Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -17975,7 +17975,7 @@
   // immediately. For all other classes, we mark their virtual members
   // at the end of the translation unit.
   if (Class->isLocalClass())
-    MarkVirtualMembersReferenced(Loc, Class);
+    MarkVirtualMembersReferenced(Loc, Class->getDefinition());
   else
     VTableUses.push_back(std::make_pair(Class, Loc));
 }
Index: clang/test/SemaCXX/warn-undefined-internal.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaCXX/warn-undefined-internal.cpp
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -fsyntax-only -Wundefined-internal -verify %s
+
+void test1() {
+  struct S { virtual void f(); };
+  // expected-warning@-1{{function 'test1()::S::f' has internal linkage but is not defined}}
+  S s;
+  // expected-note@-1{{used here}}
+}
+
+void test2() {
+  struct S;
+  struct S { virtual void f(); };
+  // expected-warning@-1{{function 'test2()::S::f' has internal linkage but is not defined}}
+  S s;
+  // expected-note@-1{{used here}}
+}