diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -155,6 +155,8 @@
 - Clang now warns by default for C++20 and later about deprecated capture of
   ``this`` with a capture default of ``=``. This warning can be disabled with
   ``-Wno-deprecated-this-capture``.
+- Clang had failed to emit some ``-Wundefined-internal`` for members of a local
+  class if that class was first introduced with a forward declaration.
 
 Bug Fixes in This Version
 -------------------------
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/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));
 }
diff --git a/clang/test/SemaCXX/warn-undefined-internal.cpp b/clang/test/SemaCXX/warn-undefined-internal.cpp
new file mode 100644
--- /dev/null
+++ b/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}}
+}