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
@@ -2505,6 +2505,10 @@
   InGroup<IgnoredAttributes>;
 def note_previous_ms_inheritance : Note<
   "previous inheritance model specified here">;
+def warn_unspecified_ms_inheritance : Warning<
+  "the 'full_generality' inheritance model was selected for this class, "
+  "this will result in very large pointers-to-members">,
+  InGroup<UnspecifiedInheritanceModel>, DefaultIgnore;
 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/SemaDeclAttr.cpp
===================================================================
--- lib/Sema/SemaDeclAttr.cpp
+++ lib/Sema/SemaDeclAttr.cpp
@@ -3917,6 +3917,14 @@
     }
   }
 
+  // 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 (MSPointerToMemberRepresentationMethod == LangOptions::PPTMK_BestCase &&
+      SemanticSpelling == MSInheritanceAttr::Keyword_unspecified_inheritance)
+    Diag(Range.getBegin(), diag::warn_unspecified_ms_inheritance);
+
   return ::new (Context)
       MSInheritanceAttr(Range, Context, BestCase, AttrSpellingListIndex);
 }
Index: test/SemaCXX/member-pointer-ms.cpp
===================================================================
--- test/SemaCXX/member-pointer-ms.cpp
+++ test/SemaCXX/member-pointer-ms.cpp
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -std=c++11 -fms-compatibility -fsyntax-only -triple=i386-pc-win32 -verify -DVMB %s
-// RUN: %clang_cc1 -std=c++11 -fms-compatibility -fsyntax-only -triple=x86_64-pc-win32 -verify -DVMB %s
+// RUN: %clang_cc1 -std=c++11 -fms-compatibility -fsyntax-only -triple=i386-pc-win32 -verify -DVMB -Wms-unspecified-inheritance-model %s
+// RUN: %clang_cc1 -std=c++11 -fms-compatibility -fsyntax-only -triple=x86_64-pc-win32 -verify -DVMB -Wms-unspecified-inheritance-model %s
 // RUN: %clang_cc1 -std=c++11 -fms-compatibility -fsyntax-only -triple=x86_64-pc-win32 -verify -DVMV -fms-memptr-rep=virtual %s
 //
 // This file should also give no diagnostics when run through cl.exe from MSVS
@@ -167,6 +167,9 @@
 
 // Re-declare to force us to iterate decls when adding attributes.
 struct ForwardDecl1;
+#ifdef VMB
+// expected-warning@-2 {{the 'full_generality' inheritance model was selected}}
+#endif
 struct ForwardDecl2;
 
 typedef int ForwardDecl1::*MemPtr1;
@@ -212,6 +215,9 @@
 };
 
 struct NewUnspecified;
+#ifdef VMB
+// expected-warning@-2 {{the 'full_generality' inheritance model was selected}}
+#endif
 SingleTemplate<void (IncSingle::*)()> tmpl_single;
 UnspecTemplate<void (NewUnspecified::*)()> tmpl_unspec;