Index: cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
@@ -133,6 +133,7 @@
     "invalid output type '%0' for use with gcc tool">;
 def err_drv_cc_print_options_failure : Error<
     "unable to open CC_PRINT_OPTIONS file: %0">;
+def err_drv_lto_without_lld : Error<"LTO requires -fuse-ld=lld">;
 def err_drv_preamble_format : Error<
     "incorrect format for -preamble-bytes=N,END">;
 def err_drv_conflicting_deployment_targets : Error<
Index: cfe/trunk/include/clang/Driver/Options.td
===================================================================
--- cfe/trunk/include/clang/Driver/Options.td
+++ cfe/trunk/include/clang/Driver/Options.td
@@ -957,7 +957,7 @@
 def flat__namespace : Flag<["-"], "flat_namespace">;
 def flax_vector_conversions : Flag<["-"], "flax-vector-conversions">, Group<f_Group>;
 def flimited_precision_EQ : Joined<["-"], "flimited-precision=">, Group<f_Group>;
-def flto_EQ : Joined<["-"], "flto=">, Flags<[CC1Option]>, Group<f_Group>,
+def flto_EQ : Joined<["-"], "flto=">, Flags<[CoreOption, CC1Option]>, Group<f_Group>,
   HelpText<"Set LTO mode to either 'full' or 'thin'">;
 def flto : Flag<["-"], "flto">, Flags<[CoreOption, CC1Option]>, Group<f_Group>,
   HelpText<"Enable LTO in 'full' mode">;
Index: cfe/trunk/lib/Driver/Driver.cpp
===================================================================
--- cfe/trunk/lib/Driver/Driver.cpp
+++ cfe/trunk/lib/Driver/Driver.cpp
@@ -2353,8 +2353,12 @@
   Arg *FinalPhaseArg;
   phases::ID FinalPhase = getFinalPhase(Args, &FinalPhaseArg);
 
-  if (FinalPhase == phases::Link && Args.hasArg(options::OPT_emit_llvm)) {
-    Diag(clang::diag::err_drv_emit_llvm_link);
+  if (FinalPhase == phases::Link) {
+    if (Args.hasArg(options::OPT_emit_llvm))
+      Diag(clang::diag::err_drv_emit_llvm_link);
+    if (IsCLMode() && LTOMode != LTOK_None &&
+        !Args.getLastArgValue(options::OPT_fuse_ld_EQ).equals_lower("lld"))
+      Diag(clang::diag::err_drv_lto_without_lld);
   }
 
   // Reject -Z* at the top level, these options should never have been exposed
Index: cfe/trunk/test/Driver/cl-options.c
===================================================================
--- cfe/trunk/test/Driver/cl-options.c
+++ cfe/trunk/test/Driver/cl-options.c
@@ -522,6 +522,15 @@
 
 // RUN: env CL="%s" _CL_="%s" not %clang --rsp-quoting=windows -c
 
+// RUN: %clang_cl -### /c -flto -- %s 2>&1 | FileCheck -check-prefix=LTO %s
+// LTO: -flto
+
+// RUN: %clang_cl -### /c -flto=thin -- %s 2>&1 | FileCheck -check-prefix=LTO-THIN %s
+// LTO-THIN: -flto=thin
+
+// RUN: %clang_cl -### -Fe%t.exe -entry:main -flto -- %s 2>&1 | FileCheck -check-prefix=LTO-WITHOUT-LLD %s
+// LTO-WITHOUT-LLD: LTO requires -fuse-ld=lld
+
 // Accept "core" clang options.
 // (/Zs is for syntax-only, -Werror makes it fail hard on unknown options)
 // RUN: %clang_cl \