Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -324,6 +324,7 @@
 def Varargs : DiagGroup<"varargs">;
 
 def Unsequenced : DiagGroup<"unsequenced">;
+def UnspecifiedInheritanceModel : DiagGroup<"ms-unspecified-inheritance-model">;
 // GCC name for -Wunsequenced
 def : DiagGroup<"sequence-point", [Unsequenced]>;
 
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -2510,6 +2510,12 @@
   InGroup<IgnoredAttributes>;
 def note_previous_ms_inheritance : Note<
   "previous inheritance model specified here">;
+def warn_unspecified_ms_inheritance : Warning<
+  "incomplete classes use the 'full_generality' pointer-to-member "
+  "representation; a different translation unit may have chosen a different "
+  "representation">,
+  InGroup<UnspecifiedInheritanceModel>;
+def note_ms_inheritance_selected : Note<"inheritance model selected here">;
 def err_machine_mode : Error<"%select{unknown|unsupported}0 machine mode %1">;
 def err_mode_not_primitive : Error<
   "mode attribute only supported for integer and floating-point types">;
Index: lib/Sema/SemaType.cpp
===================================================================
--- lib/Sema/SemaType.cpp
+++ lib/Sema/SemaType.cpp
@@ -5146,7 +5146,8 @@
 }
 
 /// Locks in the inheritance model for the given class and all of its bases.
-static void assignInheritanceModel(Sema &S, CXXRecordDecl *RD) {
+static void assignInheritanceModel(Sema &S, CXXRecordDecl *RD,
+                                   SourceLocation Loc) {
   RD = RD->getMostRecentDecl();
   if (!RD->hasAttr<MSInheritanceAttr>()) {
     MSInheritanceAttr::Spelling IM;
@@ -5166,6 +5167,18 @@
       break;
     }
 
+    // The user probably did not want the unspecified inheritance model in the
+    // best case.  Diagnose this case so that the code could be reworked by
+    // providing a definition prior to the use of a pointer-to-member or via the
+    // use of inheritance keywords.
+    if (S.MSPointerToMemberRepresentationMethod ==
+            LangOptions::PPTMK_BestCase &&
+        IM == MSInheritanceAttr::Keyword_unspecified_inheritance) {
+      S.Diag(RD->getSourceRange().getBegin(),
+             diag::warn_unspecified_ms_inheritance);
+      S.Diag(Loc, diag::note_ms_inheritance_selected);
+    }
+
     RD->addAttr(MSInheritanceAttr::CreateImplicit(
         S.getASTContext(), IM,
         /*BestCase=*/S.MSPointerToMemberRepresentationMethod ==
@@ -5180,7 +5193,7 @@
     // form pointers to members of base classes without calling
     // RequireCompleteType on the pointer to member of the base class type.
     for (const CXXBaseSpecifier &BS : RD->bases())
-      assignInheritanceModel(S, BS.getType()->getAsCXXRecordDecl());
+      assignInheritanceModel(S, BS.getType()->getAsCXXRecordDecl(), Loc);
   }
 }
 
@@ -5222,7 +5235,8 @@
       if (const MemberPointerType *MPTy = T->getAs<MemberPointerType>()) {
         if (!MPTy->getClass()->isDependentType()) {
           RequireCompleteType(Loc, QualType(MPTy->getClass(), 0), 0);
-          assignInheritanceModel(*this, MPTy->getMostRecentCXXRecordDecl());
+          assignInheritanceModel(*this, MPTy->getMostRecentCXXRecordDecl(),
+                                 Loc);
         }
       }
     }
Index: test/SemaCXX/member-pointer-ms.cpp
===================================================================
--- test/SemaCXX/member-pointer-ms.cpp
+++ test/SemaCXX/member-pointer-ms.cpp
@@ -103,7 +103,13 @@
 // An incomplete type with an unspecified inheritance model seems to take one
 // more slot than virtual.  It's not clear what it's used for yet.
 class IncUnspecified;
+#ifdef VMB
+// expected-warning@-2 {{incomplete classes use the 'full_generality' pointer-to-member representation}}
+#endif
 static_assert(sizeof(int IncUnspecified::*) == kUnspecifiedDataSize, "");
+#ifdef VMB
+// expected-note@-2 {{inheritance model selected here}}
+#endif
 static_assert(sizeof(void (IncUnspecified::*)()) == kUnspecifiedFunctionSize, "");
 
 // complete types
@@ -122,6 +128,9 @@
 // Test both declared and defined templates.
 template <typename T> class X;
 #ifdef VMB
+// expected-warning@-2 {{incomplete classes use the 'full_generality' pointer-to-member representation}}
+#endif
+#ifdef VMB
 template <> class __single_inheritance   X<IncSingle>;
 template <> class __multiple_inheritance X<IncMultiple>;
 template <> class __virtual_inheritance  X<IncVirtual>;
@@ -135,6 +144,9 @@
 static_assert(sizeof(int X<IncMultiple>::*)         == kMultipleDataSize, "");
 static_assert(sizeof(int X<IncVirtual>::*)          == kVirtualDataSize, "");
 static_assert(sizeof(int X<IncUnspecified>::*)      == kUnspecifiedDataSize, "");
+#ifdef VMB
+// expected-note@-2 {{inheritance model selected here}}
+#endif
 static_assert(sizeof(void (X<IncSingle>::*)())      == kSingleFunctionSize, "");
 static_assert(sizeof(void (X<IncMultiple>::*)())    == kMultipleFunctionSize, "");
 static_assert(sizeof(void (X<IncVirtual>::*)())     == kVirtualFunctionSize, "");
@@ -167,11 +179,17 @@
 
 // Re-declare to force us to iterate decls when adding attributes.
 struct ForwardDecl1;
+#ifdef VMB
+// expected-warning@-2 {{incomplete classes use the 'full_generality' pointer-to-member representation}}
+#endif
 struct ForwardDecl2;
 
 typedef int ForwardDecl1::*MemPtr1;
 typedef int ForwardDecl2::*MemPtr2;
 MemPtr1 variable_forces_sizing;
+#ifdef VMB
+// expected-note@-2 {{inheritance model selected here}}
+#endif
 
 struct ForwardDecl1 : B {
   virtual void foo();
@@ -208,12 +226,21 @@
 template<typename T>
 struct UnspecTemplate<void (T::*)(void)> {
   static_assert(sizeof(int T::*) == kUnspecifiedDataSize, "");
+#ifdef VMB
+// expected-note@-2 {{inheritance model selected here}}
+#endif
   static_assert(sizeof(void (T::*)()) == kUnspecifiedFunctionSize, "");
 };
 
 struct NewUnspecified;
+#ifdef VMB
+// expected-warning@-2 {{incomplete classes use the 'full_generality' pointer-to-member representation}}
+#endif
 SingleTemplate<void (IncSingle::*)()> tmpl_single;
 UnspecTemplate<void (NewUnspecified::*)()> tmpl_unspec;
+#ifdef VMB
+// expected-note@-2 {{in instantiation of template class}}
+#endif
 
 struct NewUnspecified { };