Index: include/clang/Lex/HeaderSearch.h
===================================================================
--- include/clang/Lex/HeaderSearch.h
+++ include/clang/Lex/HeaderSearch.h
@@ -693,7 +693,7 @@
 
   /// \brief Retrieve a uniqued framework name.
   StringRef getUniqueFrameworkName(StringRef Framework);
-  
+
   /// \brief Suggest a path by which the specified file could be found, for
   /// use in diagnostics to suggest a #include.
   ///
@@ -702,6 +702,15 @@
   std::string suggestPathToFileForDiagnostics(const FileEntry *File,
                                               bool *IsSystem = nullptr);
 
+  /// \brief Suggest a path by which the specified file could be found, for
+  /// use in diagnostics to suggest a #include.
+  ///
+  /// \param WorkingDir If non-empty, this will be prepended to search directory
+  /// paths that are relative.
+  std::string suggestPathToFileForDiagnostics(llvm::StringRef File,
+                                              llvm::StringRef WorkingDir,
+                                              bool *IsSystem = nullptr);
+
   void PrintStats();
   
   size_t getTotalMemory() const;
Index: lib/Lex/HeaderSearch.cpp
===================================================================
--- lib/Lex/HeaderSearch.cpp
+++ lib/Lex/HeaderSearch.cpp
@@ -1580,9 +1580,15 @@
 std::string HeaderSearch::suggestPathToFileForDiagnostics(const FileEntry *File,
                                                           bool *IsSystem) {
   // FIXME: We assume that the path name currently cached in the FileEntry is
-  // the most appropriate one for this analysis (and that it's spelled the same
-  // way as the corresponding header search path).
-  StringRef Name = File->getName();
+  // the most appropriate one for this analysis (and that it's spelled the
+  // same way as the corresponding header search path).
+  return suggestPathToFileForDiagnostics(File->getName(), /*BuildDir=*/"",
+                                         IsSystem);
+}
+
+std::string HeaderSearch::suggestPathToFileForDiagnostics(
+    llvm::StringRef File, llvm::StringRef WorkingDir, bool *IsSystem) {
+  using namespace llvm::sys;
 
   unsigned BestPrefixLength = 0;
   unsigned BestSearchDir;
@@ -1593,12 +1599,17 @@
       continue;
 
     StringRef Dir = SearchDirs[I].getDir()->getName();
-    for (auto NI = llvm::sys::path::begin(Name),
-              NE = llvm::sys::path::end(Name),
-              DI = llvm::sys::path::begin(Dir),
-              DE = llvm::sys::path::end(Dir);
+    llvm::SmallString<32> DirPath(Dir.begin(), Dir.end());
+    if (!WorkingDir.empty() && !path::is_absolute(Dir)) {
+      auto err = fs::make_absolute(WorkingDir, DirPath);
+      if (!err)
+        path::remove_dots(DirPath, /*remove_dot_dot=*/true);
+      Dir = DirPath;
+    }
+    for (auto NI = path::begin(File), NE = path::end(File),
+              DI = path::begin(Dir), DE = path::end(Dir);
          /*termination condition in loop*/; ++NI, ++DI) {
-      // '.' components in Name are ignored.
+      // '.' components in File are ignored.
       while (NI != NE && *NI == ".")
         ++NI;
       if (NI == NE)
@@ -1608,9 +1619,9 @@
       while (DI != DE && *DI == ".")
         ++DI;
       if (DI == DE) {
-        // Dir is a prefix of Name, up to '.' components and choice of path
+        // Dir is a prefix of File, up to '.' components and choice of path
         // separators.
-        unsigned PrefixLength = NI - llvm::sys::path::begin(Name);
+        unsigned PrefixLength = NI - path::begin(File);
         if (PrefixLength > BestPrefixLength) {
           BestPrefixLength = PrefixLength;
           BestSearchDir = I;
@@ -1625,5 +1636,5 @@
 
   if (IsSystem)
     *IsSystem = BestPrefixLength ? BestSearchDir >= SystemDirIdx : false;
-  return Name.drop_front(BestPrefixLength);
+  return File.drop_front(BestPrefixLength);
 }
Index: unittests/Lex/CMakeLists.txt
===================================================================
--- unittests/Lex/CMakeLists.txt
+++ unittests/Lex/CMakeLists.txt
@@ -4,6 +4,7 @@
 
 add_clang_unittest(LexTests
   HeaderMapTest.cpp
+  HeaderSearchTest.cpp
   LexerTest.cpp
   PPCallbacksTest.cpp
   PPConditionalDirectiveRecordTest.cpp
Index: unittests/Lex/HeaderSearchTest.cpp
===================================================================
--- /dev/null
+++ unittests/Lex/HeaderSearchTest.cpp
@@ -0,0 +1,96 @@
+//===- unittests/Lex/HeaderSearchTest.cpp ------ HeaderSearch tests -------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Lex/HeaderSearch.h"
+#include "clang/Basic/Diagnostic.h"
+#include "clang/Basic/DiagnosticOptions.h"
+#include "clang/Basic/FileManager.h"
+#include "clang/Basic/LangOptions.h"
+#include "clang/Basic/MemoryBufferCache.h"
+#include "clang/Basic/SourceManager.h"
+#include "clang/Basic/TargetInfo.h"
+#include "clang/Basic/TargetOptions.h"
+#include "clang/Lex/HeaderSearch.h"
+#include "clang/Lex/HeaderSearchOptions.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace {
+
+// The test fixture.
+class HeaderSearchTest : public ::testing::Test {
+protected:
+  HeaderSearchTest()
+      : VFS(new vfs::InMemoryFileSystem), FileMgr(FileMgrOpts, VFS),
+        DiagID(new DiagnosticIDs()),
+        Diags(DiagID, new DiagnosticOptions, new IgnoringDiagConsumer()),
+        SourceMgr(Diags, FileMgr), TargetOpts(new TargetOptions),
+        Search(std::make_shared<HeaderSearchOptions>(), SourceMgr, Diags,
+               LangOpts, Target.get()) {
+    TargetOpts->Triple = "x86_64-apple-darwin11.1.0";
+    Target = TargetInfo::CreateTargetInfo(Diags, TargetOpts);
+  }
+
+  void addSearchDir(llvm::StringRef Dir) {
+    VFS->addFile(Dir, 0, llvm::MemoryBuffer::getMemBuffer(""), /*User=*/None,
+                 /*Group=*/None, llvm::sys::fs::file_type::directory_file);
+    const DirectoryEntry *DE = FileMgr.getDirectory(Dir);
+    assert(DE);
+    auto DL = DirectoryLookup(DE, SrcMgr::C_User, /*isFramework=*/false);
+    Search.AddSearchPath(DL, /*isAngled=*/false);
+  }
+
+  IntrusiveRefCntPtr<vfs::InMemoryFileSystem> VFS;
+  FileSystemOptions FileMgrOpts;
+  FileManager FileMgr;
+  IntrusiveRefCntPtr<DiagnosticIDs> DiagID;
+  DiagnosticsEngine Diags;
+  SourceManager SourceMgr;
+  LangOptions LangOpts;
+  std::shared_ptr<TargetOptions> TargetOpts;
+  IntrusiveRefCntPtr<TargetInfo> Target;
+  HeaderSearch Search;
+};
+
+TEST_F(HeaderSearchTest, NoSearchDir) {
+  EXPECT_EQ(Search.search_dir_size(), 0u);
+  EXPECT_EQ(Search.suggestPathToFileForDiagnostics("/x/y/z", /*WorkingDir=*/""),
+            "/x/y/z");
+}
+
+TEST_F(HeaderSearchTest, SimpleShorten) {
+  addSearchDir("/x");
+  addSearchDir("/x/y");
+  EXPECT_EQ(Search.suggestPathToFileForDiagnostics("/x/y/z", /*WorkingDir=*/""),
+            "z");
+  addSearchDir("/a/b/");
+  EXPECT_EQ(Search.suggestPathToFileForDiagnostics("/a/b/c", /*WorkingDir=*/""),
+            "c");
+}
+
+TEST_F(HeaderSearchTest, ShortenWithWorkingDir) {
+  addSearchDir("x/y");
+  EXPECT_EQ(Search.suggestPathToFileForDiagnostics("/a/b/c/x/y/z",
+                                                   /*WorkingDir=*/"/a/b/c"),
+            "z");
+}
+
+TEST_F(HeaderSearchTest, Dots) {
+  addSearchDir("/x/./y/");
+  EXPECT_EQ(Search.suggestPathToFileForDiagnostics("/x/y/./z",
+                                                   /*WorkingDir=*/""),
+            "z");
+  addSearchDir("a/.././c/");
+  EXPECT_EQ(Search.suggestPathToFileForDiagnostics("/m/n/./c/z",
+                                                   /*WorkingDir=*/"/m/n/"),
+            "z");
+}
+
+} // namespace
+} // namespace clang