Index: lldb/include/lldb/Utility/XcodeSDK.h
===================================================================
--- lldb/include/lldb/Utility/XcodeSDK.h
+++ lldb/include/lldb/Utility/XcodeSDK.h
@@ -50,7 +50,8 @@
   bool operator==(XcodeSDK other);
 
   /// Return parsed SDK number, and SDK version number.
-  std::tuple<Type, llvm::VersionTuple> Parse() const;
+  std::tuple<Type, llvm::VersionTuple, bool> Parse() const;
+  bool IsAppleInternalSDK() const;
   llvm::VersionTuple GetVersion() const;
   Type GetType() const;
   llvm::StringRef GetString() const;
Index: lldb/source/Host/macosx/objcxx/HostInfoMacOSX.mm
===================================================================
--- lldb/source/Host/macosx/objcxx/HostInfoMacOSX.mm
+++ lldb/source/Host/macosx/objcxx/HostInfoMacOSX.mm
@@ -298,8 +298,9 @@
 }
 
 std::string HostInfoMacOSX::GetXcodeSDK(XcodeSDK sdk) {
-  std::string xcrun_cmd = "xcrun --show-sdk-path --sdk " +
-                          XcodeSDK::GetSDKNameForType(sdk.GetType()).str();
+  auto sdk_tuple = sdk.Parse();
+  auto sdk_name = XcodeSDK::GetSDKNameForType(std::get<0>(sdk_tuple));
+  std::string xcrun_cmd = "xcrun --show-sdk-path --sdk " + sdk_name.str();
   llvm::VersionTuple version = sdk.GetVersion();
   if (!version.empty())
     xcrun_cmd += version.getAsString();
@@ -327,6 +328,17 @@
   if (last_newline != llvm::StringRef::npos)
     output = output.substr(last_newline + 1);
 
+  bool internal = std::get<2>(sdk_tuple);
+  if (internal) {
+    // xcrun doesn't support searching for an internal SDK with a version.
+    if (output.endswith(".sdk")) {
+      std::string internal_sdk =
+          output.drop_back(4).str() + std::string(".Internal.sdk");
+      if (FileSystem::Instance().Exists(internal_sdk))
+        return internal_sdk;
+    }
+  }
+
   // Whatever is left in output should be a valid path.
   if (!FileSystem::Instance().Exists(output))
     return {};
Index: lldb/source/Utility/XcodeSDK.cpp
===================================================================
--- lldb/source/Utility/XcodeSDK.cpp
+++ lldb/source/Utility/XcodeSDK.cpp
@@ -64,13 +64,24 @@
   return version;
 }
 
+static bool ParseAppleInternalSDK(llvm::StringRef &name) {
+  return name.consume_front("Internal.");
+}
 
-std::tuple<XcodeSDK::Type, llvm::VersionTuple> XcodeSDK::Parse() const {
+std::tuple<XcodeSDK::Type, llvm::VersionTuple, bool> XcodeSDK::Parse() const {
   llvm::StringRef input(m_name);
   XcodeSDK::Type sdk = ParseSDKName(input);
   llvm::VersionTuple version = ParseSDKVersion(input);
+  bool internal = ParseAppleInternalSDK(input);
   return std::make_tuple<XcodeSDK::Type, llvm::VersionTuple>(
-      std::move(sdk), std::move(version));
+      std::move(sdk), std::move(version), internal);
+}
+
+bool XcodeSDK::IsAppleInternalSDK() const {
+  llvm::StringRef input(m_name);
+  ParseSDKName(input);
+  ParseSDKVersion(input);
+  return ParseAppleInternalSDK(input);
 }
 
 llvm::VersionTuple XcodeSDK::GetVersion() const {
@@ -88,8 +99,17 @@
 
 void XcodeSDK::Merge(XcodeSDK other) {
   // The "bigger" SDK always wins.
-  if (Parse() < other.Parse())
+  auto l = Parse();
+  auto r = other.Parse();
+  if (l < r)
     *this = other;
+  else {
+    // The Internal flag always wins.
+    if (llvm::StringRef(m_name).endswith(".sdk"))
+      if (!std::get<2>(l) && std::get<2>(r))
+        m_name =
+            m_name.substr(0, m_name.size() - 3) + std::string("Internal.sdk");
+  }
 }
 
 llvm::StringRef XcodeSDK::GetSDKNameForType(XcodeSDK::Type type) {
@@ -153,6 +173,7 @@
 
     auto version_part = sdk_name.drop_front(sdk_string.size());
     version_part.consume_back(".sdk");
+    version_part.consume_back(".Internal");
 
     llvm::VersionTuple version;
     if (version.tryParse(version_part))
Index: lldb/unittests/Utility/XcodeSDKTest.cpp
===================================================================
--- lldb/unittests/Utility/XcodeSDKTest.cpp
+++ lldb/unittests/Utility/XcodeSDKTest.cpp
@@ -30,6 +30,10 @@
   EXPECT_EQ(XcodeSDK("MacOSX.sdk").GetVersion(), llvm::VersionTuple());
   EXPECT_EQ(XcodeSDK("MacOSX10.9.sdk").GetVersion(), llvm::VersionTuple(10, 9));
   EXPECT_EQ(XcodeSDK("MacOSX10.15.4.sdk").GetVersion(), llvm::VersionTuple(10, 15));
+  EXPECT_EQ(XcodeSDK("MacOSX.sdk").IsAppleInternalSDK(), false);
+  EXPECT_EQ(XcodeSDK("MacOSX10.15.Internal.sdk").GetType(), XcodeSDK::MacOSX);
+  EXPECT_EQ(XcodeSDK("MacOSX10.15.Internal.sdk").GetVersion(), llvm::VersionTuple(10, 15));
+  EXPECT_EQ(XcodeSDK("MacOSX10.15.Internal.sdk").IsAppleInternalSDK(), true);
   EXPECT_EQ(XcodeSDK().GetType(), XcodeSDK::unknown);
   EXPECT_EQ(XcodeSDK().GetVersion(), llvm::VersionTuple());
 }
@@ -46,6 +50,12 @@
   EXPECT_EQ(sdk.GetVersion(), llvm::VersionTuple(1, 1));
   sdk.Merge(XcodeSDK("WatchOS2.0.sdk"));
   EXPECT_EQ(sdk.GetVersion(), llvm::VersionTuple(2, 0));
+  sdk.Merge(XcodeSDK("WatchOS1.1.Internal.sdk"));
+  EXPECT_EQ(sdk.GetVersion(), llvm::VersionTuple(2, 0));
+  EXPECT_EQ(sdk.IsAppleInternalSDK(), true);
+  XcodeSDK empty;
+  empty.Merge(XcodeSDK("MacOSX10.14.Internal.sdk"));
+  EXPECT_EQ(empty.GetString(), llvm::StringRef("MacOSX10.14.Internal.sdk"));
 }
 
 TEST(XcodeSDKTest, SDKSupportsModules) {
@@ -55,6 +65,10 @@
       FileSpec(
           base +
           "iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator12.0.sdk")));
+  EXPECT_TRUE(XcodeSDK::SDKSupportsModules(
+      XcodeSDK::Type::iPhoneSimulator,
+      FileSpec(base + "iPhoneSimulator.platform/Developer/SDKs/"
+                      "iPhoneSimulator12.0.Internal.sdk")));
   EXPECT_FALSE(XcodeSDK::SDKSupportsModules(
       XcodeSDK::Type::iPhoneSimulator,
       FileSpec(