Index: lld/trunk/COFF/DriverUtils.cpp
===================================================================
--- lld/trunk/COFF/DriverUtils.cpp
+++ lld/trunk/COFF/DriverUtils.cpp
@@ -114,9 +114,11 @@
                     uint32_t *Minor) {
   StringRef SysStr, Ver;
   std::tie(SysStr, Ver) = Arg.split(',');
-  *Sys = StringSwitch<WindowsSubsystem>(SysStr.lower())
+  std::string SysStrLower = SysStr.lower();
+  *Sys = StringSwitch<WindowsSubsystem>(SysStrLower)
     .Case("boot_application", IMAGE_SUBSYSTEM_WINDOWS_BOOT_APPLICATION)
     .Case("console", IMAGE_SUBSYSTEM_WINDOWS_CUI)
+    .Case("default", IMAGE_SUBSYSTEM_UNKNOWN)
     .Case("efi_application", IMAGE_SUBSYSTEM_EFI_APPLICATION)
     .Case("efi_boot_service_driver", IMAGE_SUBSYSTEM_EFI_BOOT_SERVICE_DRIVER)
     .Case("efi_rom", IMAGE_SUBSYSTEM_EFI_ROM)
@@ -125,7 +127,7 @@
     .Case("posix", IMAGE_SUBSYSTEM_POSIX_CUI)
     .Case("windows", IMAGE_SUBSYSTEM_WINDOWS_GUI)
     .Default(IMAGE_SUBSYSTEM_UNKNOWN);
-  if (*Sys == IMAGE_SUBSYSTEM_UNKNOWN)
+  if (*Sys == IMAGE_SUBSYSTEM_UNKNOWN && SysStrLower != "default")
     fatal("unknown subsystem: " + SysStr);
   if (!Ver.empty())
     parseVersion(Ver, Major, Minor);
Index: lld/trunk/test/COFF/subsystem-inference.test
===================================================================
--- lld/trunk/test/COFF/subsystem-inference.test
+++ lld/trunk/test/COFF/subsystem-inference.test
@@ -1,6 +1,8 @@
 # RUN: sed -e s/ENTRYNAME/main/ %s | yaml2obj > %t.obj
 # RUN: lld-link /out:%t.exe %t.obj
 # RUN: llvm-readobj --file-headers %t.exe | FileCheck -check-prefix=MAIN %s
+# RUN: lld-link /out:%t.exe %t.obj /subsystem:default,6.0
+# RUN: llvm-readobj --file-headers %t.exe | FileCheck -check-prefix=MAIN %s
 
 # RUN: sed s/ENTRYNAME/wmain/ %s | yaml2obj > %t.obj
 # RUN: lld-link /out:%t.exe %t.obj
@@ -9,6 +11,8 @@
 # RUN: sed s/ENTRYNAME/WinMain/ %s | yaml2obj > %t.obj
 # RUN: lld-link /out:%t.exe %t.obj
 # RUN: llvm-readobj --file-headers %t.exe | FileCheck -check-prefix=WINMAIN %s
+# RUN: lld-link /out:%t.exe %t.obj /subsystem:default,6.0
+# RUN: llvm-readobj --file-headers %t.exe | FileCheck -check-prefix=WINMAIN %s
 
 # RUN: sed s/ENTRYNAME/wWinMain/ %s | yaml2obj > %t.obj
 # RUN: lld-link /out:%t.exe %t.obj
Index: lld/trunk/test/COFF/subsystem.test
===================================================================
--- lld/trunk/test/COFF/subsystem.test
+++ lld/trunk/test/COFF/subsystem.test
@@ -17,3 +17,12 @@
 CHECK2: MajorSubsystemVersion: 8
 CHECK2: MinorSubsystemVersion: 9
 CHECK2: Subsystem: IMAGE_SUBSYSTEM_WINDOWS_GUI
+
+# RUN: lld-link /entry:main /out:%t.exe /subsystem:default,8.9 \
+# RUN:   %p/Inputs/ret42.obj
+# RUN: llvm-readobj --file-headers %t.exe | FileCheck -check-prefix=CHECK3 %s
+
+CHECK3: MajorOperatingSystemVersion: 8
+CHECK3: MinorOperatingSystemVersion: 9
+CHECK3: MajorSubsystemVersion: 8
+CHECK3: MinorSubsystemVersion: 9