Index: include/clang/Lex/HeaderSearchOptions.h
===================================================================
--- include/clang/Lex/HeaderSearchOptions.h
+++ include/clang/Lex/HeaderSearchOptions.h
@@ -108,6 +108,11 @@
   /// etc.).
   std::string ResourceDir;
 
+  /// Compiler install dir as detected by the Driver.
+  /// Only used to add include dirs for libc++ on Darwin. Please avoid relying
+  /// on this field for other purposes.
+  std::string InstallDir;
+
   /// The directory used for the module cache.
   std::string ModuleCachePath;
 
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -1773,6 +1773,7 @@
   if (const Arg *A = Args.getLastArg(OPT_stdlib_EQ))
     Opts.UseLibcxx = (strcmp(A->getValue(), "libc++") == 0);
   Opts.ResourceDir = Args.getLastArgValue(OPT_resource_dir);
+  Opts.InstallDir = Opts.ResourceDir;
 
   // Canonicalize -fmodules-cache-path before storing it.
   SmallString<128> P(Args.getLastArgValue(OPT_fmodules_cache_path));
Index: lib/Frontend/CreateInvocationFromCommandLine.cpp
===================================================================
--- lib/Frontend/CreateInvocationFromCommandLine.cpp
+++ lib/Frontend/CreateInvocationFromCommandLine.cpp
@@ -11,17 +11,18 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "clang/Frontend/Utils.h"
 #include "clang/Basic/DiagnosticOptions.h"
+#include "clang/Driver/Action.h"
 #include "clang/Driver/Compilation.h"
 #include "clang/Driver/Driver.h"
-#include "clang/Driver/Action.h"
 #include "clang/Driver/Options.h"
 #include "clang/Driver/Tool.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/FrontendDiagnostic.h"
+#include "clang/Frontend/Utils.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/Host.h"
+#include "llvm/Support/Path.h"
 using namespace clang;
 using namespace llvm::opt;
 
@@ -102,5 +103,8 @@
                                      CCArgs.size(),
                                      *Diags))
     return nullptr;
+  // Patch up the install dir, so we find the same standard library as the
+  // original compiler on MacOS.
+  CI->getHeaderSearchOpts().InstallDir = TheDriver.getInstalledDir();
   return CI;
 }
Index: lib/Frontend/InitHeaderSearch.cpp
===================================================================
--- lib/Frontend/InitHeaderSearch.cpp
+++ lib/Frontend/InitHeaderSearch.cpp
@@ -476,14 +476,9 @@
       if (triple.isOSDarwin()) {
         // On Darwin, libc++ may be installed alongside the compiler in
         // include/c++/v1.
-        if (!HSOpts.ResourceDir.empty()) {
-          // Remove version from foo/lib/clang/version
-          StringRef NoVer = llvm::sys::path::parent_path(HSOpts.ResourceDir);
-          // Remove clang from foo/lib/clang
-          StringRef Lib = llvm::sys::path::parent_path(NoVer);
-          // Remove lib from foo/lib
-          SmallString<128> P = llvm::sys::path::parent_path(Lib);
-
+        if (!HSOpts.InstallDir.empty()) {
+          // Get from foo/bin to foo.
+          SmallString<128> P(llvm::sys::path::parent_path(HSOpts.InstallDir));
           // Get foo/include/c++/v1
           llvm::sys::path::append(P, "include", "c++", "v1");
           AddUnmappedPath(P, CXXSystem, false);
Index: lib/Tooling/Tooling.cpp
===================================================================
--- lib/Tooling/Tooling.cpp
+++ lib/Tooling/Tooling.cpp
@@ -323,6 +323,9 @@
     Invocation->getPreprocessorOpts().addRemappedFile(It.getKey(),
                                                       Input.release());
   }
+  // Patch up the install dir, so we find the same standard library as the
+  // original compiler on MacOS.
+  Invocation->getHeaderSearchOpts().InstallDir = Driver->getInstalledDir();
   return runInvocation(BinaryName, Compilation.get(), std::move(Invocation),
                        std::move(PCHContainerOps));
 }
Index: test/Tooling/Inputs/mock-libcxx/include/c++/v1/mock_vector
===================================================================
--- /dev/null
+++ test/Tooling/Inputs/mock-libcxx/include/c++/v1/mock_vector
@@ -0,0 +1 @@
+class vector {};
Index: test/Tooling/clang-check-mac-libcxx.cpp
===================================================================
--- /dev/null
+++ test/Tooling/clang-check-mac-libcxx.cpp
@@ -0,0 +1,16 @@
+// Clang on MacOS can find libc++ living beside the installed compiler.
+// This test makes sure our libTooling-based tools emulate this properly.
+//
+// RUN: rm -rf %t
+// RUN: mkdir %t
+//
+// Install the mock libc++ (simulates the libc++ directory structure).
+// RUN: cp -r %S/Inputs/mock-libcxx %t/
+//
+// Pretend clang is installed beside the mock library that we provided.
+// RUN: echo '[{"directory":"%t","command":"%t/mock-libcxx/bin/clang++ -stdlib=libc++ -target x86_64-apple-darwin -c test.cpp","file":"test.cpp"}]' | sed -e 's/\\/\//g' > %t/compile_commands.json
+// RUN: cp "%s" "%t/test.cpp"
+// RUN: clang-check -p "%t" "%t/test.cpp"
+
+#include <mock_vector>
+vector v;