Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -6350,6 +6350,8 @@
 def err_bad_memptr_lhs : Error<
   "left hand operand to %0 must be a %select{|pointer to }1class "
   "compatible with the right hand operand, but is %2">;
+def err_memptr_incomplete : Error<
+  "member pointer has incomplete base type %0">;
 def warn_exception_caught_by_earlier_handler : Warning<
   "exception of type %0 will be caught by earlier handler">,
   InGroup<Exceptions>;
Index: include/clang/Basic/LangOptions.def
===================================================================
--- include/clang/Basic/LangOptions.def
+++ include/clang/Basic/LangOptions.def
@@ -247,6 +247,10 @@
 LANGOPT(IncludeDefaultHeader, 1, 0, "Include default header file for OpenCL")
 BENIGN_LANGOPT(DelayedTemplateParsing , 1, 0, "delayed template parsing")
 LANGOPT(BlocksRuntimeOptional , 1, 0, "optional blocks runtime")
+LANGOPT(
+    CompleteMemberPointers, 1, 0,
+    "Require member pointer base types to be complete at the point where the "
+    "type's inheritance model would be determined under the Microsoft ABI")
 
 ENUM_LANGOPT(GC, GCMode, 2, NonGC, "Objective-C Garbage Collection mode")
 ENUM_LANGOPT(ValueVisibilityMode, Visibility, 3, DefaultVisibility,
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -782,6 +782,12 @@
 def fparse_all_comments : Flag<["-"], "fparse-all-comments">, Group<f_clang_Group>, Flags<[CC1Option]>;
 def fcommon : Flag<["-"], "fcommon">, Group<f_Group>;
 def fcompile_resource_EQ : Joined<["-"], "fcompile-resource=">, Group<f_Group>;
+def fcomplete_member_pointers : Flag<["-"], "fcomplete-member-pointers">, Group<f_clang_Group>,
+   Flags<[CoreOption, CC1Option]>,
+   HelpText<"Require member pointer base types to be complete if they would be significant under the Microsoft ABI">;
+def fno_complete_member_pointers : Flag<["-"], "fno-complete-member-pointers">, Group<f_clang_Group>,
+   Flags<[CoreOption]>,
+   HelpText<"Do not require member pointer base types to be complete if they would be significant under the Microsoft ABI">;
 def fconstant_cfstrings : Flag<["-"], "fconstant-cfstrings">, Group<f_Group>;
 def fconstant_string_class_EQ : Joined<["-"], "fconstant-string-class=">, Group<f_Group>;
 def fconstexpr_depth_EQ : Joined<["-"], "fconstexpr-depth=">, Group<f_Group>;
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -4784,6 +4784,10 @@
       CmdArgs.push_back("-fforce-enable-int128");
   }
 
+  if (Args.hasFlag(options::OPT_fcomplete_member_pointers,
+                   options::OPT_fno_complete_member_pointers, false))
+    CmdArgs.push_back("-fcomplete-member-pointers");
+
   // Finally add the compile command to the compilation.
   if (Args.hasArg(options::OPT__SLASH_fallback) &&
       Output.getType() == types::TY_Object &&
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -2754,6 +2754,8 @@
           << A->getAsString(Args) << A->getValue();
     }
   }
+
+  Opts.CompleteMemberPointers = Args.hasArg(OPT_fcomplete_member_pointers);
 }
 
 static bool isStrictlyPreprocessorAction(frontend::ActionKind Action) {
Index: lib/Sema/SemaType.cpp
===================================================================
--- lib/Sema/SemaType.cpp
+++ lib/Sema/SemaType.cpp
@@ -7585,11 +7585,17 @@
   //  assert(!T->isDependentType() &&
   //         "Can't ask whether a dependent type is complete");
 
-  // We lock in the inheritance model once somebody has asked us to ensure
-  // that a pointer-to-member type is complete.
-  if (Context.getTargetInfo().getCXXABI().isMicrosoft()) {
-    if (const MemberPointerType *MPTy = T->getAs<MemberPointerType>()) {
-      if (!MPTy->getClass()->isDependentType()) {
+  if (const MemberPointerType *MPTy = T->getAs<MemberPointerType>()) {
+    if (!MPTy->getClass()->isDependentType()) {
+      if (getLangOpts().CompleteMemberPointers &&
+          !MPTy->getClass()->getAsCXXRecordDecl()->isBeingDefined() &&
+          RequireCompleteType(Loc, QualType(MPTy->getClass(), 0),
+                              diag::err_memptr_incomplete))
+        return true;
+
+      // We lock in the inheritance model once somebody has asked us to ensure
+      // that a pointer-to-member type is complete.
+      if (Context.getTargetInfo().getCXXABI().isMicrosoft()) {
         (void)isCompleteType(Loc, QualType(MPTy->getClass(), 0));
         assignInheritanceModel(*this, MPTy->getMostRecentCXXRecordDecl());
       }
Index: test/Driver/complete-member-pointers.cpp
===================================================================
--- test/Driver/complete-member-pointers.cpp
+++ test/Driver/complete-member-pointers.cpp
@@ -0,0 +1,7 @@
+// RUN: %clangxx -### -c %s -o %t.o -target x86_64-unknown-linux 2>&1 | FileCheck --check-prefix=NOFLAG %s
+// RUN: %clangxx -### -c %s -o %t.o -target x86_64-unknown-linux -fcomplete-member-pointers 2>&1 | FileCheck %s
+// RUN: %clangxx -### -c %s -o %t.o -target x86_64-unknown-linux -fcomplete-member-pointers -fno-complete-member-pointers 2>&1 | FileCheck --check-prefix=NOFLAG %s
+// RUN: %clang_cl -### /c %s /Fo%t.o -target x86_64-pc-win32 -fcomplete-member-pointers 2>&1 | FileCheck %s
+
+// CHECK: "-fcomplete-member-pointers"
+// NOFLAG-NOT: "-fcomplete-member-pointers"
Index: test/SemaCXX/complete-member-pointers.cpp
===================================================================
--- test/SemaCXX/complete-member-pointers.cpp
+++ test/SemaCXX/complete-member-pointers.cpp
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -verify -fsyntax-only -fcomplete-member-pointers %s
+
+struct S; // expected-note {{forward declaration of 'S'}}
+typedef int S::*t;
+t foo; // expected-error {{member pointer has incomplete base type 'S'}}
+
+struct S2 {
+  int S2::*foo;
+};
+int S2::*bar;
+
+template <typename T>
+struct S3 {
+  int T::*foo;
+};