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,
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