Index: lld/trunk/ELF/LTO.cpp
===================================================================
--- lld/trunk/ELF/LTO.cpp
+++ lld/trunk/ELF/LTO.cpp
@@ -105,6 +105,11 @@
 static void undefine(Symbol *S) {
   replaceBody<Undefined>(S, S->body()->getName(), /*IsLocal=*/false,
                          STV_DEFAULT, S->body()->Type, nullptr);
+  // It shouldn't normally matter what the binding is, but if a bug in the LTO
+  // implementation causes it to fail to provide a definition for a symbol,
+  // setting the binding to STB_GLOBAL will cause the linker to report an
+  // undefined symbol error, even if the definition was weak.
+  S->Binding = STB_GLOBAL;
 }
 
 void BitcodeCompiler::add(BitcodeFile &F) {