diff -ur llvm-project-apple-stable-20191106/clang/lib/Driver/ToolChains/Darwin.cpp llvm-project-apple-stable-20191106+iOS/clang/lib/Driver/ToolChains/Darwin.cpp
--- llvm-project-apple-stable-20191106/clang/lib/Driver/ToolChains/Darwin.cpp	2019-12-03 19:28:47.000000000 -1000
+++ llvm-project-apple-stable-20191106+iOS/clang/lib/Driver/ToolChains/Darwin.cpp	2019-12-05 11:22:40.000000000 -1000
@@ -1655,10 +1655,8 @@
           << Triple.getOSName();
     break;
   case llvm::Triple::IOS:
-    Triple.getiOSVersion(Major, Minor, Micro);
-    break;
   case llvm::Triple::TvOS:
-    Triple.getOSVersion(Major, Minor, Micro);
+    Triple.getiOSVersion(Major, Minor, Micro);
     break;
   case llvm::Triple::WatchOS:
     Triple.getWatchOSVersion(Major, Minor, Micro);
@@ -1682,9 +1680,12 @@
 
   StringRef MachOArchName = Toolchain.getMachOArchName(Args);
   if (MachOArchName == "armv7" || MachOArchName == "armv7s" ||
-      MachOArchName == "arm64" || MachOArchName == "arm64e")
-    OSTy = llvm::Triple::IOS;
-  else if (MachOArchName == "armv7k")
+      MachOArchName == "arm64" || MachOArchName == "arm64e") {
+    if (Triple.getOS() == llvm::Triple::TvOS)
+      OSTy = llvm::Triple::TvOS;
+    else
+      OSTy = llvm::Triple::IOS;
+  } else if (MachOArchName == "armv7k")
     OSTy = llvm::Triple::WatchOS;
   else if (MachOArchName != "armv6m" && MachOArchName != "armv7m" &&
            MachOArchName != "armv7em")
@@ -1879,6 +1880,27 @@
     Environment = Simulator;
 
   setTarget(Platform, Environment, Major, Minor, Micro);
+  if (!Args.getLastArg(options::OPT_isysroot)) {
+      llvm::Triple DefaultTriple(LLVM_DEFAULT_TARGET_TRIPLE);
+      switch (Platform) {
+          case DarwinPlatformKind::MacOS:
+              if (DefaultTriple.getOS() != llvm::Triple::MacOSX)
+                  Args.append(Args.MakeSeparateArg(nullptr, Opts.getOption(options::OPT_isysroot), "/usr/share/SDKs/MacOSX.sdk"));
+              break;
+          case DarwinPlatformKind::IPhoneOS:
+              if (DefaultTriple.getOS() != llvm::Triple::IOS)
+                  Args.append(Args.MakeSeparateArg(nullptr, Opts.getOption(options::OPT_isysroot), "/usr/share/SDKs/iPhoneOS.sdk"));
+              break;
+          case DarwinPlatformKind::TvOS:
+              if (DefaultTriple.getOS() != llvm::Triple::TvOS)
+                  Args.append(Args.MakeSeparateArg(nullptr, Opts.getOption(options::OPT_isysroot), "/usr/share/SDKs/AppleTVOS.sdk"));
+              break;
+          case DarwinPlatformKind::WatchOS:
+              if (DefaultTriple.getOS() != llvm::Triple::WatchOS)
+                  Args.append(Args.MakeSeparateArg(nullptr, Opts.getOption(options::OPT_isysroot), "/usr/share/SDKs/WatchOS.sdk"));
+              break;
+      }
+  }
 
   if (const Arg *A = Args.getLastArg(options::OPT_isysroot)) {
     StringRef SDK = getSDKName(A->getValue());
@@ -1944,6 +1966,21 @@
     SmallString<128> P(Sysroot);
     llvm::sys::path::append(P, "usr", "include");
     addExternCSystemInclude(DriverArgs, CC1Args, P.str());
+    if (Sysroot == "/") {
+      switch (getTriple().getOS()) {
+          case llvm::Triple::IOS:
+              addExternCSystemInclude(DriverArgs, CC1Args, "/usr/share/SDKs/iPhoneOS.sdk/usr/include");
+              break;
+          case llvm::Triple::TvOS:
+              addExternCSystemInclude(DriverArgs, CC1Args, "/usr/share/SDKs/AppleTVOS.sdk/usr/include");
+              break;
+          case llvm::Triple::WatchOS:
+              addExternCSystemInclude(DriverArgs, CC1Args, "/usr/share/SDKs/WatchOS.sdk/usr/include");
+              break;
+          default:
+              break;
+      }
+    }
   }
 }
 
diff -ur llvm-project-apple-stable-20191106/clang/lib/Frontend/InitHeaderSearch.cpp llvm-project-apple-stable-20191106+iOS/clang/lib/Frontend/InitHeaderSearch.cpp
--- llvm-project-apple-stable-20191106/clang/lib/Frontend/InitHeaderSearch.cpp	2019-12-03 19:28:47.000000000 -1000
+++ llvm-project-apple-stable-20191106+iOS/clang/lib/Frontend/InitHeaderSearch.cpp	2019-12-05 11:10:22.000000000 -1000
@@ -445,6 +445,24 @@
   if (triple.isOSDarwin()) {
     if (HSOpts.UseStandardSystemIncludes) {
       // Add the default framework include paths on Darwin.
+      if (!HasSysroot) {
+        switch (triple.getOS()) {
+            case llvm::Triple::IOS:
+                AddPath("/usr/share/SDKs/iPhoneOS.sdk/System/Library/Frameworks", System, true);
+                AddPath("/usr/share/SDKs/iPhoneOS.sdk/Library/Frameworks", System, true);
+                break;
+            case llvm::Triple::TvOS:
+                AddPath("/usr/share/SDKs/AppleTVOS.sdk/System/Library/Frameworks", System, true);
+                AddPath("/usr/share/SDKs/AppleTVOS.sdk/Library/Frameworks", System, true);
+                break;
+            case llvm::Triple::WatchOS:
+                AddPath("/usr/share/SDKs/WatchOS.sdk/System/Library/Frameworks", System, true);
+                AddPath("/usr/share/SDKs/WatchOS.sdk/Library/Frameworks", System, true);
+                break;
+            default:
+                break;
+        }
+      }
       AddPath("/System/Library/Frameworks", System, true);
       AddPath("/Library/Frameworks", System, true);
     }