Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6476,6 +6476,9 @@
 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 warn_memptr_incomplete : Warning<
+  "member pointer has incomplete base type %0">,
+  InGroup<DiagGroup<"incomplete-member-pointers">>, DefaultIgnore;
 def warn_exception_caught_by_earlier_handler : Warning<
   "exception of type %0 will be caught by earlier handler">,
   InGroup<Exceptions>;
Index: clang/lib/Sema/SemaType.cpp
===================================================================
--- clang/lib/Sema/SemaType.cpp
+++ clang/lib/Sema/SemaType.cpp
@@ -7585,14 +7585,23 @@
   //  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()) {
+      // 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());
       }
+
+      // We only test for this warning if enabled because testing whether a type
+      // is complete has side effects.
+      if (!Context.getDiagnostics().isIgnored(diag::warn_memptr_incomplete,
+                                              Loc) &&
+          !MPTy->getClass()->getAsCXXRecordDecl()->isBeingDefined() &&
+          !isCompleteType(Loc, QualType(MPTy->getClass(), 0)))
+        Diag(Loc, diag::warn_memptr_incomplete)
+            << QualType(MPTy->getClass(), 0);
     }
   }
 
Index: clang/test/SemaCXX/warn-incomplete-member-pointers.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaCXX/warn-incomplete-member-pointers.cpp
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -verify -fsyntax-only -Wincomplete-member-pointers %s
+
+struct S;
+typedef int S::*t;
+t foo; // expected-warning {{member pointer has incomplete base type 'S'}}
+
+struct S2 {
+  int S2::*foo;
+};
+int S2::*bar;
+
+template <typename T>
+struct S3 {
+  int T::*foo;
+};