Index: lib/LTO/LTO.cpp
===================================================================
--- lib/LTO/LTO.cpp
+++ lib/LTO/LTO.cpp
@@ -338,7 +338,8 @@
 
     GlobalValue *GV = Obj->getSymbolGV(Sym.I->getRawDataRefImpl());
     if (Sym.getFlags() & object::BasicSymbolRef::SF_Undefined)
-      continue;
+      assert(!Res.Prevailing &&
+             "An undefined symbol cannot have prevailing resolution");
     if (Res.Prevailing && GV) {
       Keep.push_back(GV);
       switch (GV->getLinkage()) {