diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 9802370a6056..12bf16372f71 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -175,6 +175,7 @@ def InfiniteRecursion : DiagGroup<"infinite-recursion">;
 def GNUImaginaryConstant : DiagGroup<"gnu-imaginary-constant">;
 def IgnoredQualifiers : DiagGroup<"ignored-qualifiers">;
 def : DiagGroup<"import">;
+def IncompatibleMSStruct : DiagGroup<"incompatible-ms-struct">;
 def IncompatiblePointerTypesDiscardsQualifiers 
   : DiagGroup<"incompatible-pointer-types-discards-qualifiers">;
 def IncompatiblePointerTypes
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 059a28ab88da..6e1484c1ca82 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -500,8 +500,10 @@ def warn_pragma_pack_pop_identifer_and_alignment : Warning<
   "specifying both a name and alignment to 'pop' is undefined">;
 def warn_pragma_pop_failed : Warning<"#pragma %0(pop, ...) failed: %1">,
   InGroup<IgnoredPragmas>;
-def warn_pragma_ms_struct_failed : Warning<"#pramga ms_struct can not be used with dynamic classes or structures">,
-  InGroup<IgnoredPragmas>;
+def warn_cxx_ms_struct :
+  Warning<"ms_struct may not produce MSVC-compatible layouts for classes "
+          "with base classes or virtual functions">,
+  DefaultError, InGroup<IncompatibleMSStruct>;
 
 def warn_pragma_unused_undeclared_var : Warning<
   "undeclared variable %0 used as an argument for '#pragma unused'">,
diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index d27255ecc8de..11835c901cbb 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -1502,6 +1502,10 @@ void RecordLayoutBuilder::LayoutBitField(const FieldDecl *D) {
   // __attribute__((packed))) always uses the next available bit
   // offset.
 
+  // In an ms_struct struct, the alignment of a fundamental type is
+  // always equal to its size.  This is necessary in order to mimic
+  // the i386 alignment rules on targets which might not fully align
+  // all types (e.g. Darwin PPC32, where alignof(long long) == 4).
 
   // First, some simple bookkeeping to perform for ms_struct structs.
   if (IsMsStruct) {
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index b9c7cca326a6..74ab1ee84990 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -4520,11 +4520,18 @@ void Sema::CheckCompletedCXXClass(CXXRecordDecl *Record) {
     }
   }
 
-  // Check to see if we're trying to lay out a struct using the ms_struct
-  // attribute that is dynamic.
-  if (Record->isMsStruct(Context) && Record->isDynamicClass()) {
-    Diag(Record->getLocation(), diag::warn_pragma_ms_struct_failed);
-    Record->dropAttr<MsStructAttr>();
+  // ms_struct is a request to use the same ABI rules as MSVC.  Check
+  // whether this class uses any C++ features that are implemented
+  // completely differently in MSVC, and if so, emit a diagnostic.
+  // That diagnostic defaults to an error, but we allow projects to
+  // map it down to a warning (or ignore it).  It's a fairly common
+  // practice among users of the ms_struct pragma to mass-annotate
+  // headers, sweeping up a bunch of types that the project doesn't
+  // really rely on MSVC-compatible layout for.  We must therefore
+  // support "ms_struct except for C++ stuff" as a secondary ABI.
+  if (Record->isMsStruct(Context) &&
+      (Record->isPolymorphic() || Record->getNumBases())) {
+    Diag(Record->getLocation(), diag::warn_cxx_ms_struct);
   }
 
   // Declare inheriting constructors. We do this eagerly here because:
diff --git a/clang/test/SemaCXX/ms_struct.cpp b/clang/test/SemaCXX/ms_struct.cpp
index 37fa9a7c687c..2832b5620f3b 100644
--- a/clang/test/SemaCXX/ms_struct.cpp
+++ b/clang/test/SemaCXX/ms_struct.cpp
@@ -1,5 +1,6 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -triple i686-apple-darwin9 -std=c++11 %s
-// expected-no-diagnostics
+// RUN: %clang_cc1 -fsyntax-only -Wno-error=incompatible-ms-struct -verify -triple i686-apple-darwin9 -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -Wno-error=incompatible-ms-struct -verify -triple armv7-apple-darwin9 -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -DTEST_FOR_ERROR -verify -triple armv7-apple-darwin9 -std=c++11 %s
 
 #pragma ms_struct on
 
@@ -9,10 +10,29 @@ struct A {
 };
 
 struct B : public A {
+#ifdef TEST_FOR_ERROR
+  // expected-error@-2 {{ms_struct may not produce MSVC-compatible layouts for classes with base classes or virtual functions}}
+#else
+  // expected-warning@-4 {{ms_struct may not produce MSVC-compatible layouts for classes with base classes or virtual functions}}
+#endif
   unsigned long c:16;
 	int d;
   B();
 };
 
 static_assert(__builtin_offsetof(B, d) == 12,
-  "We can't allocate the bitfield into the padding under ms_struct");
\ No newline at end of file
+  "We can't allocate the bitfield into the padding under ms_struct");
+
+// rdar://16178895
+struct C {
+#ifdef TEST_FOR_ERROR
+  // expected-error@-2 {{ms_struct may not produce MSVC-compatible layouts for classes with base classes or virtual functions}}
+#else
+  // expected-warning@-4 {{ms_struct may not produce MSVC-compatible layouts for classes with base classes or virtual functions}}
+#endif
+  virtual void foo();
+  long long n;
+};
+
+static_assert(__builtin_offsetof(C, n) == 8,
+              "long long field in ms_struct should be 8-byte aligned");