Index: lib/Driver/ToolChains/Darwin.cpp
===================================================================
--- lib/Driver/ToolChains/Darwin.cpp
+++ lib/Driver/ToolChains/Darwin.cpp
@@ -1752,10 +1752,11 @@
       break;
     // On Darwin, libc++ may be installed alongside the compiler in
     // include/c++/v1.
-    // Get from 'foo/bin' to 'foo'.
-    SmallString<128> P = llvm::sys::path::parent_path(InstallDir);
-    // Get to 'foo/include/c++/v1'.
-    llvm::sys::path::append(P, "include", "c++", "v1");
+    // Get from 'foo/bin' to 'foo/include/c++/v1'.
+    SmallString<128> P = InstallDir;
+    // Note that InstallDir can be relative, so we have to '..' and not
+    // parent_path.
+    llvm::sys::path::append(P, "..", "include", "c++", "v1");
     addSystemInclude(DriverArgs, CC1Args, P);
     break;
   }
Index: test/Driver/darwin-stdlib.cpp
===================================================================
--- test/Driver/darwin-stdlib.cpp
+++ test/Driver/darwin-stdlib.cpp
@@ -14,7 +14,7 @@
 // optional absolute include for libc++ from InitHeaderSearch.cpp also fires.
 
 // CHECK-LIBCXX: "-stdlib=libc++"
-// CHECK-LIBCXX: "-internal-isystem" "{{[^"]*}}{{/|\\\\}}Inputs{{/|\\\\}}darwin_toolchain_tree{{/|\\\\}}bin{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}v1"
+// CHECK-LIBCXX: "-internal-isystem" "{{[^"]*}}{{/|\\\\}}Inputs{{/|\\\\}}darwin_toolchain_tree{{/|\\\\}}bin{{/|\\\\}}..{{/|\\\\}}include{{/|\\\\}}c++{{/|\\\\}}v1"
 
 // CHECK-LIBSTDCXX-NOT: -stdlib=libc++
 // CHECK-LIBSTDCXX-NOT: -stdlib=libstdc++
Index: test/Tooling/Inputs/mock-libcxx/bin/clang
===================================================================
--- test/Tooling/Inputs/mock-libcxx/bin/clang
+++ test/Tooling/Inputs/mock-libcxx/bin/clang
@@ -0,0 +1 @@
+This file is a placeholder to keep its parent directory in git.