Index: cfe/trunk/lib/Driver/ToolChains/Darwin.cpp
===================================================================
--- cfe/trunk/lib/Driver/ToolChains/Darwin.cpp
+++ cfe/trunk/lib/Driver/ToolChains/Darwin.cpp
@@ -1118,6 +1118,25 @@
   }
 }
 
+/// Returns the most appropriate macOS target version for the current process.
+///
+/// If the macOS SDK version is the same or earlier than the system version,
+/// then the SDK version is returned. Otherwise the system version is returned.
+static std::string getSystemOrSDKMacOSVersion(StringRef MacOSSDKVersion) {
+  unsigned Major, Minor, Micro;
+  llvm::Triple(llvm::sys::getProcessTriple())
+      .getMacOSXVersion(Major, Minor, Micro);
+  VersionTuple SystemVersion(Major, Minor, Micro);
+  bool HadExtra;
+  if (!Driver::GetReleaseVersion(MacOSSDKVersion, Major, Minor, Micro,
+                                 HadExtra))
+    return MacOSSDKVersion;
+  VersionTuple SDKVersion(Major, Minor, Micro);
+  if (SDKVersion > SystemVersion)
+    return SystemVersion.getAsString();
+  return MacOSSDKVersion;
+}
+
 void Darwin::AddDeploymentTarget(DerivedArgList &Args) const {
   const OptTable &Opts = getDriver().getOpts();
 
@@ -1210,7 +1229,7 @@
                 SDK.startswith("iPhoneSimulator"))
               iOSTarget = Version;
             else if (SDK.startswith("MacOSX"))
-              OSXTarget = Version;
+              OSXTarget = getSystemOrSDKMacOSVersion(Version);
             else if (SDK.startswith("WatchOS") ||
                      SDK.startswith("WatchSimulator"))
               WatchOSTarget = Version;
Index: cfe/trunk/test/Driver/darwin-sdkroot.c
===================================================================
--- cfe/trunk/test/Driver/darwin-sdkroot.c
+++ cfe/trunk/test/Driver/darwin-sdkroot.c
@@ -74,3 +74,12 @@
 // CHECK-MACOSX: "-triple" "x86_64-apple-macosx10.10.0"
 // CHECK-MACOSX: ld
 // CHECK-MACOSX: "-macosx_version_min" "10.10.0"
+
+// Ensure that we never pick a version that's based on the SDK that's newer than
+// the system version:
+// RUN: rm -rf %t/SDKs/MacOSX10.99.99.sdk
+// RUN: mkdir -p %t/SDKs/MacOSX10.99.99.sdk
+// RUN: %clang -target x86_64-apple-darwin -isysroot %t/SDKs/MacOSX10.99.99.sdk %s -### 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-MACOSX-SYSTEM-VERSION %s
+
+// CHECK-MACOSX-SYSTEM-VERSION-NOT: 10.99.99"