diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -2020,21 +2020,42 @@
 
   switch (GetCXXStdlibType(DriverArgs)) {
   case ToolChain::CST_Libcxx: {
-    // On Darwin, libc++ is installed alongside the compiler in
-    // include/c++/v1, so get from '<install>/bin' to '<install>/include/c++/v1'.
-    {
-      llvm::SmallString<128> P = llvm::StringRef(getDriver().getInstalledDir());
-      // Note that P can be relative, so we have to '..' and not parent_path.
-      llvm::sys::path::append(P, "..", "include", "c++", "v1");
-      addSystemInclude(DriverArgs, CC1Args, P);
+    // On Darwin, libc++ can be installed in one of the following two places:
+    // 1. Alongside the compiler in         <install>/include/c++/v1
+    // 2. In a SDK (or a custom sysroot) in <sysroot>/usr/include/c++/v1
+    //
+    // The precendence of paths is as listed above, i.e. we take the first path
+    // that exists. Also note that we never include libc++ twice -- we take the
+    // first path that exists and don't send the other paths to CC1 (otherwise
+    // include_next could break).
+
+    // Check for (1)
+    // Get from '<install>/bin' to '<install>/include/c++/v1'.
+    // Note that InstallBin can be relative, so we use '..' instead of
+    // parent_path.
+    llvm::SmallString<128> InstallBin =
+        llvm::StringRef(getDriver().getInstalledDir()); // <install>/bin
+    llvm::sys::path::append(InstallBin, "..", "include", "c++", "v1");
+    if (getVFS().exists(InstallBin)) {
+      addSystemInclude(DriverArgs, CC1Args, InstallBin);
+      return;
+    } else if (DriverArgs.hasArg(options::OPT_v)) {
+      llvm::errs() << "ignoring nonexistent directory \"" << InstallBin
+                   << "\"\n";
     }
-    // Also add <sysroot>/usr/include/c++/v1 unless -nostdinc is used,
-    // to match the legacy behavior in CC1.
-    if (!DriverArgs.hasArg(options::OPT_nostdinc)) {
-      llvm::SmallString<128> P = Sysroot;
-      llvm::sys::path::append(P, "usr", "include", "c++", "v1");
-      addSystemInclude(DriverArgs, CC1Args, P);
+
+    // Otherwise, check for (2)
+    llvm::SmallString<128> SysrootUsr = Sysroot;
+    llvm::sys::path::append(SysrootUsr, "usr", "include", "c++", "v1");
+    if (getVFS().exists(SysrootUsr)) {
+      addSystemInclude(DriverArgs, CC1Args, SysrootUsr);
+      return;
+    } else if (DriverArgs.hasArg(options::OPT_v)) {
+      llvm::errs() << "ignoring nonexistent directory \"" << SysrootUsr
+                   << "\"\n";
     }
+
+    // Otherwise, don't add any path.
     break;
   }
 
diff --git a/clang/test/Driver/Inputs/basic_darwin_sdk_usr_cxx_v1/usr/include/c++/v1/.keep b/clang/test/Driver/Inputs/basic_darwin_sdk_usr_cxx_v1/usr/include/c++/v1/.keep
new file mode 100644
diff --git a/clang/test/Driver/Inputs/basic_darwin_sdk_usr_cxx_v1/usr/lib/.keep b/clang/test/Driver/Inputs/basic_darwin_sdk_usr_cxx_v1/usr/lib/.keep
new file mode 100644
diff --git a/clang/test/Driver/darwin-header-search-libcxx.cpp b/clang/test/Driver/darwin-header-search-libcxx.cpp
--- a/clang/test/Driver/darwin-header-search-libcxx.cpp
+++ b/clang/test/Driver/darwin-header-search-libcxx.cpp
@@ -13,39 +13,57 @@
 // RUN:   | FileCheck --check-prefix=CHECK-LIBCXX-NONE %s
 // CHECK-LIBCXX-NONE: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 
-// Check with only headers alongside the installation (those should be used,
-// but we should still add /usr/include/c++/v1 after to preserve legacy).
+// Check with only headers alongside the installation (those should be used).
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin \
 // RUN:     -stdlib=libc++ \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
 // RUN:     --sysroot="" \
-// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain --check-prefix=CHECK-LIBCXX-TOOLCHAIN-1 %s
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
+// RUN:               --check-prefix=CHECK-LIBCXX-TOOLCHAIN-1 %s
 // CHECK-LIBCXX-TOOLCHAIN-1: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-LIBCXX-TOOLCHAIN-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
-// CHECK-LIBCXX-TOOLCHAIN-1: "-internal-isystem" "/usr/include/c++/v1"
+// CHECK-LIBCXX-TOOLCHAIN-1-NOT: "-internal-isystem" "/usr/include/c++/v1"
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin \
 // RUN:     -stdlib=libc++ \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
 // RUN:     -isysroot %S/Inputs/basic_darwin_sdk_no_libcxx \
-// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain --check-prefix=CHECK-LIBCXX-TOOLCHAIN-2 %s
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
+// RUN:               -DSYSROOT=%S/Inputs/basic_darwin_sdk_no_libcxx \
+// RUN:               --check-prefix=CHECK-LIBCXX-TOOLCHAIN-2 %s
 // CHECK-LIBCXX-TOOLCHAIN-2: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-LIBCXX-TOOLCHAIN-2: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
+// CHECK-LIBCXX-TOOLCHAIN-2-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
+
+// Check with only headers in the sysroot (those should be used).
+//
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -stdlib=libc++ \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
+// RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx \
+// RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT-1 %s
+// CHECK-LIBCXX-SYSROOT-1: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
+// CHECK-LIBCXX-SYSROOT-1: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
+// CHECK-LIBCXX-SYSROOT-1-NOT: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 
 // Check with both headers in the sysroot and headers alongside the installation
-// (the headers in <sysroot> should be added after the toolchain headers).
-// Ensure that both -isysroot and --sysroot work, and that isysroot has precedence.
+// (the headers in the toolchain should be preferred over the <sysroot> headers).
+// Ensure that both -isysroot and --sysroot work, and that isysroot has precedence
+// over --sysroot.
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin \
 // RUN:     -stdlib=libc++ \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir \
-// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr \
-// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
 // RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1 %s
 //
@@ -54,8 +72,8 @@
 // RUN:     -stdlib=libc++ \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir \
-// RUN:     --sysroot %S/Inputs/basic_darwin_sdk_usr \
-// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN:     --sysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
 // RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1 %s
 //
@@ -64,32 +82,46 @@
 // RUN:     -stdlib=libc++ \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir \
-// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:     --sysroot %S/Inputs/basic_darwin_sdk_no_libcxx \
-// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
 // RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1 %s
 //
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
-// CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
+// CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
 
-// Make sure that using -nostdinc will drop the sysroot C++ library include
-// path, but not the toolchain one.
+// Make sure that using -nostdinc does not drop any C++ library include path.
+// This behavior is strange, but it is compatible with the legacy CC1 behavior.
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin16 \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir \
-// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:     -stdlib=platform \
 // RUN:     -nostdinc \
-// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
-// RUN:               --check-prefix=CHECK-LIBCXX-NOSTDINC %s
-// CHECK-LIBCXX-NOSTDINC: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
-// CHECK-LIBCXX-NOSTDINC: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
-// CHECK-LIBCXX-NOSTDINC-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
+// RUN:               --check-prefix=CHECK-LIBCXX-NOSTDINC-1 %s
+// CHECK-LIBCXX-NOSTDINC-1: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
+// CHECK-LIBCXX-NOSTDINC-1-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
+// CHECK-LIBCXX-NOSTDINC-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
+//
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-apple-darwin16 \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_no_libcxx \
+// RUN:     -stdlib=platform \
+// RUN:     -nostdinc \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_no_libcxx \
+// RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
+// RUN:               --check-prefix=CHECK-LIBCXX-NOSTDINC-2 %s
+// CHECK-LIBCXX-NOSTDINC-2: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
+// CHECK-LIBCXX-NOSTDINC-2: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
+// CHECK-LIBCXX-NOSTDINC-2-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
 
 // Make sure that using -nostdinc++ or -nostdlib will drop both the toolchain
 // C++ include path and the sysroot one.
@@ -98,7 +130,7 @@
 // RUN:     -target x86_64-apple-darwin16 \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir \
-// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_cxx_v1 \
 // RUN:     -stdlib=platform \
 // RUN:     -nostdinc++ \
 // RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
@@ -121,3 +153,26 @@
 // CHECK-LIBCXX-NOSTDLIBINC: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-LIBCXX-NOSTDLIBINC-NOT: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 // CHECK-LIBCXX-NOSTDLIBINC-NOT: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
+
+// Make sure we explain that we considered a path but didn't add it when it
+// doesn't exist.
+//
+// RUN: %clang -no-canonical-prefixes %s -fsyntax-only -v 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk \
+// RUN:     -stdlib=libc++ \
+// RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx \
+// RUN:               --check-prefix=CHECK-LIBCXX-MISSING-TOOLCHAIN %s
+// CHECK-LIBCXX-MISSING-TOOLCHAIN: ignoring nonexistent directory "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
+//
+// RUN: %clang -no-canonical-prefixes %s -fsyntax-only -v 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_no_libcxx \
+// RUN:     -stdlib=libc++ \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_no_libcxx \
+// RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain_no_libcxx \
+// RUN:               --check-prefix=CHECK-LIBCXX-MISSING-BOTH %s
+// CHECK-LIBCXX-MISSING-BOTH: ignoring nonexistent directory "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
+// CHECK-LIBCXX-MISSING-BOTH: ignoring nonexistent directory "[[SYSROOT]]/usr/include/c++/v1"