Index: ELF/InputFiles.h
===================================================================
--- ELF/InputFiles.h
+++ ELF/InputFiles.h
@@ -240,6 +240,7 @@
 public:
   explicit ArchiveFile(MemoryBufferRef M) : InputFile(ArchiveKind, M) {}
   static bool classof(const InputFile *F) { return F->kind() == ArchiveKind; }
+  static bool archiveWithoutSymbolsSeen() { return ArchiveWithoutSymbolsSeen; }
   template <class ELFT> void parse();
 
   // Returns a memory buffer for a given symbol and the offset in the archive
@@ -249,6 +250,7 @@
   std::pair<MemoryBufferRef, uint64_t> getMember(const Archive::Symbol *Sym);
 
 private:
+  static bool ArchiveWithoutSymbolsSeen;
   std::unique_ptr<Archive> File;
   llvm::DenseSet<uint64_t> Seen;
 };
Index: ELF/InputFiles.cpp
===================================================================
--- ELF/InputFiles.cpp
+++ ELF/InputFiles.cpp
@@ -55,6 +55,8 @@
 };
 }
 
+bool ArchiveFile::ArchiveWithoutSymbolsSeen = false;
+
 Optional<MemoryBufferRef> elf::readFile(StringRef Path) {
   log(Path);
   auto MBOrErr = MemoryBuffer::getFile(Path);
@@ -556,8 +558,13 @@
                MB.getBufferIdentifier() + ": failed to parse archive");
 
   // Read the symbol table to construct Lazy objects.
-  for (const Archive::Symbol &Sym : File->symbols())
+  bool NoSymbols = true;
+  for (const Archive::Symbol &Sym : File->symbols()) {
     Symtab<ELFT>::X->addLazyArchive(this, Sym);
+    NoSymbols = false;
+  }
+
+  if (NoSymbols) ArchiveWithoutSymbolsSeen = true;
 }
 
 // Returns a buffer pointing to a member file containing a given symbol.
Index: ELF/Relocations.cpp
===================================================================
--- ELF/Relocations.cpp
+++ ELF/Relocations.cpp
@@ -611,10 +611,18 @@
                     toString(Sym) + "'";
 
   if (Config->UnresolvedSymbols == UnresolvedPolicy::WarnAll ||
-      (Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal))
+      (Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal)) {
     warn(Msg);
-  else
+  } else {
     error(Msg);
+    if (ArchiveFile::archiveWithoutSymbolsSeen())
+      message("At least one archive listed no symbols in its index."
+              " This can happen when creating archives with a version"
+              " of ar that does not understand the object files in"
+              " the archive. For example, if you are using LLVM"
+              " bitcode objects (such as created by -flto), you may "
+              " need to use llvm-ar or GNU ar with a plugin.\n");
+  }
 }
 
 template <class RelTy>
Index: test/ELF/lto/archive-no-index.ll
===================================================================
--- /dev/null
+++ test/ELF/lto/archive-no-index.ll
@@ -0,0 +1,39 @@
+; REQUIRES: x86
+; Tests that we suggest that LTO symbols missing from an archive index
+; may be the cause of undefined references, but only if we both
+; encountered an empty archive index and undefined references (to prevent
+; noisy false alarms).
+
+; RUN: rm -fr %T/archive-no-index
+; RUN: mkdir %T/archive-no-index
+; RUN: llvm-as %S/Inputs/archive.ll -o %T/archive-no-index/f.o
+; RUN: llvm-ar cr %T/archive-no-index/libf.a
+; RUN: llvm-ar qS %T/archive-no-index/libf.a %T/archive-no-index/f.o
+; RUN: llvm-as %s -o %t.o
+; RUN: not ld.lld -emain -m elf_x86_64 %t.o -o %t %T/archive-no-index/libf.a \
+; RUN:     2>&1 | FileCheck --check-prefix=NOTE %s
+
+; RUN: llvm-ar crs %T/archive-no-index/libfs.a %T/archive-no-index/f.o
+; RUN: ld.lld -emain -m elf_x86_64 %t.o -o %t %T/archive-no-index/libf.a \
+; RUN:     %T/archive-no-index/libfs.a
+
+; RUN: llvm-as %S/Inputs/archive-3.ll -o %T/archive-no-index/foo.o
+; RUN: llvm-ar crs %T/archive-no-index/libfoo.a %T/archive-no-index/foo.o
+; RUN: not ld.lld -emain -m elf_x86_64 %t.o -o %t %T/archive-no-index/libfoo.a \
+; RUN:     2>&1 | FileCheck --check-prefix=NO-NOTE %s
+
+; NOTE: undefined symbol 'f'
+; NOTE: archive listed no symbols
+
+; NO-NOTE: undefined symbol 'f'
+; NO-NOTE-NOT: archive listed no symbols
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @f()
+
+define i32 @main() {
+  call void @f()
+  ret i32 0
+}