diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
index a5057c9d3200..9811bab2936f 100644
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -20,7 +20,9 @@ def warn_file_asm_volatile : Warning<
 
 let CategoryName = "Parse Issue" in {
 
-def ext_empty_source_file : Extension<"ISO C forbids an empty source file">;
+def ext_empty_translation_unit : Extension<
+  "ISO C requires a translation unit to contain at least one declaration.">,
+  InGroup<DiagGroup<"empty-translation-unit">>;
 def warn_cxx98_compat_top_level_semi : Warning<
   "extra ';' outside of a function is incompatible with C++98">,
   InGroup<CXX98CompatPedantic>, DefaultIgnore;
diff --git a/clang/include/clang/Basic/SourceManager.h b/clang/include/clang/Basic/SourceManager.h
index 3164f874f152..f7fb1f523140 100644
--- a/clang/include/clang/Basic/SourceManager.h
+++ b/clang/include/clang/Basic/SourceManager.h
@@ -584,6 +584,9 @@ class SourceManager : public RefCountedBase<SourceManager> {
   /// \brief The file ID for the precompiled preamble there is one.
   FileID PreambleFileID;
 
+  /// \brief The file ID for the preprocessor's predefines.
+  FileID PredefinesFileID;
+
   // Statistics for -print-stats.
   mutable unsigned NumLinearScans, NumBinaryProbes;
 
@@ -628,6 +631,14 @@ public:
     MainFileID = createFileIDForMemBuffer(Buffer);
     return MainFileID;
   }
+  
+  /// \brief Create the FileID for a memory buffer that contains the
+  /// preprocessor's predefines.
+  FileID createPredefinesFileIDForMemBuffer(const llvm::MemoryBuffer *Buffer) {
+    assert(PredefinesFileID.isInvalid() && "PredefinesFileID already set!");
+    PredefinesFileID = createFileIDForMemBuffer(Buffer);
+    return PredefinesFileID;
+  }
 
   //===--------------------------------------------------------------------===//
   // MainFileID creation and querying methods.
@@ -636,6 +647,9 @@ public:
   /// getMainFileID - Returns the FileID of the main source file.
   FileID getMainFileID() const { return MainFileID; }
 
+  /// \brief Returns the FileID of the preprocessor predefines buffer.
+  FileID getPredefinesFileID() const { return PredefinesFileID; }
+
   /// createMainFileID - Create the FileID for the main source file.
   FileID createMainFileID(const FileEntry *SourceFile, 
                           SrcMgr::CharacteristicKind Kind = SrcMgr::C_User) {
@@ -1113,6 +1127,12 @@ public:
     return getFileID(Loc) == getMainFileID();
   }
 
+  /// isFromPredefines - Returns true if the provided SourceLocation is
+  ///   within the processor's predefines buffer.
+  bool isFromPredefines(SourceLocation Loc) const {
+    return getFileID(Loc) == getPredefinesFileID();
+  }
+
   /// isInSystemHeader - Returns if a SourceLocation is in a system header.
   bool isInSystemHeader(SourceLocation Loc) const {
     return getFileCharacteristic(Loc) != SrcMgr::C_User;
diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
index 07d632090112..2222e781e802 100644
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -730,6 +730,9 @@ private:
 public:
   DiagnosticBuilder Diag(SourceLocation Loc, unsigned DiagID);
   DiagnosticBuilder Diag(const Token &Tok, unsigned DiagID);
+  DiagnosticBuilder Diag(unsigned DiagID) {
+    return Diag(Tok, DiagID);
+  }
 
 private:
   void SuggestParentheses(SourceLocation Loc, unsigned DK,
diff --git a/clang/lib/Basic/SourceManager.cpp b/clang/lib/Basic/SourceManager.cpp
index ed920eb48800..783e073e665d 100644
--- a/clang/lib/Basic/SourceManager.cpp
+++ b/clang/lib/Basic/SourceManager.cpp
@@ -407,6 +407,7 @@ SourceManager::~SourceManager() {
 
 void SourceManager::clearIDTables() {
   MainFileID = FileID();
+  PredefinesFileID = FileID();
   LocalSLocEntryTable.clear();
   LoadedSLocEntryTable.clear();
   SLocEntryLoaded.clear();
diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
index d387f4392deb..955c39cfe78d 100644
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -425,7 +425,7 @@ void Preprocessor::EnterMainSourceFile() {
   llvm::MemoryBuffer *SB =
     llvm::MemoryBuffer::getMemBufferCopy(Predefines, "<built-in>");
   assert(SB && "Cannot create predefined source buffer");
-  FileID FID = SourceMgr.createFileIDForMemBuffer(SB);
+  FileID FID = SourceMgr.createPredefinesFileIDForMemBuffer(SB);
   assert(!FID.isInvalid() && "Could not create FileID for predefines?");
 
   // Start parsing the predefines.
diff --git a/clang/lib/Parse/ParseAST.cpp b/clang/lib/Parse/ParseAST.cpp
index d1c2624f8cb3..3f86c4d2c89b 100644
--- a/clang/lib/Parse/ParseAST.cpp
+++ b/clang/lib/Parse/ParseAST.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Parse/ParseAST.h"
+#include "clang/Parse/ParseDiagnostic.h"
 #include "clang/Sema/Sema.h"
 #include "clang/Sema/CodeCompleteConsumer.h"
 #include "clang/Sema/SemaConsumer.h"
@@ -77,27 +78,50 @@ void clang::ParseAST(Sema &S, bool PrintStats, bool SkipFunctionBodies) {
   S.getPreprocessor().EnterMainSourceFile();
   P.Initialize();
   S.Initialize();
-  
-  if (ExternalASTSource *External = S.getASTContext().getExternalSource())
+
+  // C11 6.9p1 says translation units must have at least one top-level
+  // declaration. C++ doesn't have this restriction. We also don't want to
+  // complain if we have a precompiled header, although technically if the PCH
+  // is empty we should still emit the (pedantic) diagnostic.
+  bool WarnForEmptyTU = !S.getLangOpts().CPlusPlus;
+  if (ExternalASTSource *External = S.getASTContext().getExternalSource()) {
     External->StartTranslationUnit(Consumer);
-  
-  bool Abort = false;
+    WarnForEmptyTU = false;
+  }
+
+  // Clang's predefines contain top-level declarations for things like va_list,
+  // making it hard to tell if the /user's/ translation unit has at least one
+  // top-level declaration. So we parse cautiously, looking for a declaration
+  // that doesn't come from our predefines.
+  // Note that ParseTopLevelDecl returns 'true' at EOF.
+  SourceManager &SM = S.getSourceManager();
   Parser::DeclGroupPtrTy ADecl;
-  
-  while (!P.ParseTopLevelDecl(ADecl)) {  // Not end of file.
-    // If we got a null return and something *was* parsed, ignore it.  This
-    // is due to a top-level semicolon, an action override, or a parse error
-    // skipping something.
+  while (WarnForEmptyTU && !P.ParseTopLevelDecl(ADecl)) {
     if (ADecl) {
-      if (!Consumer->HandleTopLevelDecl(ADecl.get())) {
-        Abort = true;
-        break;
+      if (!Consumer->HandleTopLevelDecl(ADecl.get()))
+        return;
+      if (DeclGroupRef::iterator FirstDecl = ADecl.get().begin()) {
+        SourceLocation DeclLoc = (*FirstDecl)->getLocation();
+        WarnForEmptyTU = SM.isFromPredefines(DeclLoc);
       }
     }
-  };
+  }
 
-  if (Abort)
-    return;
+  // If we ended up seeing EOF before any top-level declarations, emit our
+  // diagnostic. Otherwise, parse the rest of the file normally.
+  if (WarnForEmptyTU) {
+    P.Diag(diag::ext_empty_translation_unit);
+  } else {
+    while (!P.ParseTopLevelDecl(ADecl)) {  // Not end of file.
+      // If we got a null return and something *was* parsed, ignore it.  This
+      // is due to a top-level semicolon, an action override, or a parse error
+      // skipping something.
+      if (ADecl) {
+        if (!Consumer->HandleTopLevelDecl(ADecl.get())) 
+          return;
+      }
+    };
+  }
   
   // Process any TopLevelDecls generated by #pragma weak.
   for (SmallVector<Decl*,2>::iterator
diff --git a/clang/lib/Parse/Parser.cpp b/clang/lib/Parse/Parser.cpp
index 504071405b73..f0e2b3aa85f4 100644
--- a/clang/lib/Parse/Parser.cpp
+++ b/clang/lib/Parse/Parser.cpp
@@ -439,10 +439,6 @@ void Parser::Initialize() {
   // Prime the lexer look-ahead.
   ConsumeToken();
 
-  if (Tok.is(tok::eof) &&
-      !getLangOpts().CPlusPlus)  // Empty source file is an extension in C
-    Diag(Tok, diag::ext_empty_source_file);
-
   // Initialization for Objective-C context sensitive keywords recognition.
   // Referenced in Parser::ParseObjCTypeQualifierList.
   if (getLangOpts().ObjC1) {
diff --git a/clang/test/Misc/warning-flags.c b/clang/test/Misc/warning-flags.c
index 98130c5e27ab..cdfb38fb4166 100644
--- a/clang/test/Misc/warning-flags.c
+++ b/clang/test/Misc/warning-flags.c
@@ -17,7 +17,7 @@ This test serves two purposes:
 
 The list of warnings below should NEVER grow.  It should gradually shrink to 0.
 
-CHECK: Warnings without flags (242):
+CHECK: Warnings without flags (241):
 CHECK-NEXT:   ext_anonymous_struct_union_qualified
 CHECK-NEXT:   ext_binary_literal
 CHECK-NEXT:   ext_cast_fn_obj
@@ -26,7 +26,6 @@ CHECK-NEXT:   ext_designated_init
 CHECK-NEXT:   ext_duplicate_declspec
 CHECK-NEXT:   ext_ellipsis_exception_spec
 CHECK-NEXT:   ext_empty_fnmacro_arg
-CHECK-NEXT:   ext_empty_source_file
 CHECK-NEXT:   ext_enum_friend
 CHECK-NEXT:   ext_enum_value_not_int
 CHECK-NEXT:   ext_enumerator_list_comma
diff --git a/clang/test/PCH/empty-with-headers.c b/clang/test/PCH/empty-with-headers.c
new file mode 100644
index 000000000000..751be1c9eb39
--- /dev/null
+++ b/clang/test/PCH/empty-with-headers.c
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c99 -pedantic-errors %s
+// RUN: %clang_cc1 -fsyntax-only -std=c99 -emit-pch -o %t %s
+// RUN: %clang_cc1 -fsyntax-only -std=c99 -pedantic-errors -include-pch %t %s
+
+// RUN: %clang_cc1 -fsyntax-only -std=c99 -pedantic-errors -DINCLUDED %s -verify
+// This last one should warn for -Wempty-translation-unit (C99 6.9p1).
+
+#if defined(INCLUDED)
+
+// empty except for the prefix header
+
+#elif defined(HEADER)
+
+typedef int my_int;
+#define INCLUDED
+
+#else
+
+#define HEADER
+#include "empty-with-headers.c"
+// empty except for the header
+
+#endif
+
+// This should only fire if the header is not included,
+// either explicitly or as a prefix header.
+// expected-error{{ISO C requires a translation unit to contain at least one declaration.}}
diff --git a/clang/test/Parser/completely-empty-header-file.h b/clang/test/Parser/completely-empty-header-file.h
new file mode 100644
index 000000000000..e69de29bb2d1
diff --git a/clang/test/Parser/empty-translation-unit.c b/clang/test/Parser/empty-translation-unit.c
new file mode 100644
index 000000000000..0dbf37e447c3
--- /dev/null
+++ b/clang/test/Parser/empty-translation-unit.c
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c99 -pedantic -W -verify %s
+// RUN: %clang_cc1 -fsyntax-only -x c++ -std=c++03 -pedantic-errors -W %s
+
+#include "completely-empty-header-file.h"
+// no-warning -- an empty file is OK
+
+#define A_MACRO_IS_NOT_GOOD_ENOUGH 1
+
+// In C we should get this warning, but in C++ we shouldn't.
+// expected-warning{{ISO C requires a translation unit to contain at least one declaration.}}
diff --git a/clang/test/Parser/opencl-pragma.cl b/clang/test/Parser/opencl-pragma.cl
index 19460771137b..4c48b2a496f7 100644
--- a/clang/test/Parser/opencl-pragma.cl
+++ b/clang/test/Parser/opencl-pragma.cl
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 %s -verify -pedantic -fsyntax-only
+// RUN: %clang_cc1 %s -verify -pedantic -Wno-empty-translation-unit -fsyntax-only
 
 #pragma OPENCL EXTENSION cl_khr_fp16 : enable
 
diff --git a/clang/test/Preprocessor/undef-error.c b/clang/test/Preprocessor/undef-error.c
index ad611decedda..959c163e031d 100644
--- a/clang/test/Preprocessor/undef-error.c
+++ b/clang/test/Preprocessor/undef-error.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 %s -pedantic-errors -verify 
+// RUN: %clang_cc1 %s -pedantic-errors -Wno-empty-translation-unit -verify 
 // PR2045
 
 #define b
diff --git a/clang/test/Sema/c89-2.c b/clang/test/Sema/c89-2.c
index f6f6bd972090..14b955a6a4b3 100644
--- a/clang/test/Sema/c89-2.c
+++ b/clang/test/Sema/c89-2.c
@@ -1,4 +1,4 @@
-/* RUN: %clang_cc1 %s -std=c89 -pedantic-errors -verify
+/* RUN: %clang_cc1 %s -std=c89 -pedantic-errors -Wno-empty-translation-unit -verify
  */
 
 #if 1LL        /* expected-error {{long long}} */