5 files changed, 34 insertions, 0 deletions
diff --git a/include/clang/Basic/DiagnosticGroups.td b/include/clang/Basic/DiagnosticGroups.td
index 87c6c2d19e..d3cc08319a 100644
--- a/include/clang/Basic/DiagnosticGroups.td
+++ b/include/clang/Basic/DiagnosticGroups.td
@@ -109,6 +109,7 @@ def : DiagGroup<"stack-protector">;
 def : DiagGroup<"switch-default">;
 def : DiagGroup<"synth">;
 def TautologicalCompare : DiagGroup<"tautological-compare">;
+def HeaderHygiene : DiagGroup<"header-hygiene">;
 
 // Preprocessor warnings.
 def : DiagGroup<"builtin-macro-redefined">;
diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index c0d0642d63..a722f0514f 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2928,6 +2928,9 @@ def warn_overloaded_virtual : Warning<
   InGroup<OverloadedVirtual>, DefaultIgnore;
 def note_hidden_overloaded_virtual_declared_here : Note<
   "hidden overloaded virtual function %q0 declared here">;
+def warn_using_directive_in_header : Warning<
+  "using namespace directive in global context in header">,
+  InGroup<HeaderHygiene>, DefaultIgnore;
 
 def err_conditional_void_nonvoid : Error<
   "%select{left|right}1 operand to ? is void, but %select{right|left}1 operand "
diff --git a/lib/Sema/SemaDeclCXX.cpp b/lib/Sema/SemaDeclCXX.cpp
index 858a5f9251..8b7bbb6954 100644
--- a/lib/Sema/SemaDeclCXX.cpp
+++ b/lib/Sema/SemaDeclCXX.cpp
@@ -3922,6 +3922,12 @@ Decl *Sema::ActOnUsingDirective(Scope *S,
     UDir = UsingDirectiveDecl::Create(Context, CurContext, UsingLoc, NamespcLoc,
                                       SS.getWithLocInContext(Context),
                                       IdentLoc, Named, CommonAncestor);
+
+    if (CurContext->getDeclKind() == Decl::TranslationUnit &&
+        !SourceMgr.isFromMainFile(IdentLoc)) {
+      Diag(IdentLoc, diag::warn_using_directive_in_header);
+    }
+
     PushUsingDirective(S, UDir);
   } else {
     Diag(IdentLoc, diag::err_expected_namespace_name) << SS.getRange();
diff --git a/test/SemaCXX/warn-using-namespace-in-header.cpp b/test/SemaCXX/warn-using-namespace-in-header.cpp
new file mode 100644
index 0000000000..393e097df5
--- /dev/null
+++ b/test/SemaCXX/warn-using-namespace-in-header.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fsyntax-only -Wheader-hygiene -verify %s
+
+#include "warn-using-namespace-in-header.h"
+
+namespace dont_warn {}
+using namespace dont_warn;
+
+// Warning is actually in the header but only the cpp file gets scanned.
+// expected-warning {{using namespace directive in global context in header}}
diff --git a/test/SemaCXX/warn-using-namespace-in-header.h b/test/SemaCXX/warn-using-namespace-in-header.h
new file mode 100644
index 0000000000..677c4ac51a
--- /dev/null
+++ b/test/SemaCXX/warn-using-namespace-in-header.h
@@ -0,0 +1,15 @@
+
+
+
+
+
+// Lots of vertical space to make the error line match up with the line of the
+// expected line in the source file.
+namespace warn_in_header_in_global_context {}
+using namespace warn_in_header_in_global_context;
+
+// While we want to error on the previous using directive, we don't when we are
+// inside a namespace
+namespace dont_warn_here {
+using namespace warn_in_header_in_global_context;
+}