Index: clang-tidy/ClangTidy.h
===================================================================
--- clang-tidy/ClangTidy.h
+++ clang-tidy/ClangTidy.h
@@ -223,7 +223,8 @@
 /// \brief Displays the found \p Errors to the users. If \p Fix is true, \p
 /// Errors containing fixes are automatically applied.
 void handleErrors(const std::vector<ClangTidyError> &Errors, bool Fix,
-                  unsigned &WarningsAsErrorsCount);
+                  unsigned &WarningsAsErrorsCount,
+                  StringRef BuildDirectory);
 
 /// \brief Serializes replacements into YAML and writes them to the specified
 /// output stream.
Index: clang-tidy/ClangTidy.cpp
===================================================================
--- clang-tidy/ClangTidy.cpp
+++ clang-tidy/ClangTidy.cpp
@@ -95,7 +95,7 @@
 
 class ErrorReporter {
 public:
-  ErrorReporter(bool ApplyFixes)
+  ErrorReporter(bool ApplyFixes, StringRef BuildDirectory)
       : Files(FileSystemOptions()), DiagOpts(new DiagnosticOptions()),
         DiagPrinter(new TextDiagnosticPrinter(llvm::outs(), &*DiagOpts)),
         Diags(IntrusiveRefCntPtr<DiagnosticIDs>(new DiagnosticIDs), &*DiagOpts,
@@ -103,6 +103,12 @@
         SourceMgr(Diags, Files), Rewrite(SourceMgr, LangOpts),
         ApplyFixes(ApplyFixes), TotalFixes(0), AppliedFixes(0),
         WarningsAsErrors(0) {
+    // By default, the working directory of file system is the current
+    // clang-tidy running directory.
+    //
+    // Change the directory to the one used during the analysis.
+    if (!BuildDirectory.empty())
+      Files.getVirtualFileSystem()->setCurrentWorkingDirectory(BuildDirectory);
     DiagOpts->ShowColors = llvm::sys::Process::StandardOutHasColors();
     DiagPrinter->BeginSourceFile(LangOpts);
   }
@@ -231,6 +237,12 @@
   Context.setSourceManager(&Compiler.getSourceManager());
   Context.setCurrentFile(File);
   Context.setASTContext(&Compiler.getASTContext());
+  auto WorkingDir = Compiler.getSourceManager()
+                        .getFileManager()
+                        .getVirtualFileSystem()
+                        ->getCurrentWorkingDirectory();
+  if (WorkingDir)
+    Context.setBuildDirectoryForStats(WorkingDir.get());
 
   std::vector<std::unique_ptr<ClangTidyCheck>> Checks;
   CheckFactories->createChecks(&Context, Checks);
@@ -444,8 +456,9 @@
 }
 
 void handleErrors(const std::vector<ClangTidyError> &Errors, bool Fix,
-                  unsigned &WarningsAsErrorsCount) {
-  ErrorReporter Reporter(Fix);
+                  unsigned &WarningsAsErrorsCount,
+                  StringRef BuildDirectory) {
+  ErrorReporter Reporter(Fix, BuildDirectory);
   for (const ClangTidyError &Error : Errors)
     Reporter.reportDiagnostic(Error);
   Reporter.Finish();
Index: clang-tidy/ClangTidyDiagnosticConsumer.h
===================================================================
--- clang-tidy/ClangTidyDiagnosticConsumer.h
+++ clang-tidy/ClangTidyDiagnosticConsumer.h
@@ -101,6 +101,14 @@
   unsigned ErrorsIgnoredNOLINT;
   unsigned ErrorsIgnoredNonUserCode;
   unsigned ErrorsIgnoredLineFilter;
+  // A build directory of the source file containing the error.
+  //
+  // It's an absolute path which is `directory` field of the source file in
+  // compilation database. If users don't specify compilation database
+  // directory, it is the current directory where clang-tidy runs.
+  //
+  // It can be empty in some cases, e.g. the source file does not exist.
+  std::string BuildDirectory;
 
   unsigned errorsIgnored() const {
     return ErrorsIgnoredNOLINT + ErrorsIgnoredCheckFilter +
@@ -185,6 +193,8 @@
   /// counters.
   const ClangTidyStats &getStats() const { return Stats; }
 
+  void setBuildDirectoryForStats(StringRef BuildDirectory);
+
   /// \brief Returns all collected errors.
   const std::vector<ClangTidyError> &getErrors() const { return Errors; }
 
Index: clang-tidy/ClangTidyDiagnosticConsumer.cpp
===================================================================
--- clang-tidy/ClangTidyDiagnosticConsumer.cpp
+++ clang-tidy/ClangTidyDiagnosticConsumer.cpp
@@ -229,6 +229,10 @@
       OptionsProvider->getOptions(File));
 }
 
+void ClangTidyContext::setBuildDirectoryForStats(StringRef BuildDirectory) {
+  Stats.BuildDirectory = BuildDirectory;
+}
+
 void ClangTidyContext::setCheckProfileData(ProfileData *P) { Profile = P; }
 
 GlobList &ClangTidyContext::getChecksFilter() {
Index: clang-tidy/tool/ClangTidyMain.cpp
===================================================================
--- clang-tidy/tool/ClangTidyMain.cpp
+++ clang-tidy/tool/ClangTidyMain.cpp
@@ -357,7 +357,8 @@
   unsigned WErrorCount = 0;
 
   // -fix-errors implies -fix.
-  handleErrors(Errors, (FixErrors || Fix) && !DisableFixes, WErrorCount);
+  handleErrors(Errors, (FixErrors || Fix) && !DisableFixes, WErrorCount,
+               Stats.BuildDirectory);
 
   if (!ExportFixes.empty() && !Errors.empty()) {
     std::error_code EC;
Index: test/clang-tidy/Inputs/compilation-database/header.h
===================================================================
--- /dev/null
+++ test/clang-tidy/Inputs/compilation-database/header.h
@@ -0,0 +1,3 @@
+#define NULL 0
+
+int *a = NULL;
Index: test/clang-tidy/Inputs/compilation-database/template.json
===================================================================
--- /dev/null
+++ test/clang-tidy/Inputs/compilation-database/template.json
@@ -0,0 +1,7 @@
+[
+{
+  "directory": "test_dir",
+  "command": "clang++ -o test.o clang-tidy-run-with-database.cpp",
+  "file": "test_dir/clang-tidy-run-with-database.cpp"
+}
+]
Index: test/clang-tidy/clang-tidy-run-with-database.cpp
===================================================================
--- /dev/null
+++ test/clang-tidy/clang-tidy-run-with-database.cpp
@@ -0,0 +1,6 @@
+// REQUIRES: shell
+// RUN: sed 's|test_dir|%S|g' %S/Inputs/compilation-database/template.json > %T/compile_commands.json
+// RUN: clang-tidy --checks=-*,modernize-use-nullptr -p %T %s -header-filter=.* | FileCheck %s -implicit-check-not="{{warning:}}"
+
+#include "./Inputs/compilation-database/header.h"
+// CHECK: warning: use nullptr [modernize-use-nullptr]