Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -341,6 +341,13 @@
   if (Config->Relocatable)
     return UnresolvedPolicy::Ignore;
 
+  if (auto *Arg = Args.getLastArg(OPT_warn_undef, OPT_error_undef)) {
+    if (Arg->getOption().getID() == OPT_warn_undef)
+      return UnresolvedPolicy::Warn;
+
+    return UnresolvedPolicy::ReportError;
+  }
+
   if (auto *Arg = Args.getLastArg(OPT_unresolved_symbols)) {
     StringRef S = Arg->getValue();
     if (S == "ignore-all" || S == "ignore-in-object-files")
Index: ELF/Options.td
===================================================================
--- ELF/Options.td
+++ ELF/Options.td
@@ -79,6 +79,9 @@
 def error_limit: S<"error-limit">,
   HelpText<"Maximum number of errors to emit before stopping (0 = no limit)">;
 
+def error_undef: F<"error-unresolved-symbols">,
+  HelpText<"Report unresolved symbols as errors">;
+
 def export_dynamic: F<"export-dynamic">,
   HelpText<"Put symbols in the dynamic symbol table">;
 
@@ -248,6 +251,9 @@
 def warn_common: F<"warn-common">,
   HelpText<"Warn about duplicate common symbols">;
 
+def warn_undef: F<"warn-unresolved-symbols">,
+  HelpText<"Report unresolved symbols as warnings">;
+
 def whole_archive: F<"whole-archive">,
   HelpText<"Force load of all members in a static library">;
 
Index: test/ELF/warn-unresolved-symbols.s
===================================================================
--- /dev/null
+++ test/ELF/warn-unresolved-symbols.s
@@ -0,0 +1,39 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t1.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/unresolved-symbols.s -o %t2.o
+
+## The link should fail with an undef error by default
+# RUN: not ld.lld %t1.o %t2.o -o %t3 2>&1 | \
+# RUN:	FileCheck -check-prefix=ERRUND %s
+
+## --error-unresolved-symbols should generate an error
+# RUN: not ld.lld %t1.o %t2.o -o %t4 --error-unresolved-symbols 2>&1 | \
+# RUN:	FileCheck -check-prefix=ERRUND %s
+
+## --warn-unresolved-symbols should generate a warning
+# RUN: ld.lld %t1.o %t2.o -o %t5 --warn-unresolved-symbols 2>&1 | \
+# RUN:	FileCheck -check-prefix=WARNUND %s
+
+## Test that the last option wins
+# RUN: ld.lld %t1.o %t2.o -o %t5 --error-unresolved-symbols --warn-unresolved-symbols 2>&1 | \
+# RUN:  FileCheck -check-prefix=WARNUND %s
+# RUN: not ld.lld %t1.o %t2.o -o %t6 --warn-unresolved-symbols --error-unresolved-symbols 2>&1 | \
+# RUN:  FileCheck -check-prefix=ERRUND %s
+
+## Do not report undefines if linking relocatable or shared.
+# RUN: ld.lld -r %t1.o %t2.o -o %t7 --error-unresolved-symbols 2>&1 | \
+# RUN:  FileCheck -allow-empty -check-prefix=NOERR %s
+# RUN: ld.lld -shared %t1.o %t2.o -o %t8.so --error-unresolved-symbols 2>&1 | \
+# RUN:  FileCheck -allow-empty -check-prefix=NOERR %s
+# RUN: ld.lld -r %t1.o %t2.o -o %t9 --warn-unresolved-symbols 2>&1 | \
+# RUN:  FileCheck -allow-empty -check-prefix=NOWARN %s
+# RUN: ld.lld -shared %t1.o %t2.o -o %t10.so --warn-unresolved-symbols 2>&1 | \
+# RUN:  FileCheck -allow-empty -check-prefix=NOWARN %s
+
+# ERRUND: error: {{.*}}:(.text+0x1): undefined symbol 'undef'
+# WARNUND: warning: {{.*}}:(.text+0x1): undefined symbol 'undef'
+# NOERR-NOT: error: {{.*}}:(.text+0x1): undefined symbol 'undef'
+# NOWARN-NOT: warning: {{.*}}:(.text+0x1): undefined symbol 'undef'
+
+.globl _start
+_start: