Fix false positive on anonymous namespaces in headers.

[sbenza]

Summary

Anynoumous namespaces inject a using directive into the AST to import
the names into the containing namespace.
We should not have them in headers, but there is another warning for
that.

[alexfh]

One more case where not matching implicit code would be useful.

Anyway, the patch looks good!

clang-tools-extra/trunk/clang-tidy/google/GlobalNamesInHeadersCheck.cpp

if (Result.SourceManager->isInMainFile(
     // unless that file is a header.
     StringRef Filename = Result.SourceManager->getFilename(
         Result.SourceManager->getSpellingLoc(D->getLocStart()));
 
     if (!Filename.endswith(".h"))
       return;
   }
 
+  if (const auto* UsingDirective = dyn_cast<UsingDirectiveDecl>(D)) {
+    if (UsingDirective->getNominatedNamespace()->isAnonymousNamespace()) {
+      // Anynoumous namespaces inject a using directive into the AST to import
+      // the names into the containing namespace.
+      // We should not have them in headers, but there is another warning for
+      // that.
+      return;
+    }
+  }
+
   diag(D->getLocStart(),
        "using declarations in the global namespace in headers are prohibited");
 }
 
 } // namespace readability
 } // namespace google
 } // namespace tidy
 } // namespace clang

clang-tools-extra/trunk/unittests/clang-tidy/GoogleModuleTest.cpp

TEST_F(GlobalNamesInHeadersCheckTest, UsingDirectives) {
   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"));
 }
 
+TEST_F(GlobalNamesInHeadersCheckTest, RegressionAnonymousNamespace) {
+  EXPECT_FALSE(runCheckOnCode("namespace {}", "foo.h"));
+}
+
 } // namespace test
 } // namespace tidy
 } // namespace clang