Index: lld/ELF/Relocations.cpp
===================================================================
--- lld/ELF/Relocations.cpp
+++ lld/ELF/Relocations.cpp
@@ -663,6 +663,10 @@
     Msg += Src + "\n>>>               ";
   Msg += Sec.getObjMsg(Offset);
 
+  if (Sym.getName().startswith("_ZTV"))
+    Msg += "\nthe vtable symbol may be undefined because the class is "
+           "missing its key function";
+
   if ((Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal) ||
       Config->NoinhibitExec) {
     warn(Msg);
Index: lld/test/ELF/undef.s
===================================================================
--- lld/test/ELF/undef.s
+++ lld/test/ELF/undef.s
@@ -20,6 +20,9 @@
 # CHECK: >>> referenced by undef.s
 # CHECK: >>>               {{.*}}:(.text+0x10)
 
+# CHECK: error: undefined symbol: vtable for Foo
+# CHECK: the vtable symbol may be undefined because the class is missing its key function
+
 # CHECK: error: undefined symbol: zed2
 # CHECK: >>> referenced by {{.*}}.o:(.text+0x0) in archive {{.*}}2.a
 
@@ -60,3 +63,4 @@
   call bar
   call zed1
   call _Z3fooi
+  call _ZTV3Foo