy
+} // namespace clang
+
+#endif  // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_GOOGLE_GLOBAL_NAMES_IN_HEADERS_CHECK_H
+
Index: clang-tools-extra/trunk/clang-tidy/google/GlobalNamesInHeadersCheck.cpp
===================================================================
--- clang-tools-extra/trunk/clang-tidy/google/GlobalNamesInHeadersCheck.cpp
+++ clang-tools-extra/trunk/clang-tidy/google/GlobalNamesInHeadersCheck.cpp
@@ -0,0 +1,53 @@
+//===--- GlobalNamesInHeadersCheck.cpp - clang-tidy -----------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "GlobalNamesInHeadersCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Lex/Lexer.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace readability {
+
+void
+GlobalNamesInHeadersCheck::registerMatchers(ast_matchers::MatchFinder *Finder) {
+  Finder->addMatcher(
+      decl(anyOf(usingDecl(), usingDirectiveDecl()),
+           hasDeclContext(translationUnitDecl())).bind("using_decl"),
+      this);
+}
+
+void GlobalNamesInHeadersCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *D = Result.Nodes.getNodeAs<Decl>("using_decl");
+  // If it comes from a macro, we'll assume it is fine.
+  if (D->getLocStart().isMacroID())
+    return;
+
+  // Ignore if it comes from the "main" file ...
+  if (Result.SourceManager->isInMainFile(
+          Result.SourceManager->getExpansionLoc(D->getLocStart()))) {
+    // unless that file is a header.
+    StringRef Filename = Result.SourceManager->getFilename(
+        Result.SourceManager->getSpellingLoc(D->getLocStart()));
+
+    if (!Filename.endswith(".h"))
+      return;
+  }
+
+  diag(D->getLocStart(),
+       "using declarations in the global namespace in headers are prohibited");
+}
+
+} // namespace readability
+} // namespace tidy
+} // namespace clang
Index: clang-tools-extra/trunk/clang-tidy/google/GoogleTidyModule.cpp
===================================================================
--- clang-tools-extra/trunk/clang-tidy/google/GoogleTidyModule.cpp
+++ clang-tools-extra/trunk/clang-tidy/google/GoogleTidyModule.cpp
@@ -17,6 +17,7 @@
 #include "AvoidCStyleCastsCheck.h"
 #include "ExplicitConstructorCheck.h"
 #include "ExplicitMakePairCheck.h"
+#include "GlobalNamesInHeadersCheck.h"
 #include "IntegerTypesCheck.h"
 #include "MemsetZeroLengthCheck.h"
 #include "NamedParameterCheck.h"
@@ -58,6 +59,8 @@
         "google-readability-todo");
     CheckFactories.registerCheck<readability::BracesAroundStatementsCheck>(
         "google-readability-braces-around-statements");
+    CheckFactories.registerCheck<readability::GlobalNamesInHeadersCheck>(
+        "google-global-names-in-headers");
     CheckFactories.registerCheck<readability::FunctionSizeCheck>(
         "google-readability-function-size");
     CheckFactories.registerCheck<readability::NamespaceCommentCheck>(
Index: clang-tools-extra/trunk/unittests/clang-tidy/GoogleModuleTest.cpp
===================================================================
--- clang-tools-extra/trunk/unittests/clang-tidy/GoogleModuleTest.cpp
+++ clang-tools-extra/trunk/unittests/clang-tidy/GoogleModuleTest.cpp
@@ -1,5 +1,6 @@
 #include "ClangTidyTest.h"
 #include "google/ExplicitConstructorCheck.h"
+#include "google/GlobalNamesInHeadersCheck.h"
 #include "gtest/gtest.h"
 
 namespace clang {
@@ -56,6 +57,51 @@
           "A(Foo);"));
 }
 
+class GlobalNamesInHeadersCheckTest : public ::testing::Test {
+protected:
+  bool runCheckOnCode(const std::string &Code, const std::string &Filename) {
+    static const char *const Header = "namespace std {\n"
+                                      "class string {};\n"
+                                      "}  // namespace std\n"
+                                      "\n"
+                                      "#define SOME_MACRO(x) using x\n";
+    std::vector<ClangTidyError> Errors;
+    std::vector<std::string> Args;
+    if (!StringRef(Filename).endswith(".cpp")) {
+      Args.emplace_back("-xc++-header");
+    }
+    test::runCheckOnCode<readability::GlobalNamesInHeadersCheck>(
+        Header + Code, &Errors, Filename, Args);
+    if (Errors.empty())
+      return false;
+    assert(Errors.size() == 1);
+    assert(
+        Errors[0].Message.Message ==
+        "using declarations in the global namespace in headers are prohibited");
+    return true;
+  }
+};
+
+TEST_F(GlobalNamesInHeadersCheckTest, UsingDeclarations) {
+  EXPECT_TRUE(runCheckOnCode("using std::string;", "foo.h"));
+  EXPECT_FALSE(runCheckOnCode("using std::string;", "foo.cpp"));
+  EXPECT_FALSE(runCheckOnCode("namespace my_namespace {\n"
+                              "using std::string;\n"
+                              "}  // my_namespace\n",
+                              "foo.h"));
+  EXPECT_FALSE(runCheckOnCode("SOME_MACRO(std::string);", "foo.h"));
+}
+
+TEST_F(GlobalNamesInHeadersCheckTest, UsingDirectives) {
+  EXPECT_TRUE(runCheckOnCode("using namespace std;", "foo.h"));
+  EXPECT_FALSE(runCheckOnCode("using namespace std;", "foo.cpp"));
+  EXPECT_FALSE(runCheckOnCode("namespace my_namespace {\n"
+                              "using namespace std;\n"
+                              "}  // my_namespace\n",
+                              "foo.h"));
+  EXPECT_FALSE(runCheckOnCode("SOME_MACRO(namespace std);", "foo.h"));
+}
+
 } // namespace test
 } // namespace tidy
 } // namespace clang