Index: lib/Driver/ToolChains/Darwin.cpp
===================================================================
--- lib/Driver/ToolChains/Darwin.cpp
+++ lib/Driver/ToolChains/Darwin.cpp
@@ -1512,9 +1512,18 @@
   else if (MachOArchName == "armv7k")
     OSTy = llvm::Triple::WatchOS;
   else if (MachOArchName != "armv6m" && MachOArchName != "armv7m" &&
-           MachOArchName != "armv7em")
+           MachOArchName != "armv7em") {
+    // Default to macOS for other architectures.
     OSTy = llvm::Triple::MacOSX;
 
+    // If there is no version specified on triple, and both host and target are
+    // macos, use the host triple to infer OS version.
+    llvm::Triple SystemTriple(llvm::sys::getProcessTriple());
+    if (SystemTriple.isMacOSX() && !Triple.getOSMajorVersion())
+      return DarwinPlatform::createFromArch(
+          OSTy, getOSVersion(OSTy, SystemTriple, TheDriver));
+  }
+
   if (OSTy == llvm::Triple::UnknownOS)
     return None;
   return DarwinPlatform::createFromArch(OSTy,
@@ -1529,7 +1538,17 @@
   if (Triple.getOS() == llvm::Triple::Darwin ||
       Triple.getOS() == llvm::Triple::UnknownOS)
     return None;
-  std::string OSVersion = getOSVersion(Triple.getOS(), Triple, TheDriver);
+
+  // If the host and target are both macos, and the OS version is not set in
+  // the target, infer the os version from host triple.
+  std::string OSVersion;
+  llvm::Triple SystemTriple(llvm::sys::getProcessTriple());
+  if (Triple.isMacOSX() && SystemTriple.isMacOSX() &&
+      !Triple.getOSMajorVersion())
+    OSVersion = getOSVersion(SystemTriple.getOS(), SystemTriple, TheDriver);
+  else
+    OSVersion = getOSVersion(Triple.getOS(), Triple, TheDriver);
+
   return DarwinPlatform::createFromTarget(Triple, OSVersion,
                                           Args.getLastArg(options::OPT_target));
 }
Index: test/Driver/clang-g-opts.c
===================================================================
--- test/Driver/clang-g-opts.c
+++ test/Driver/clang-g-opts.c
@@ -3,7 +3,7 @@
 // RUN:             | FileCheck --check-prefix=CHECK-WITH-G %s
 
 // Assert that the toolchains which should default to a lower Dwarf version do so.
-// RUN: %clang -### -S %s -g -target x86_64-apple-darwin 2>&1 \
+// RUN: %clang -### -S %s -g -target x86_64-apple-darwin8 2>&1 \
 // RUN:             | FileCheck --check-prefix=CHECK-WITH-G-DWARF2 %s
 // RUN: %clang -### -S %s -g -target i686-pc-openbsd 2>&1 \
 // RUN:             | FileCheck --check-prefix=CHECK-WITH-G-DWARF2 %s
@@ -21,7 +21,7 @@
 //
 // RUN: %clang -### -S %s -g0 -g -target x86_64-linux-gnu 2>&1 \
 // RUN:             | FileCheck --check-prefix=CHECK-WITH-G %s
-// RUN: %clang -### -S %s -g0 -g -target x86_64-apple-darwin 2>&1 \
+// RUN: %clang -### -S %s -g0 -g -target x86_64-apple-darwin8 2>&1 \
 // RUN:             | FileCheck --check-prefix=CHECK-WITH-G-STANDALONE %s
 // RUN: %clang -### -S %s -g0 -g -target i686-pc-openbsd 2>&1 \
 // RUN:             | FileCheck --check-prefix=CHECK-WITH-G-DWARF2 %s
Index: test/Driver/target-triple-deployment.c
===================================================================
--- test/Driver/target-triple-deployment.c
+++ test/Driver/target-triple-deployment.c
@@ -1,5 +1,5 @@
 // RUN: touch %t.o
-// RUN: %clang -target x86_64-apple-macosx -### %t.o 2> %t.log
+// RUN: %clang -target x86_64-apple-macosx10.4 -### %t.o 2> %t.log
 // RUN: %clang -target x86_64-apple-darwin9 -### %t.o 2>> %t.log
 // RUN: %clang -target x86_64-apple-macosx10.7 -### %t.o 2>> %t.log
 //