Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -2373,6 +2373,35 @@
   return true;
 }
 
+/// \brief Tests if the __interface base is public.
+static bool IsRecordPublicInterface(const CXXRecordDecl *RD,
+                                  AccessSpecifier spec) {
+  return RD->isInterface() && spec == AS_public;
+}
+
+/// \brief Test if record is an uuid Unknown.
+/// This is an MS SDK specific type that has a special
+/// behavior in the CL compiler.
+static bool IsUnknownType(const CXXRecordDecl *RD) {
+  auto *Uuid = RD->getAttr<UuidAttr>();
+  
+  return RD->isStruct() && RD->getName() == "IUnknown" && RD->isEmpty() &&
+         Uuid && Uuid->getGuid() =="00000000-0000-0000-C000-000000000046";
+}
+
+/// \brief Test if record and records in inheritance tree is a an Unknown.
+static bool IsOrInheritsFromUnknown(const CXXRecordDecl *RD) {
+  if (RD->getNumBases()) {
+    const CXXRecordDecl *Base =
+      RD->bases_begin()->getType()->getAsCXXRecordDecl();
+
+    return Base && IsUnknownType(Base) && !IsUnknownType(RD) &&
+           IsOrInheritsFromUnknown(Base);
+  } else {
+    return IsUnknownType(RD);
+  }
+}
+
 /// Use small set to collect indirect bases.  As this is only used
 /// locally, there's no need to abstract the small size parameter.
 typedef llvm::SmallPtrSet<QualType, 4> IndirectBaseSet;
@@ -2450,8 +2479,8 @@
       if (const RecordType *Record = NewBaseType->getAs<RecordType>()) {
         const CXXRecordDecl *RD = cast<CXXRecordDecl>(Record->getDecl());
         if (Class->isInterface() &&
-              (!RD->isInterface() ||
-               KnownBase->getAccessSpecifier() != AS_public)) {
+            !IsRecordPublicInterface(RD, KnownBase->getAccessSpecifier()) &&
+            !IsOrInheritsFromUnknown(RD)) {
           // The Microsoft extension __interface does not permit bases that
           // are not themselves public interfaces.
           Diag(KnownBase->getLocStart(), diag::err_invalid_base_in_interface)
Index: test/SemaCXX/ms-uuid.cpp
===================================================================
--- test/SemaCXX/ms-uuid.cpp
+++ test/SemaCXX/ms-uuid.cpp
@@ -93,3 +93,27 @@
 [uuid("000000A0-0000-0000-C000-000000000049"),
  uuid("000000A0-0000-0000-C000-000000000049")] class C10;
 }
+
+struct __declspec(uuid("00000000-0000-0000-C000-000000000046")) IUnknown {};
+__interface ISfFileIOPropertyPage : public IUnknown {};
+
+struct __declspec(uuid("00000000-0000-0000-C000-000000000045")) IUnknown1 {};
+__interface ISfFileIOPropertyPage1 : public IUnknown1 {};  // expected-error {{interface type cannot inherit from}}
+
+struct __declspec(uuid("00000000-0000-0000-C000-000000000046")) IUnknown2 {};
+struct IPropertyPage2 : public IUnknown2 {};
+__interface ISfFileIOPropertyPage2 : public IPropertyPage2 {}; // expected-error {{interface type cannot inherit from}}
+
+struct IPropertyPage3 : public IUnknown {};
+__interface ISfFileIOPropertyPage3 : public IPropertyPage3 {};
+
+__interface __declspec(dllimport) ISfFileIOPropertyPage33 : public IUnknown {};
+
+struct __declspec(uuid("00000000-0000-0000-C000-000000000046")) IUnknown4 {};
+__interface ISfFileIOPropertyPage4 : public IUnknown4 {}; // expected-error {{interface type cannot inherit from}}
+
+class __declspec(uuid("00000000-0000-0000-C000-000000000046")) IUnknown5 {};
+__interface ISfFileIOPropertyPage5 : public IUnknown5 {}; // expected-error {{interface type cannot inherit from}}
+
+struct __declspec(dllexport) IUnknown6{};
+__interface foo : public IUnknown6{}; // expected-error {{interface type cannot inherit from}}