Index: include/clang/AST/DeclCXX.h
===================================================================
--- include/clang/AST/DeclCXX.h
+++ include/clang/AST/DeclCXX.h
@@ -1831,6 +1831,10 @@
     return getLambdaData().MethodTyInfo;
   }
 
+  // \brief Determine whether this type is an Interface Like type for
+  // __interface inheritence purposes.
+  bool isInterfaceLike() const;
+
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) {
     return K >= firstCXXRecord && K <= lastCXXRecord;
Index: lib/AST/DeclCXX.cpp
===================================================================
--- lib/AST/DeclCXX.cpp
+++ lib/AST/DeclCXX.cpp
@@ -1470,6 +1470,61 @@
   return false;
 }
 
+bool CXXRecordDecl::isInterfaceLike() const {
+  // All __interfaces are inheritently interface like.
+  if (isInterface())
+    return true;
+
+  // No interface-like types can have a user declared constructor, destructor,
+  // friends, VBases, conersion functions, or fields.  Additionally, lambdas
+  // cannot be interface types.
+  if (isLambda() || hasUserDeclaredConstructor() ||
+      hasUserDeclaredDestructor() || !field_empty() || hasFriends() ||
+      getNumVBases() > 0 || conversion_end() - conversion_begin() > 0)
+    return false;
+
+  // No interface-like type can have a method with a definition.
+  for (const auto *const Method : methods())
+    if (Method->isDefined())
+      return false;
+
+  // Check "Special" types.
+  const auto *Uuid = getAttr<UuidAttr>();
+  if (Uuid && isStruct() && getDeclContext()->isTranslationUnit() &&
+      ((getName() == "IUnknown" &&
+        Uuid->getGuid() == "00000000-0000-0000-C000-000000000046") ||
+       (getName() == "IDispatch" &&
+        Uuid->getGuid() == "00020400-0000-0000-C000-000000000046"))) {
+    if (getNumBases() > 0)
+      return false;
+    return true;
+  }
+
+  // FIXME: Any access specifiers is supposed to make this no longer interface
+  // like.
+
+  // If this isn't a 'special' type, it must have exclusively interface &
+  // interface-like bases.  It must have a single interface-like base.
+  int NumInterfaceLikeBases = 0;
+  if (getNumBases() == 0)
+    return false;
+
+  for (const auto BS : bases()) {
+    if (BS.isVirtual() || BS.getAccessSpecifier() != AS_public)
+      return false;
+    const auto *Base = BS.getType()->getAsCXXRecordDecl();
+    if (Base->isInterface())
+      continue;
+    if (Base->isInterfaceLike()) {
+      ++NumInterfaceLikeBases;
+    } else {
+      return false;
+    }
+  }
+
+  return NumInterfaceLikeBases == 1;
+}
+
 void CXXRecordDecl::completeDefinition() {
   completeDefinition(nullptr);
 }
Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -2388,7 +2388,7 @@
       if (const RecordType *Record = NewBaseType->getAs<RecordType>()) {
         const CXXRecordDecl *RD = cast<CXXRecordDecl>(Record->getDecl());
         if (Class->isInterface() &&
-              (!RD->isInterface() ||
+              (!RD->isInterfaceLike() ||
                KnownBase->getAccessSpecifier() != AS_public)) {
           // The Microsoft extension __interface does not permit bases that
           // are not themselves public interfaces.
Index: test/SemaCXX/ms-iunknown-inline-def.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/ms-iunknown-inline-def.cpp
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -fms-extensions %s 
+
+struct __declspec(uuid("00000000-0000-0000-C000-000000000046")) IUnknown {
+  void foo() {}
+};
+
+// expected-error@+1{{interface type cannot inherit from}}
+__interface HasError : public IUnknown {}; 
Index: test/SemaCXX/ms-iunknown-outofline-def.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/ms-iunknown-outofline-def.cpp
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -fms-extensions %s 
+
+struct __declspec(uuid("00000000-0000-0000-C000-000000000046")) IUnknown {
+  void foo();
+};
+
+__interface NoError : public IUnknown {};
+void IUnknown::foo() {}
+// expected-error@+1{{interface type cannot inherit from}}
+__interface HasError : public IUnknown {}; 
Index: test/SemaCXX/ms-iunknown.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/ms-iunknown.cpp
@@ -0,0 +1,38 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -fms-extensions %s 
+
+struct __declspec(uuid("00000000-0000-0000-C000-000000000046")) IUnknown {
+  void foo();
+};
+struct IPropertyPageBase : public IUnknown {};
+struct IPropertyPage : public IPropertyPageBase {};
+__interface ISfFileIOPropertyPage : public IPropertyPage {};
+
+
+namespace NS {
+  struct __declspec(uuid("00000000-0000-0000-C000-000000000046")) IUnknown {};
+  // expected-error@+1 {{interface type cannot inherit from}}
+  __interface IPropertyPageBase : public IUnknown {}; 
+}
+// expected-error@+1 {{interface type cannot inherit from}}
+__interface IPropertyPageBase2 : public NS::IUnknown {}; 
+
+__interface temp_iface {};
+struct bad_base : temp_iface {};
+// expected-error@+1 {{interface type cannot inherit from}}
+__interface cant_inherit: public bad_base{};
+
+struct fine_base : temp_iface, IUnknown {};
+__interface can_inherit: public fine_base{};
+
+struct PageBase : public IUnknown {};
+struct Page3 : public PageBase {};
+struct Page4 : public PageBase {};
+__interface PropertyPage : public Page4 {};
+
+struct Page5 : public Page3, Page4{};
+// expected-error@+1 {{interface type cannot inherit from}}
+__interface PropertyPage2 : public Page5 {}; 
+
+__interface IF1 {};
+__interface PP : IUnknown, IF1{}; 
+__interface PP2 : PP, Page3, Page4{};