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diff --git a/clang-tools-extra/clangd/Diagnostics.cpp b/clang-tools-extra/clangd/Diagnostics.cpp
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+++ b/clang-tools-extra/clangd/Diagnostics.cpp
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+//===--- Diagnostics.cpp ----------------------------------------*- C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===---------------------------------------------------------------------===//
+
+#include "Diagnostics.h"
+#include "Compiler.h"
+#include "Logger.h"
+#include "SourceCode.h"
+#include "clang/Basic/SourceManager.h"
+#include "clang/Lex/Lexer.h"
+#include "llvm/Support/Capacity.h"
+#include "llvm/Support/Path.h"
+#include <algorithm>
+
+namespace clang {
+namespace clangd {
+
+namespace {
+
+bool mentionsMainFile(const Diag &D) {
+  if (D.InsideMainFile)
+    return true;
+  // Fixes are always in the main file.
+  if (!D.Fixes.empty())
+    return true;
+  for (auto &N : D.Notes) {
+    if (N.InsideMainFile)
+      return true;
+  }
+  return false;
+}
+
+// Checks whether a location is within a half-open range.
+// Note that clang also uses closed source ranges, which this can't handle!
+bool locationInRange(SourceLocation L, CharSourceRange R,
+                     const SourceManager &M) {
+  assert(R.isCharRange());
+  if (!R.isValid() || M.getFileID(R.getBegin()) != M.getFileID(R.getEnd()) ||
+      M.getFileID(R.getBegin()) != M.getFileID(L))
+    return false;
+  return L != R.getEnd() && M.isPointWithin(L, R.getBegin(), R.getEnd());
+}
+
+// Clang diags have a location (shown as ^) and 0 or more ranges (~~~~).
+// LSP needs a single range.
+Range diagnosticRange(const clang::Diagnostic &D, const LangOptions &L) {
+  auto &M = D.getSourceManager();
+  auto Loc = M.getFileLoc(D.getLocation());
+  // Accept the first range that contains the location.
+  for (const auto &CR : D.getRanges()) {
+    auto R = Lexer::makeFileCharRange(CR, M, L);
+    if (locationInRange(Loc, R, M))
+      return halfOpenToRange(M, R);
+  }
+  // The range may be given as a fixit hint instead.
+  for (const auto &F : D.getFixItHints()) {
+    auto R = Lexer::makeFileCharRange(F.RemoveRange, M, L);
+    if (locationInRange(Loc, R, M))
+      return halfOpenToRange(M, R);
+  }
+  // If no suitable range is found, just use the token at the location.
+  auto R = Lexer::makeFileCharRange(CharSourceRange::getTokenRange(Loc), M, L);
+  if (!R.isValid()) // Fall back to location only, let the editor deal with it.
+    R = CharSourceRange::getCharRange(Loc);
+  return halfOpenToRange(M, R);
+}
+
+TextEdit toTextEdit(const FixItHint &FixIt, const SourceManager &M,
+                    const LangOptions &L) {
+  TextEdit Result;
+  Result.range =
+      halfOpenToRange(M, Lexer::makeFileCharRange(FixIt.RemoveRange, M, L));
+  Result.newText = FixIt.CodeToInsert;
+  return Result;
+}
+
+bool isInsideMainFile(const SourceLocation Loc, const SourceManager &M) {
+  return Loc.isValid() && M.isInMainFile(Loc);
+}
+
+bool isInsideMainFile(const clang::Diagnostic &D) {
+  if (!D.hasSourceManager())
+    return false;
+
+  return isInsideMainFile(D.getLocation(), D.getSourceManager());
+}
+
+bool isNote(DiagnosticsEngine::Level L) {
+  return L == DiagnosticsEngine::Note || L == DiagnosticsEngine::Remark;
+}
+
+llvm::StringRef diagLeveltoString(DiagnosticsEngine::Level Lvl) {
+  switch (Lvl) {
+  case DiagnosticsEngine::Ignored:
+    return "ignored";
+  case DiagnosticsEngine::Note:
+    return "note";
+  case DiagnosticsEngine::Remark:
+    return "remark";
+  case DiagnosticsEngine::Warning:
+    return "warning";
+  case DiagnosticsEngine::Error:
+    return "error";
+  case DiagnosticsEngine::Fatal:
+    return "fatal error";
+  }
+  llvm_unreachable("unhandled DiagnosticsEngine::Level");
+}
+
+/// Prints a single diagnostic in a clang-like manner, the output includes
+/// location, severity and error message. An example of the output message is:
+///
+///     main.cpp:12:23: error: undeclared identifier
+///
+/// For main file we only print the basename and for all other files we print
+/// the filename on a separate line to provide a slightly more readable output
+/// in the editors:
+///
+///     dir1/dir2/dir3/../../dir4/header.h:12:23
+///     error: undeclared identifier
+void printDiag(llvm::raw_string_ostream &OS, const DiagBase &D) {
+  if (D.InsideMainFile) {
+    // Paths to main files are often taken from compile_command.json, where they
+    // are typically absolute. To reduce noise we print only basename for them,
+    // it should not be confusing and saves space.
+    OS << llvm::sys::path::filename(D.File) << ":";
+  } else {
+    OS << D.File << ":";
+  }
+  // Note +1 to line and character. clangd::Range is zero-based, but when
+  // printing for users we want one-based indexes.
+  auto Pos = D.Range.start;
+  OS << (Pos.line + 1) << ":" << (Pos.character + 1) << ":";
+  // The non-main-file paths are often too long, putting them on a separate
+  // line improves readability.
+  if (D.InsideMainFile)
+    OS << " ";
+  else
+    OS << "\n";
+  OS << diagLeveltoString(D.Severity) << ": " << D.Message;
+}
+
+/// Returns a message sent to LSP for the main diagnostic in \p D.
+/// The message includes all the notes with their corresponding locations.
+/// However, notes with fix-its are excluded as those usually only contain a
+/// fix-it message and just add noise if included in the message for diagnostic.
+/// Example output:
+///
+///     no matching function for call to 'foo'
+///
+///     main.cpp:3:5: note: candidate function not viable: requires 2 arguments
+///
+///     dir1/dir2/dir3/../../dir4/header.h:12:23
+///     note: candidate function not viable: requires 3 arguments
+std::string mainMessage(const Diag &D) {
+  std::string Result;
+  llvm::raw_string_ostream OS(Result);
+  OS << D.Message;
+  for (auto &Note : D.Notes) {
+    OS << "\n\n";
+    printDiag(OS, Note);
+  }
+  OS.flush();
+  return Result;
+}
+
+/// Returns a message sent to LSP for the note of the main diagnostic.
+/// The message includes the main diagnostic to provide the necessary context
+/// for the user to understand the note.
+std::string noteMessage(const Diag &Main, const DiagBase &Note) {
+  std::string Result;
+  llvm::raw_string_ostream OS(Result);
+  OS << Note.Message;
+  OS << "\n\n";
+  printDiag(OS, Main);
+  OS.flush();
+  return Result;
+}
+} // namespace
+
+llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const DiagBase &D) {
+  if (!D.InsideMainFile)
+    OS << "[in " << D.File << "] ";
+  return OS << D.Message;
+}
+
+llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const Fix &F) {
+  OS << F.Message << " {";
+  const char *Sep = "";
+  for (const auto &Edit : F.Edits) {
+    OS << Sep << Edit;
+    Sep = ", ";
+  }
+  return OS << "}";
+}
+
+llvm::raw_ostream &operator<<(llvm::raw_ostream &OS, const Diag &D) {
+  OS << static_cast<const DiagBase &>(D);
+  if (!D.Notes.empty()) {
+    OS << ", notes: {";
+    const char *Sep = "";
+    for (auto &Note : D.Notes) {
+      OS << Sep << Note;
+      Sep = ", ";
+    }
+    OS << "}";
+  }
+  if (!D.Fixes.empty()) {
+    OS << ", fixes: {";
+    const char *Sep = "";
+    for (auto &Fix : D.Fixes) {
+      OS << Sep << Fix;
+      Sep = ", ";
+    }
+  }
+  return OS;
+}
+
+void toLSPDiags(
+    const Diag &D,
+    llvm::function_ref<void(clangd::Diagnostic, llvm::ArrayRef<Fix>)> OutFn) {
+  auto FillBasicFields = [](const DiagBase &D) -> clangd::Diagnostic {
+    clangd::Diagnostic Res;
+    Res.range = D.Range;
+    Res.severity = getSeverity(D.Severity);
+    return Res;
+  };
+
+  {
+    clangd::Diagnostic Main = FillBasicFields(D);
+    Main.message = mainMessage(D);
+    OutFn(std::move(Main), D.Fixes);
+  }
+
+  for (auto &Note : D.Notes) {
+    if (!Note.InsideMainFile)
+      continue;
+    clangd::Diagnostic Res = FillBasicFields(Note);
+    Res.message = noteMessage(D, Note);
+    OutFn(std::move(Res), llvm::ArrayRef<Fix>());
+  }
+}
+
+int getSeverity(DiagnosticsEngine::Level L) {
+  switch (L) {
+  case DiagnosticsEngine::Remark:
+    return 4;
+  case DiagnosticsEngine::Note:
+    return 3;
+  case DiagnosticsEngine::Warning:
+    return 2;
+  case DiagnosticsEngine::Fatal:
+  case DiagnosticsEngine::Error:
+    return 1;
+  case DiagnosticsEngine::Ignored:
+    return 0;
+  }
+  llvm_unreachable("Unknown diagnostic level!");
+}
+
+std::vector<Diag> StoreDiags::take() { return std::move(Output); }
+
+void StoreDiags::BeginSourceFile(const LangOptions &Opts,
+                                 const Preprocessor *) {
+  LangOpts = Opts;
+}
+
+void StoreDiags::EndSourceFile() {
+  flushLastDiag();
+  LangOpts = llvm::None;
+}
+
+void StoreDiags::HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
+                                  const clang::Diagnostic &Info) {
+  DiagnosticConsumer::HandleDiagnostic(DiagLevel, Info);
+
+  if (!LangOpts || !Info.hasSourceManager()) {
+    IgnoreDiagnostics::log(DiagLevel, Info);
+    return;
+  }
+
+  bool InsideMainFile = isInsideMainFile(Info);
+
+  auto FillDiagBase = [&](DiagBase &D) {
+    D.Range = diagnosticRange(Info, *LangOpts);
+    llvm::SmallString<64> Message;
+    Info.FormatDiagnostic(Message);
+    D.Message = Message.str();
+    D.InsideMainFile = InsideMainFile;
+    D.File = Info.getSourceManager().getFilename(Info.getLocation());
+    D.Severity = DiagLevel;
+    return D;
+  };
+
+  auto AddFix = [&]() -> bool {
+    assert(!Info.getFixItHints().empty() &&
+           "diagnostic does not have attached fix-its");
+    if (!InsideMainFile)
+      return false;
+
+    llvm::SmallVector<TextEdit, 1> Edits;
+    for (auto &FixIt : Info.getFixItHints()) {
+      if (!isInsideMainFile(FixIt.RemoveRange.getBegin(),
+                            Info.getSourceManager()))
+        return false;
+      Edits.push_back(toTextEdit(FixIt, Info.getSourceManager(), *LangOpts));
+    }
+
+    llvm::SmallString<64> Message;
+    Info.FormatDiagnostic(Message);
+    LastDiag->Fixes.push_back(Fix{Message.str(), std::move(Edits)});
+    return true;
+  };
+
+  if (!isNote(DiagLevel)) {
+    // Handle the new main diagnostic.
+    flushLastDiag();
+
+    LastDiag = Diag();
+    FillDiagBase(*LastDiag);
+
+    if (!Info.getFixItHints().empty())
+      AddFix();
+  } else {
+    // Handle a note to an existing diagnostic.
+    if (!LastDiag) {
+      assert(false && "Adding a note without main diagnostic");
+      IgnoreDiagnostics::log(DiagLevel, Info);
+      return;
+    }
+
+    if (!Info.getFixItHints().empty()) {
+      // A clang note with fix-it is not a separate diagnostic in clangd. We
+      // attach it as a Fix to the main diagnostic instead.
+      if (!AddFix())
+        IgnoreDiagnostics::log(DiagLevel, Info);
+    } else {
+      // A clang note without fix-its corresponds to clangd::Note.
+      Note N;
+      FillDiagBase(N);
+
+      LastDiag->Notes.push_back(std::move(N));
+    }
+  }
+}
+
+void StoreDiags::flushLastDiag() {
+  if (!LastDiag)
+    return;
+  if (mentionsMainFile(*LastDiag))
+    Output.push_back(std::move(*LastDiag));
+  else
+    log(Twine("Dropped diagnostic outside main file:") + LastDiag->File + ":" +
+        LastDiag->Message);
+  LastDiag.reset();
+}
+
+} // namespace clangd
+} // namespace clang