Index: include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- include/clang/Basic/DiagnosticDriverKinds.td
+++ include/clang/Basic/DiagnosticDriverKinds.td
@@ -195,6 +195,7 @@
   "precompiled header '%0' was ignored because '%1' is not first '-include'">;
 def warn_missing_sysroot : Warning<"no such sysroot directory: '%0'">,
   InGroup<DiagGroup<"missing-sysroot">>;
+def warn_incompatible_sdk : Warning<"using SDK for '%0' but deploying to '%1'">;
 def warn_debug_compression_unavailable : Warning<"cannot compress debug sections (zlib not installed)">,
   InGroup<DiagGroup<"debug-compression-unavailable">>;
 def warn_drv_enabling_rtti_with_exceptions : Warning<
Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -1087,6 +1087,8 @@
 def Wlarger_than_EQ : Joined<["-"], "Wlarger-than=">, Group<clang_ignored_f_Group>;
 def Wlarger_than_ : Joined<["-"], "Wlarger-than-">, Alias<Wlarger_than_EQ>;
 def Wframe_larger_than_EQ : Joined<["-"], "Wframe-larger-than=">, Group<f_Group>, Flags<[DriverOption]>;
+def Wincompatible_sdk : Flag<["-"], "Wincompatible-sdk">, Group<f_Group>,
+  Flags<[DriverOption]>;
 
 def : Flag<["-"], "fterminated-vtables">, Alias<fapple_kext>;
 def fthreadsafe_statics : Flag<["-"], "fthreadsafe-statics">, Group<f_Group>;
Index: lib/Driver/ToolChains.h
===================================================================
--- lib/Driver/ToolChains.h
+++ lib/Driver/ToolChains.h
@@ -496,6 +496,7 @@
     return TargetVersion < VersionTuple(V0, V1, V2);
   }
 
+  StringRef getPlatformFamily() const;
   StringRef getOSLibraryNameSuffix() const;
 
 public:
Index: lib/Driver/ToolChains.cpp
===================================================================
--- lib/Driver/ToolChains.cpp
+++ lib/Driver/ToolChains.cpp
@@ -329,6 +329,23 @@
   }
 }
 
+StringRef Darwin::getPlatformFamily() const {
+  switch(TargetPlatform) {
+  case DarwinPlatformKind::MacOS:
+    return "MacOSX";
+  case DarwinPlatformKind::IPhoneOS:
+  case DarwinPlatformKind::IPhoneOSSimulator:
+    return "iPhone";
+  case DarwinPlatformKind::TvOS:
+  case DarwinPlatformKind::TvOSSimulator:
+    return "AppleTV";
+  case DarwinPlatformKind::WatchOS:
+  case DarwinPlatformKind::WatchOSSimulator:
+    return "Watch";
+  }
+  llvm_unreachable("Unsupported platform");
+}
+
 StringRef Darwin::getOSLibraryNameSuffix() const {
   switch(TargetPlatform) {
   case DarwinPlatformKind::MacOS:
@@ -721,6 +738,21 @@
     Platform = WatchOSSimulator;
 
   setTarget(Platform, Major, Minor, Micro);
+
+  if(Args.hasArg(options::OPT_Wincompatible_sdk)) {
+    if (const Arg *A = Args.getLastArg(options::OPT_isysroot)) {
+      StringRef isysroot = A->getValue();
+      // Assume SDK has path: SOME_PATH/SDKs/PlatformXX.YY.sdk
+      size_t BeginSDK = isysroot.rfind("SDKs/");
+      size_t EndSDK = isysroot.rfind(".sdk");
+      if (BeginSDK != StringRef::npos && EndSDK != StringRef::npos) {
+        StringRef SDK = isysroot.slice(BeginSDK + 5, EndSDK);
+        if(!SDK.startswith(getPlatformFamily()))
+          getDriver().Diag(diag::warn_incompatible_sdk) << SDK
+            << getPlatformFamily();
+      }
+    }
+  }
 }
 
 void DarwinClang::AddCXXStdlibLibArgs(const ArgList &Args,
Index: test/Driver/incompatible_sdk.c
===================================================================
--- /dev/null
+++ test/Driver/incompatible_sdk.c
@@ -0,0 +1,10 @@
+// RUN: %clang -Wincompatible-sdk -isysroot SDKs/MacOSX10.9.sdk -mios-version-min=9.0 -S -o - %s 2>&1 | FileCheck -check-prefix CHECK-OSX-IOS %s
+// RUN: %clang -Wincompatible-sdk -isysroot SDKs/iPhoneOS9.2.sdk -mwatchos-version-min=2.0 -S -o - %s 2>&1 | FileCheck -check-prefix CHECK-IOS-WATCHOS %s
+// RUN: %clang -Wincompatible-sdk -isysroot SDKs/iPhoneOS9.2.sdk -mtvos-version-min=9.0 -S -o - %s 2>&1 | FileCheck -check-prefix CHECK-IOS-TVOS %s
+// RUN: %clang -Wincompatible-sdk -isysroot SDKs/iPhoneSimulator9.2.sdk -mios-version-min=9.0 -S -o - %s 2>&1 | FileCheck -check-prefix CHECK-IOS-IOSSIM %s
+
+int main() { return 0; }
+// CHECK-OSX-IOS: warning: using SDK for 'MacOSX10.9' but deploying to 'iPhone'
+// CHECK-IOS-WATCHOS: warning: using SDK for 'iPhoneOS9.2' but deploying to 'Watch'
+// CHECK-IOS-TVOS: warning: using SDK for 'iPhoneOS9.2' but deploying to 'AppleTV'
+// CHECK-IOS-IOSSIM-NOT: warning: using SDK for '{{.*}}' but deploying to '{{.*}}'