Index: llvm/trunk/include/llvm/Support/JSON.h
===================================================================
--- llvm/trunk/include/llvm/Support/JSON.h
+++ llvm/trunk/include/llvm/Support/JSON.h
@@ -0,0 +1,627 @@
+//===--- JSON.h - JSON values, parsing and serialization -------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===---------------------------------------------------------------------===//
+///
+/// \file
+/// This file supports working with JSON data.
+///
+/// It comprises:
+///
+/// - classes which hold dynamically-typed parsed JSON structures
+///   These are value types that can be composed, inspected, and modified.
+///   See json::Value, and the related types json::Object and json::Array.
+///
+/// - functions to parse JSON text into Values, and to serialize Values to text.
+///   See parse(), operator<<, and format_provider.
+///
+/// - a convention and helpers for mapping between json::Value and user-defined
+///   types. See fromJSON(), ObjectMapper, and the class comment on Value.
+///
+/// Typically, JSON data would be read from an external source, parsed into
+/// a Value, and then converted into some native data structure before doing
+/// real work on it. (And vice versa when writing).
+///
+/// Other serialization mechanisms you may consider:
+///
+/// - YAML is also text-based, and more human-readable than JSON. It's a more
+///   complex format and data model, and YAML parsers aren't ubiquitous.
+///   YAMLParser.h is a streaming parser suitable for parsing large documents
+///   (including JSON, as YAML is a superset). It can be awkward to use
+///   directly. YAML I/O (YAMLTraits.h) provides data mapping that is more
+///   declarative than the toJSON/fromJSON conventions here.
+///
+/// - LLVM bitstream is a space- and CPU- efficient binary format. Typically it
+///   encodes LLVM IR ("bitcode"), but it can be a container for other data.
+///   Low-level reader/writer libraries are in Bitcode/Bitstream*.h
+///
+//===---------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_JSON_H
+#define LLVM_SUPPORT_JSON_H
+
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Error.h"
+#include "llvm/Support/FormatVariadic.h"
+#include "llvm/Support/raw_ostream.h"
+#include <map>
+
+namespace llvm {
+namespace json {
+class Array;
+class ObjectKey;
+class Value;
+
+/// An Object is a JSON object, which maps strings to heterogenous JSON values.
+/// It simulates DenseMap<ObjectKey, Value>. ObjectKey is a maybe-owned string.
+class Object {
+  using Storage = DenseMap<ObjectKey, Value, llvm::DenseMapInfo<StringRef>>;
+  Storage M;
+
+public:
+  using key_type = ObjectKey;
+  using mapped_type = Value;
+  using value_type = Storage::value_type;
+  using iterator = Storage::iterator;
+  using const_iterator = Storage::const_iterator;
+
+  explicit Object() = default;
+  // KV is a trivial key-value struct for list-initialization.
+  // (using std::pair forces extra copies).
+  struct KV;
+  explicit Object(std::initializer_list<KV> Properties);
+
+  iterator begin() { return M.begin(); }
+  const_iterator begin() const { return M.begin(); }
+  iterator end() { return M.end(); }
+  const_iterator end() const { return M.end(); }
+
+  bool empty() const { return M.empty(); }
+  size_t size() const { return M.size(); }
+
+  void clear() { M.clear(); }
+  std::pair<iterator, bool> insert(KV E);
+  template <typename... Ts>
+  std::pair<iterator, bool> try_emplace(const ObjectKey &K, Ts &&... Args) {
+    return M.try_emplace(K, std::forward<Ts>(Args)...);
+  }
+  template <typename... Ts>
+  std::pair<iterator, bool> try_emplace(ObjectKey &&K, Ts &&... Args) {
+    return M.try_emplace(std::move(K), std::forward<Ts>(Args)...);
+  }
+
+  iterator find(StringRef K) { return M.find_as(K); }
+  const_iterator find(StringRef K) const { return M.find_as(K); }
+  // operator[] acts as if Value was default-constructible as null.
+  Value &operator[](const ObjectKey &K);
+  Value &operator[](ObjectKey &&K);
+  // Look up a property, returning nullptr if it doesn't exist.
+  Value *get(StringRef K);
+  const Value *get(StringRef K) const;
+  // Typed accessors return None/nullptr if
+  //   - the property doesn't exist
+  //   - or it has the wrong type
+  llvm::Optional<std::nullptr_t> getNull(StringRef K) const;
+  llvm::Optional<bool> getBoolean(StringRef K) const;
+  llvm::Optional<double> getNumber(StringRef K) const;
+  llvm::Optional<int64_t> getInteger(StringRef K) const;
+  llvm::Optional<llvm::StringRef> getString(StringRef K) const;
+  const json::Object *getObject(StringRef K) const;
+  json::Object *getObject(StringRef K);
+  const json::Array *getArray(StringRef K) const;
+  json::Array *getArray(StringRef K);
+};
+bool operator==(const Object &LHS, const Object &RHS);
+inline bool operator!=(const Object &LHS, const Object &RHS) {
+  return !(LHS == RHS);
+}
+
+/// An Array is a JSON array, which contains heterogeneous JSON values.
+/// It simulates std::vector<Value>.
+class Array {
+  std::vector<Value> V;
+
+public:
+  using value_type = Value;
+  using iterator = std::vector<Value>::iterator;
+  using const_iterator = std::vector<Value>::const_iterator;
+
+  explicit Array() = default;
+  explicit Array(std::initializer_list<Value> Elements);
+  template <typename Collection> explicit Array(const Collection &C) {
+    for (const auto &V : C)
+      emplace_back(V);
+  }
+
+  Value &operator[](size_t I) { return V[I]; }
+  const Value &operator[](size_t I) const { return V[I]; }
+  Value &front() { return V.front(); }
+  const Value &front() const { return V.front(); }
+  Value &back() { return V.back(); }
+  const Value &back() const { return V.back(); }
+  Value *data() { return V.data(); }
+  const Value *data() const { return V.data(); }
+
+  iterator begin() { return V.begin(); }
+  const_iterator begin() const { return V.begin(); }
+  iterator end() { return V.end(); }
+  const_iterator end() const { return V.end(); }
+
+  bool empty() const { return V.empty(); }
+  size_t size() const { return V.size(); }
+
+  void clear() { V.clear(); }
+  void push_back(const Value &E) { V.push_back(E); }
+  void push_back(Value &&E) { V.push_back(std::move(E)); }
+  template <typename... Args> void emplace_back(Args &&... A) {
+    V.emplace_back(std::forward<Args>(A)...);
+  }
+  void pop_back() { V.pop_back(); }
+  iterator insert(const_iterator P, const Value &E) { return V.insert(P, E); }
+  iterator insert(const_iterator P, Value &&E) {
+    return V.insert(P, std::move(E));
+  }
+  template <typename It> iterator insert(const_iterator P, It A, It Z) {
+    return V.insert(P, A, Z);
+  }
+  template <typename... Args> iterator emplace(const_iterator P, Args &&... A) {
+    return V.emplace(P, std::forward<Args>(A)...);
+  }
+
+  friend bool operator==(const Array &L, const Array &R) { return L.V == R.V; }
+};
+inline bool operator!=(const Array &L, const Array &R) { return !(L == R); }
+
+/// A Value is an JSON value of unknown type.
+/// They can be copied, but should generally be moved.
+///
+/// === Composing values ===
+///
+/// You can implicitly construct Values from:
+///   - strings: std::string, SmallString, formatv, StringRef, char*
+///              (char*, and StringRef are references, not copies!)
+///   - numbers
+///   - booleans
+///   - null: nullptr
+///   - arrays: {"foo", 42.0, false}
+///   - serializable things: types with toJSON(const T&)->Value, found by ADL
+///
+/// They can also be constructed from object/array helpers:
+///   - json::Object is a type like map<ObjectKey, Value>
+///   - json::Array is a type like vector<Value>
+/// These can be list-initialized, or used to build up collections in a loop.
+/// json::ary(Collection) converts all items in a collection to Values.
+///
+/// === Inspecting values ===
+///
+/// Each Value is one of the JSON kinds:
+///   null    (nullptr_t)
+///   boolean (bool)
+///   number  (double)
+///   string  (StringRef)
+///   array   (json::Array)
+///   object  (json::Object)
+///
+/// The kind can be queried directly, or implicitly via the typed accessors:
+///   if (Optional<StringRef> S = E.getAsString()
+///     assert(E.kind() == Value::String);
+///
+/// Array and Object also have typed indexing accessors for easy traversal:
+///   Expected<Value> E = parse(R"( {"options": {"font": "sans-serif"}} )");
+///   if (Object* O = E->getAsObject())
+///     if (Object* Opts = O->getObject("options"))
+///       if (Optional<StringRef> Font = Opts->getString("font"))
+///         assert(Opts->at("font").kind() == Value::String);
+///
+/// === Converting JSON values to C++ types ===
+///
+/// The convention is to have a deserializer function findable via ADL:
+///     fromJSON(const json::Value&, T&)->bool
+/// Deserializers are provided for:
+///   - bool
+///   - int
+///   - double
+///   - std::string
+///   - vector<T>, where T is deserializable
+///   - map<string, T>, where T is deserializable
+///   - Optional<T>, where T is deserializable
+/// ObjectMapper can help writing fromJSON() functions for object types.
+///
+/// For conversion in the other direction, the serializer function is:
+///    toJSON(const T&) -> json::Value
+/// If this exists, then it also allows constructing Value from T, and can
+/// be used to serialize vector<T>, map<string, T>, and Optional<T>.
+///
+/// === Serialization ===
+///
+/// Values can be serialized to JSON:
+///   1) raw_ostream << Value                    // Basic formatting.
+///   2) raw_ostream << formatv("{0}", Value)    // Basic formatting.
+///   3) raw_ostream << formatv("{0:2}", Value)  // Pretty-print with indent 2.
+///
+/// And parsed:
+///   Expected<Value> E = json::parse("[1, 2, null]");
+///   assert(E && E->kind() == Value::Array);
+class Value {
+public:
+  enum Kind {
+    Null,
+    Boolean,
+    Number,
+    String,
+    Array,
+    Object,
+  };
+
+  // It would be nice to have Value() be null. But that would make {} null too.
+  Value(const Value &M) { copyFrom(M); }
+  Value(Value &&M) { moveFrom(std::move(M)); }
+  Value(std::initializer_list<Value> Elements);
+  Value(json::Array &&Elements) : Type(T_Array) {
+    create<json::Array>(std::move(Elements));
+  }
+  Value(json::Object &&Properties) : Type(T_Object) {
+    create<json::Object>(std::move(Properties));
+  }
+  // Strings: types with value semantics.
+  Value(std::string &&V) : Type(T_String) { create<std::string>(std::move(V)); }
+  Value(const std::string &V) : Type(T_String) { create<std::string>(V); }
+  Value(const llvm::SmallVectorImpl<char> &V) : Type(T_String) {
+    create<std::string>(V.begin(), V.end());
+  }
+  Value(const llvm::formatv_object_base &V) : Value(V.str()){};
+  // Strings: types with reference semantics.
+  Value(llvm::StringRef V) : Type(T_StringRef) { create<llvm::StringRef>(V); }
+  Value(const char *V) : Type(T_StringRef) { create<llvm::StringRef>(V); }
+  Value(std::nullptr_t) : Type(T_Null) {}
+  // Prevent implicit conversions to boolean.
+  template <typename T, typename = typename std::enable_if<
+                            std::is_same<T, bool>::value>::type>
+  Value(T B) : Type(T_Boolean) {
+    create<bool>(B);
+  }
+  // Numbers: arithmetic types that are not boolean.
+  template <
+      typename T,
+      typename = typename std::enable_if<std::is_arithmetic<T>::value>::type,
+      typename = typename std::enable_if<!std::is_same<T, bool>::value>::type>
+  Value(T D) : Type(T_Number) {
+    create<double>(D);
+  }
+  // Serializable types: with a toJSON(const T&)->Value function, found by ADL.
+  template <typename T,
+            typename = typename std::enable_if<std::is_same<
+                Value, decltype(toJSON(*(const T *)nullptr))>::value>>
+  Value(const T &V) : Value(toJSON(V)) {}
+
+  Value &operator=(const Value &M) {
+    destroy();
+    copyFrom(M);
+    return *this;
+  }
+  Value &operator=(Value &&M) {
+    destroy();
+    moveFrom(std::move(M));
+    return *this;
+  }
+  ~Value() { destroy(); }
+
+  Kind kind() const {
+    switch (Type) {
+    case T_Null:
+      return Null;
+    case T_Boolean:
+      return Boolean;
+    case T_Number:
+      return Number;
+    case T_String:
+    case T_StringRef:
+      return String;
+    case T_Object:
+      return Object;
+    case T_Array:
+      return Array;
+    }
+    llvm_unreachable("Unknown kind");
+  }
+
+  // Typed accessors return None/nullptr if the Value is not of this type.
+  llvm::Optional<std::nullptr_t> getAsNull() const {
+    if (LLVM_LIKELY(Type == T_Null))
+      return nullptr;
+    return llvm::None;
+  }
+  llvm::Optional<bool> getAsBoolean() const {
+    if (LLVM_LIKELY(Type == T_Boolean))
+      return as<bool>();
+    return llvm::None;
+  }
+  llvm::Optional<double> getAsNumber() const {
+    if (LLVM_LIKELY(Type == T_Number))
+      return as<double>();
+    return llvm::None;
+  }
+  llvm::Optional<int64_t> getAsInteger() const {
+    if (LLVM_LIKELY(Type == T_Number)) {
+      double D = as<double>();
+      if (LLVM_LIKELY(std::modf(D, &D) == 0.0 &&
+                      D >= double(std::numeric_limits<int64_t>::min()) &&
+                      D <= double(std::numeric_limits<int64_t>::max())))
+        return D;
+    }
+    return llvm::None;
+  }
+  llvm::Optional<llvm::StringRef> getAsString() const {
+    if (Type == T_String)
+      return llvm::StringRef(as<std::string>());
+    if (LLVM_LIKELY(Type == T_StringRef))
+      return as<llvm::StringRef>();
+    return llvm::None;
+  }
+  const json::Object *getAsObject() const {
+    return LLVM_LIKELY(Type == T_Object) ? &as<json::Object>() : nullptr;
+  }
+  json::Object *getAsObject() {
+    return LLVM_LIKELY(Type == T_Object) ? &as<json::Object>() : nullptr;
+  }
+  const json::Array *getAsArray() const {
+    return LLVM_LIKELY(Type == T_Array) ? &as<json::Array>() : nullptr;
+  }
+  json::Array *getAsArray() {
+    return LLVM_LIKELY(Type == T_Array) ? &as<json::Array>() : nullptr;
+  }
+
+  /// Serializes this Value to JSON, writing it to the provided stream.
+  /// The formatting is compact (no extra whitespace) and deterministic.
+  /// For pretty-printing, use the formatv() format_provider below.
+  friend llvm::raw_ostream &operator<<(llvm::raw_ostream &, const Value &);
+
+private:
+  void destroy();
+  void copyFrom(const Value &M);
+  // We allow moving from *const* Values, by marking all members as mutable!
+  // This hack is needed to support initializer-list syntax efficiently.
+  // (std::initializer_list<T> is a container of const T).
+  void moveFrom(const Value &&M);
+  friend class Array;
+  friend class Object;
+
+  template <typename T, typename... U> void create(U &&... V) {
+    new (reinterpret_cast<T *>(Union.buffer)) T(std::forward<U>(V)...);
+  }
+  template <typename T> T &as() const {
+    return *reinterpret_cast<T *>(Union.buffer);
+  }
+
+  template <typename Indenter>
+  void print(llvm::raw_ostream &, const Indenter &) const;
+  friend struct llvm::format_provider<llvm::json::Value>;
+
+  enum ValueType : char {
+    T_Null,
+    T_Boolean,
+    // FIXME: splitting Number into Double and Integer would allow us to
+    //        round-trip 64-bit integers.
+    T_Number,
+    T_StringRef,
+    T_String,
+    T_Object,
+    T_Array,
+  };
+  // All members mutable, see moveFrom().
+  mutable ValueType Type;
+  mutable llvm::AlignedCharArrayUnion<bool, double, llvm::StringRef,
+                                      std::string, json::Array, json::Object>
+      Union;
+};
+
+bool operator==(const Value &, const Value &);
+inline bool operator!=(const Value &L, const Value &R) { return !(L == R); }
+llvm::raw_ostream &operator<<(llvm::raw_ostream &, const Value &);
+
+/// ObjectKey is a used to capture keys in Object. Like Value but:
+///   - only strings are allowed
+///   - it's optimized for the string literal case (Owned == nullptr)
+class ObjectKey {
+public:
+  ObjectKey(const char *S) : Data(S) {}
+  ObjectKey(llvm::StringRef S) : Data(S) {}
+  ObjectKey(std::string &&V)
+      : Owned(new std::string(std::move(V))), Data(*Owned) {}
+  ObjectKey(const std::string &V) : Owned(new std::string(V)), Data(*Owned) {}
+  ObjectKey(const llvm::SmallVectorImpl<char> &V)
+      : ObjectKey(std::string(V.begin(), V.end())) {}
+  ObjectKey(const llvm::formatv_object_base &V) : ObjectKey(V.str()) {}
+
+  ObjectKey(const ObjectKey &C) { *this = C; }
+  ObjectKey(ObjectKey &&C) : ObjectKey(static_cast<const ObjectKey &&>(C)) {}
+  ObjectKey &operator=(const ObjectKey &C) {
+    if (C.Owned) {
+      Owned.reset(new std::string(*C.Owned));
+      Data = *Owned;
+    } else {
+      Data = C.Data;
+    }
+    return *this;
+  }
+  ObjectKey &operator=(ObjectKey &&) = default;
+
+  operator llvm::StringRef() const { return Data; }
+  std::string str() const { return Data.str(); }
+
+private:
+  // FIXME: this is unneccesarily large (3 pointers). Pointer + length + owned
+  // could be 2 pointers at most.
+  std::unique_ptr<std::string> Owned;
+  llvm::StringRef Data;
+};
+
+inline bool operator==(const ObjectKey &L, const ObjectKey &R) {
+  return llvm::StringRef(L) == llvm::StringRef(R);
+}
+inline bool operator!=(const ObjectKey &L, const ObjectKey &R) {
+  return !(L == R);
+}
+inline bool operator<(const ObjectKey &L, const ObjectKey &R) {
+  return StringRef(L) < StringRef(R);
+}
+
+struct Object::KV {
+  ObjectKey K;
+  Value V;
+};
+
+inline Object::Object(std::initializer_list<KV> Properties) {
+  for (const auto &P : Properties) {
+    auto R = try_emplace(P.K, nullptr);
+    if (R.second)
+      R.first->getSecond().moveFrom(std::move(P.V));
+  }
+}
+inline std::pair<Object::iterator, bool> Object::insert(KV E) {
+  return try_emplace(std::move(E.K), std::move(E.V));
+}
+
+// Standard deserializers are provided for primitive types.
+// See comments on Value.
+inline bool fromJSON(const Value &E, std::string &Out) {
+  if (auto S = E.getAsString()) {
+    Out = *S;
+    return true;
+  }
+  return false;
+}
+inline bool fromJSON(const Value &E, int &Out) {
+  if (auto S = E.getAsInteger()) {
+    Out = *S;
+    return true;
+  }
+  return false;
+}
+inline bool fromJSON(const Value &E, double &Out) {
+  if (auto S = E.getAsNumber()) {
+    Out = *S;
+    return true;
+  }
+  return false;
+}
+inline bool fromJSON(const Value &E, bool &Out) {
+  if (auto S = E.getAsBoolean()) {
+    Out = *S;
+    return true;
+  }
+  return false;
+}
+template <typename T> bool fromJSON(const Value &E, llvm::Optional<T> &Out) {
+  if (E.getAsNull()) {
+    Out = llvm::None;
+    return true;
+  }
+  T Result;
+  if (!fromJSON(E, Result))
+    return false;
+  Out = std::move(Result);
+  return true;
+}
+template <typename T> bool fromJSON(const Value &E, std::vector<T> &Out) {
+  if (auto *A = E.getAsArray()) {
+    Out.clear();
+    Out.resize(A->size());
+    for (size_t I = 0; I < A->size(); ++I)
+      if (!fromJSON((*A)[I], Out[I]))
+        return false;
+    return true;
+  }
+  return false;
+}
+template <typename T>
+bool fromJSON(const Value &E, std::map<std::string, T> &Out) {
+  if (auto *O = E.getAsObject()) {
+    Out.clear();
+    for (const auto &KV : *O)
+      if (!fromJSON(KV.second, Out[llvm::StringRef(KV.first)]))
+        return false;
+    return true;
+  }
+  return false;
+}
+
+/// Helper for mapping JSON objects onto protocol structs.
+///
+/// Example:
+/// \code
+///   bool fromJSON(const Value &E, MyStruct &R) {
+///     ObjectMapper O(E);
+///     if (!O || !O.map("mandatory_field", R.MandatoryField))
+///       return false;
+///     O.map("optional_field", R.OptionalField);
+///     return true;
+///   }
+/// \endcode
+class ObjectMapper {
+public:
+  ObjectMapper(const Value &E) : O(E.getAsObject()) {}
+
+  /// True if the expression is an object.
+  /// Must be checked before calling map().
+  operator bool() { return O; }
+
+  /// Maps a property to a field, if it exists.
+  template <typename T> bool map(StringRef Prop, T &Out) {
+    assert(*this && "Must check this is an object before calling map()");
+    if (const Value *E = O->get(Prop))
+      return fromJSON(*E, Out);
+    return false;
+  }
+
+  /// Maps a property to a field, if it exists.
+  /// (Optional requires special handling, because missing keys are OK).
+  template <typename T> bool map(StringRef Prop, llvm::Optional<T> &Out) {
+    assert(*this && "Must check this is an object before calling map()");
+    if (const Value *E = O->get(Prop))
+      return fromJSON(*E, Out);
+    Out = llvm::None;
+    return true;
+  }
+
+private:
+  const Object *O;
+};
+
+/// Parses the provided JSON source, or returns a ParseError.
+/// The returned Value is self-contained and owns its strings (they do not refer
+/// to the original source).
+llvm::Expected<Value> parse(llvm::StringRef JSON);
+
+class ParseError : public llvm::ErrorInfo<ParseError> {
+  const char *Msg;
+  unsigned Line, Column, Offset;
+
+public:
+  static char ID;
+  ParseError(const char *Msg, unsigned Line, unsigned Column, unsigned Offset)
+      : Msg(Msg), Line(Line), Column(Column), Offset(Offset) {}
+  void log(llvm::raw_ostream &OS) const override {
+    OS << llvm::formatv("[{0}:{1}, byte={2}]: {3}", Line, Column, Offset, Msg);
+  }
+  std::error_code convertToErrorCode() const override {
+    return llvm::inconvertibleErrorCode();
+  }
+};
+} // namespace json
+
+/// Allow printing json::Value with formatv().
+/// The default style is basic/compact formatting, like operator<<.
+/// A format string like formatv("{0:2}", Value) pretty-prints with indent 2.
+template <> struct format_provider<llvm::json::Value> {
+  static void format(const llvm::json::Value &, raw_ostream &, StringRef);
+};
+} // namespace llvm
+
+#endif
Index: llvm/trunk/lib/Support/CMakeLists.txt
===================================================================
--- llvm/trunk/lib/Support/CMakeLists.txt
+++ llvm/trunk/lib/Support/CMakeLists.txt
@@ -83,6 +83,7 @@
   IntEqClasses.cpp
   IntervalMap.cpp
   JamCRC.cpp
+  JSON.cpp
   KnownBits.cpp
   LEB128.cpp
   LineIterator.cpp
Index: llvm/trunk/lib/Support/JSON.cpp
===================================================================
--- llvm/trunk/lib/Support/JSON.cpp
+++ llvm/trunk/lib/Support/JSON.cpp
@@ -0,0 +1,642 @@
+//=== JSON.cpp - JSON value, parsing and serialization - C++ -----------*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===---------------------------------------------------------------------===//
+
+#include "llvm/Support/JSON.h"
+#include "llvm/Support/Format.h"
+#include <cctype>
+
+namespace llvm {
+namespace json {
+
+Value &Object::operator[](const ObjectKey &K) {
+  return try_emplace(K, nullptr).first->getSecond();
+}
+Value &Object::operator[](ObjectKey &&K) {
+  return try_emplace(std::move(K), nullptr).first->getSecond();
+}
+Value *Object::get(StringRef K) {
+  auto I = find(K);
+  if (I == end())
+    return nullptr;
+  return &I->second;
+}
+const Value *Object::get(StringRef K) const {
+  auto I = find(K);
+  if (I == end())
+    return nullptr;
+  return &I->second;
+}
+llvm::Optional<std::nullptr_t> Object::getNull(StringRef K) const {
+  if (auto *V = get(K))
+    return V->getAsNull();
+  return llvm::None;
+}
+llvm::Optional<bool> Object::getBoolean(StringRef K) const {
+  if (auto *V = get(K))
+    return V->getAsBoolean();
+  return llvm::None;
+}
+llvm::Optional<double> Object::getNumber(StringRef K) const {
+  if (auto *V = get(K))
+    return V->getAsNumber();
+  return llvm::None;
+}
+llvm::Optional<int64_t> Object::getInteger(StringRef K) const {
+  if (auto *V = get(K))
+    return V->getAsInteger();
+  return llvm::None;
+}
+llvm::Optional<llvm::StringRef> Object::getString(StringRef K) const {
+  if (auto *V = get(K))
+    return V->getAsString();
+  return llvm::None;
+}
+const json::Object *Object::getObject(StringRef K) const {
+  if (auto *V = get(K))
+    return V->getAsObject();
+  return nullptr;
+}
+json::Object *Object::getObject(StringRef K) {
+  if (auto *V = get(K))
+    return V->getAsObject();
+  return nullptr;
+}
+const json::Array *Object::getArray(StringRef K) const {
+  if (auto *V = get(K))
+    return V->getAsArray();
+  return nullptr;
+}
+json::Array *Object::getArray(StringRef K) {
+  if (auto *V = get(K))
+    return V->getAsArray();
+  return nullptr;
+}
+bool operator==(const Object &LHS, const Object &RHS) {
+  if (LHS.size() != RHS.size())
+    return false;
+  for (const auto &L : LHS) {
+    auto R = RHS.find(L.first);
+    if (R == RHS.end() || L.second != R->second)
+      return false;
+  }
+  return true;
+}
+
+Array::Array(std::initializer_list<Value> Elements) {
+  V.reserve(Elements.size());
+  for (const Value &V : Elements) {
+    emplace_back(nullptr);
+    back().moveFrom(std::move(V));
+  }
+}
+
+Value::Value(std::initializer_list<Value> Elements)
+    : Value(json::Array(Elements)) {}
+
+void Value::copyFrom(const Value &M) {
+  Type = M.Type;
+  switch (Type) {
+  case T_Null:
+  case T_Boolean:
+  case T_Number:
+    memcpy(Union.buffer, M.Union.buffer, sizeof(Union.buffer));
+    break;
+  case T_StringRef:
+    create<StringRef>(M.as<StringRef>());
+    break;
+  case T_String:
+    create<std::string>(M.as<std::string>());
+    break;
+  case T_Object:
+    create<json::Object>(M.as<json::Object>());
+    break;
+  case T_Array:
+    create<json::Array>(M.as<json::Array>());
+    break;
+  }
+}
+
+void Value::moveFrom(const Value &&M) {
+  Type = M.Type;
+  switch (Type) {
+  case T_Null:
+  case T_Boolean:
+  case T_Number:
+    memcpy(Union.buffer, M.Union.buffer, sizeof(Union.buffer));
+    break;
+  case T_StringRef:
+    create<StringRef>(M.as<StringRef>());
+    break;
+  case T_String:
+    create<std::string>(std::move(M.as<std::string>()));
+    M.Type = T_Null;
+    break;
+  case T_Object:
+    create<json::Object>(std::move(M.as<json::Object>()));
+    M.Type = T_Null;
+    break;
+  case T_Array:
+    create<json::Array>(std::move(M.as<json::Array>()));
+    M.Type = T_Null;
+    break;
+  }
+}
+
+void Value::destroy() {
+  switch (Type) {
+  case T_Null:
+  case T_Boolean:
+  case T_Number:
+    break;
+  case T_StringRef:
+    as<StringRef>().~StringRef();
+    break;
+  case T_String:
+    as<std::string>().~basic_string();
+    break;
+  case T_Object:
+    as<json::Object>().~Object();
+    break;
+  case T_Array:
+    as<json::Array>().~Array();
+    break;
+  }
+}
+
+bool operator==(const Value &L, const Value &R) {
+  if (L.kind() != R.kind())
+    return false;
+  switch (L.kind()) {
+  case Value::Null:
+    return *L.getAsNull() == *R.getAsNull();
+  case Value::Boolean:
+    return *L.getAsBoolean() == *R.getAsBoolean();
+  case Value::Number:
+    return *L.getAsNumber() == *R.getAsNumber();
+  case Value::String:
+    return *L.getAsString() == *R.getAsString();
+  case Value::Array:
+    return *L.getAsArray() == *R.getAsArray();
+  case Value::Object:
+    return *L.getAsObject() == *R.getAsObject();
+  }
+  llvm_unreachable("Unknown value kind");
+}
+
+namespace {
+// Simple recursive-descent JSON parser.
+class Parser {
+public:
+  Parser(StringRef JSON)
+      : Start(JSON.begin()), P(JSON.begin()), End(JSON.end()) {}
+
+  bool parseValue(Value &Out);
+
+  bool assertEnd() {
+    eatWhitespace();
+    if (P == End)
+      return true;
+    return parseError("Text after end of document");
+  }
+
+  Error takeError() {
+    assert(Err);
+    return std::move(*Err);
+  }
+
+private:
+  void eatWhitespace() {
+    while (P != End && (*P == ' ' || *P == '\r' || *P == '\n' || *P == '\t'))
+      ++P;
+  }
+
+  // On invalid syntax, parseX() functions return false and set Err.
+  bool parseNumber(char First, double &Out);
+  bool parseString(std::string &Out);
+  bool parseUnicode(std::string &Out);
+  bool parseError(const char *Msg); // always returns false
+
+  char next() { return P == End ? 0 : *P++; }
+  char peek() { return P == End ? 0 : *P; }
+  static bool isNumber(char C) {
+    return C == '0' || C == '1' || C == '2' || C == '3' || C == '4' ||
+           C == '5' || C == '6' || C == '7' || C == '8' || C == '9' ||
+           C == 'e' || C == 'E' || C == '+' || C == '-' || C == '.';
+  }
+
+  Optional<Error> Err;
+  const char *Start, *P, *End;
+};
+
+bool Parser::parseValue(Value &Out) {
+  eatWhitespace();
+  if (P == End)
+    return parseError("Unexpected EOF");
+  switch (char C = next()) {
+  // Bare null/true/false are easy - first char identifies them.
+  case 'n':
+    Out = nullptr;
+    return (next() == 'u' && next() == 'l' && next() == 'l') ||
+           parseError("Invalid JSON value (null?)");
+  case 't':
+    Out = true;
+    return (next() == 'r' && next() == 'u' && next() == 'e') ||
+           parseError("Invalid JSON value (true?)");
+  case 'f':
+    Out = false;
+    return (next() == 'a' && next() == 'l' && next() == 's' && next() == 'e') ||
+           parseError("Invalid JSON value (false?)");
+  case '"': {
+    std::string S;
+    if (parseString(S)) {
+      Out = std::move(S);
+      return true;
+    }
+    return false;
+  }
+  case '[': {
+    Out = Array{};
+    Array &A = *Out.getAsArray();
+    eatWhitespace();
+    if (peek() == ']') {
+      ++P;
+      return true;
+    }
+    for (;;) {
+      A.emplace_back(nullptr);
+      if (!parseValue(A.back()))
+        return false;
+      eatWhitespace();
+      switch (next()) {
+      case ',':
+        eatWhitespace();
+        continue;
+      case ']':
+        return true;
+      default:
+        return parseError("Expected , or ] after array element");
+      }
+    }
+  }
+  case '{': {
+    Out = Object{};
+    Object &O = *Out.getAsObject();
+    eatWhitespace();
+    if (peek() == '}') {
+      ++P;
+      return true;
+    }
+    for (;;) {
+      if (next() != '"')
+        return parseError("Expected object key");
+      std::string K;
+      if (!parseString(K))
+        return false;
+      eatWhitespace();
+      if (next() != ':')
+        return parseError("Expected : after object key");
+      eatWhitespace();
+      if (!parseValue(O[std::move(K)]))
+        return false;
+      eatWhitespace();
+      switch (next()) {
+      case ',':
+        eatWhitespace();
+        continue;
+      case '}':
+        return true;
+      default:
+        return parseError("Expected , or } after object property");
+      }
+    }
+  }
+  default:
+    if (isNumber(C)) {
+      double Num;
+      if (parseNumber(C, Num)) {
+        Out = Num;
+        return true;
+      } else {
+        return false;
+      }
+    }
+    return parseError("Invalid JSON value");
+  }
+}
+
+bool Parser::parseNumber(char First, double &Out) {
+  SmallString<24> S;
+  S.push_back(First);
+  while (isNumber(peek()))
+    S.push_back(next());
+  char *End;
+  Out = std::strtod(S.c_str(), &End);
+  return End == S.end() || parseError("Invalid JSON value (number?)");
+}
+
+bool Parser::parseString(std::string &Out) {
+  // leading quote was already consumed.
+  for (char C = next(); C != '"'; C = next()) {
+    if (LLVM_UNLIKELY(P == End))
+      return parseError("Unterminated string");
+    if (LLVM_UNLIKELY((C & 0x1f) == C))
+      return parseError("Control character in string");
+    if (LLVM_LIKELY(C != '\\')) {
+      Out.push_back(C);
+      continue;
+    }
+    // Handle escape sequence.
+    switch (C = next()) {
+    case '"':
+    case '\\':
+    case '/':
+      Out.push_back(C);
+      break;
+    case 'b':
+      Out.push_back('\b');
+      break;
+    case 'f':
+      Out.push_back('\f');
+      break;
+    case 'n':
+      Out.push_back('\n');
+      break;
+    case 'r':
+      Out.push_back('\r');
+      break;
+    case 't':
+      Out.push_back('\t');
+      break;
+    case 'u':
+      if (!parseUnicode(Out))
+        return false;
+      break;
+    default:
+      return parseError("Invalid escape sequence");
+    }
+  }
+  return true;
+}
+
+static void encodeUtf8(uint32_t Rune, std::string &Out) {
+  if (Rune < 0x80) {
+    Out.push_back(Rune & 0x7F);
+  } else if (Rune < 0x800) {
+    uint8_t FirstByte = 0xC0 | ((Rune & 0x7C0) >> 6);
+    uint8_t SecondByte = 0x80 | (Rune & 0x3F);
+    Out.push_back(FirstByte);
+    Out.push_back(SecondByte);
+  } else if (Rune < 0x10000) {
+    uint8_t FirstByte = 0xE0 | ((Rune & 0xF000) >> 12);
+    uint8_t SecondByte = 0x80 | ((Rune & 0xFC0) >> 6);
+    uint8_t ThirdByte = 0x80 | (Rune & 0x3F);
+    Out.push_back(FirstByte);
+    Out.push_back(SecondByte);
+    Out.push_back(ThirdByte);
+  } else if (Rune < 0x110000) {
+    uint8_t FirstByte = 0xF0 | ((Rune & 0x1F0000) >> 18);
+    uint8_t SecondByte = 0x80 | ((Rune & 0x3F000) >> 12);
+    uint8_t ThirdByte = 0x80 | ((Rune & 0xFC0) >> 6);
+    uint8_t FourthByte = 0x80 | (Rune & 0x3F);
+    Out.push_back(FirstByte);
+    Out.push_back(SecondByte);
+    Out.push_back(ThirdByte);
+    Out.push_back(FourthByte);
+  } else {
+    llvm_unreachable("Invalid codepoint");
+  }
+}
+
+// Parse a UTF-16 \uNNNN escape sequence. "\u" has already been consumed.
+// May parse several sequential escapes to ensure proper surrogate handling.
+// We do not use ConvertUTF.h, it can't accept and replace unpaired surrogates.
+// These are invalid Unicode but valid JSON (RFC 8259, section 8.2).
+bool Parser::parseUnicode(std::string &Out) {
+  // Invalid UTF is not a JSON error (RFC 8529§8.2). It gets replaced by U+FFFD.
+  auto Invalid = [&] { Out.append(/* UTF-8 */ {'\xef', '\xbf', '\xbd'}); };
+  // Decodes 4 hex digits from the stream into Out, returns false on error.
+  auto Parse4Hex = [this](uint16_t &Out) -> bool {
+    Out = 0;
+    char Bytes[] = {next(), next(), next(), next()};
+    for (unsigned char C : Bytes) {
+      if (!std::isxdigit(C))
+        return parseError("Invalid \\u escape sequence");
+      Out <<= 4;
+      Out |= (C > '9') ? (C & ~0x20) - 'A' + 10 : (C - '0');
+    }
+    return true;
+  };
+  uint16_t First; // UTF-16 code unit from the first \u escape.
+  if (!Parse4Hex(First))
+    return false;
+
+  // We loop to allow proper surrogate-pair error handling.
+  while (true) {
+    // Case 1: the UTF-16 code unit is already a codepoint in the BMP.
+    if (LLVM_LIKELY(First < 0xD800 || First >= 0xE000)) {
+      encodeUtf8(First, Out);
+      return true;
+    }
+
+    // Case 2: it's an (unpaired) trailing surrogate.
+    if (LLVM_UNLIKELY(First >= 0xDC00)) {
+      Invalid();
+      return true;
+    }
+
+    // Case 3: it's a leading surrogate. We expect a trailing one next.
+    // Case 3a: there's no trailing \u escape. Don't advance in the stream.
+    if (!LLVM_LIKELY(P + 2 <= End && *P == '\\' && *(P + 1) == 'u')) {
+      Invalid(); // Leading surrogate was unpaired.
+      return true;
+    }
+    P += 2;
+    uint16_t Second;
+    if (!Parse4Hex(Second))
+      return false;
+    // Case 3b: there was another \u escape, but it wasn't a trailing surrogate.
+    if (LLVM_UNLIKELY(Second < 0xDC00 || Second >= 0xE000)) {
+      Invalid();      // Leading surrogate was unpaired.
+      First = Second; // Second escape still needs to be processed.
+      continue;
+    }
+    // Case 3c: a valid surrogate pair encoding an astral codepoint.
+    encodeUtf8(0x10000 | ((First - 0xD800) << 10) | (Second - 0xDC00), Out);
+    return true;
+  }
+}
+
+bool Parser::parseError(const char *Msg) {
+  int Line = 1;
+  const char *StartOfLine = Start;
+  for (const char *X = Start; X < P; ++X) {
+    if (*X == 0x0A) {
+      ++Line;
+      StartOfLine = X + 1;
+    }
+  }
+  Err.emplace(
+      llvm::make_unique<ParseError>(Msg, Line, P - StartOfLine, P - Start));
+  return false;
+}
+} // namespace
+
+Expected<Value> parse(StringRef JSON) {
+  Parser P(JSON);
+  Value E = nullptr;
+  if (P.parseValue(E))
+    if (P.assertEnd())
+      return std::move(E);
+  return P.takeError();
+}
+char ParseError::ID = 0;
+
+static std::vector<const Object::value_type *> sortedElements(const Object &O) {
+  std::vector<const Object::value_type *> Elements;
+  for (const auto &E : O)
+    Elements.push_back(&E);
+  llvm::sort(Elements.begin(), Elements.end(),
+             [](const Object::value_type *L, const Object::value_type *R) {
+               return L->first < R->first;
+             });
+  return Elements;
+}
+
+} // namespace json
+} // namespace llvm
+
+static void quote(llvm::raw_ostream &OS, llvm::StringRef S) {
+  OS << '\"';
+  for (unsigned char C : S) {
+    if (C == 0x22 || C == 0x5C)
+      OS << '\\';
+    if (C >= 0x20) {
+      OS << C;
+      continue;
+    }
+    OS << '\\';
+    switch (C) {
+    // A few characters are common enough to make short escapes worthwhile.
+    case '\t':
+      OS << 't';
+      break;
+    case '\n':
+      OS << 'n';
+      break;
+    case '\r':
+      OS << 'r';
+      break;
+    default:
+      OS << 'u';
+      llvm::write_hex(OS, C, llvm::HexPrintStyle::Lower, 4);
+      break;
+    }
+  }
+  OS << '\"';
+}
+
+enum IndenterAction {
+  Indent,
+  Outdent,
+  Newline,
+  Space,
+};
+
+// Prints JSON. The indenter can be used to control formatting.
+template <typename Indenter>
+void llvm::json::Value::print(raw_ostream &OS, const Indenter &I) const {
+  switch (Type) {
+  case T_Null:
+    OS << "null";
+    break;
+  case T_Boolean:
+    OS << (as<bool>() ? "true" : "false");
+    break;
+  case T_Number:
+    OS << format("%g", as<double>());
+    break;
+  case T_StringRef:
+    quote(OS, as<StringRef>());
+    break;
+  case T_String:
+    quote(OS, as<std::string>());
+    break;
+  case T_Object: {
+    bool Comma = false;
+    OS << '{';
+    I(Indent);
+    for (const auto *P : sortedElements(as<json::Object>())) {
+      if (Comma)
+        OS << ',';
+      Comma = true;
+      I(Newline);
+      quote(OS, P->first);
+      OS << ':';
+      I(Space);
+      P->second.print(OS, I);
+    }
+    I(Outdent);
+    if (Comma)
+      I(Newline);
+    OS << '}';
+    break;
+  }
+  case T_Array: {
+    bool Comma = false;
+    OS << '[';
+    I(Indent);
+    for (const auto &E : as<json::Array>()) {
+      if (Comma)
+        OS << ',';
+      Comma = true;
+      I(Newline);
+      E.print(OS, I);
+    }
+    I(Outdent);
+    if (Comma)
+      I(Newline);
+    OS << ']';
+    break;
+  }
+  }
+}
+
+void llvm::format_provider<llvm::json::Value>::format(
+    const llvm::json::Value &E, raw_ostream &OS, StringRef Options) {
+  if (Options.empty()) {
+    OS << E;
+    return;
+  }
+  unsigned IndentAmount = 0;
+  if (Options.getAsInteger(/*Radix=*/10, IndentAmount))
+    llvm_unreachable("json::Value format options should be an integer");
+  unsigned IndentLevel = 0;
+  E.print(OS, [&](IndenterAction A) {
+    switch (A) {
+    case Newline:
+      OS << '\n';
+      OS.indent(IndentLevel);
+      break;
+    case Space:
+      OS << ' ';
+      break;
+    case Indent:
+      IndentLevel += IndentAmount;
+      break;
+    case Outdent:
+      IndentLevel -= IndentAmount;
+      break;
+    };
+  });
+}
+
+llvm::raw_ostream &llvm::json::operator<<(raw_ostream &OS, const Value &E) {
+  E.print(OS, [](IndenterAction A) { /*ignore*/ });
+  return OS;
+}
Index: llvm/trunk/unittests/Support/CMakeLists.txt
===================================================================
--- llvm/trunk/unittests/Support/CMakeLists.txt
+++ llvm/trunk/unittests/Support/CMakeLists.txt
@@ -30,6 +30,7 @@
   FormatVariadicTest.cpp
   GlobPatternTest.cpp
   Host.cpp
+  JSONTest.cpp
   LEB128Test.cpp
   LineIteratorTest.cpp
   LockFileManagerTest.cpp
Index: llvm/trunk/unittests/Support/JSONTest.cpp
===================================================================
--- llvm/trunk/unittests/Support/JSONTest.cpp
+++ llvm/trunk/unittests/Support/JSONTest.cpp
@@ -0,0 +1,292 @@
+//===-- JSONTest.cpp - JSON unit tests --------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/JSON.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace llvm {
+namespace json {
+
+namespace {
+
+std::string s(const Value &E) { return llvm::formatv("{0}", E).str(); }
+std::string sp(const Value &E) { return llvm::formatv("{0:2}", E).str(); }
+
+TEST(JSONTest, Types) {
+  EXPECT_EQ("true", s(true));
+  EXPECT_EQ("null", s(nullptr));
+  EXPECT_EQ("2.5", s(2.5));
+  EXPECT_EQ(R"("foo")", s("foo"));
+  EXPECT_EQ("[1,2,3]", s({1, 2, 3}));
+  EXPECT_EQ(R"({"x":10,"y":20})", s(Object{{"x", 10}, {"y", 20}}));
+}
+
+TEST(JSONTest, Constructors) {
+  // Lots of edge cases around empty and singleton init lists.
+  EXPECT_EQ("[[[3]]]", s({{{3}}}));
+  EXPECT_EQ("[[[]]]", s({{{}}}));
+  EXPECT_EQ("[[{}]]", s({{Object{}}}));
+  EXPECT_EQ(R"({"A":{"B":{}}})", s(Object{{"A", Object{{"B", Object{}}}}}));
+  EXPECT_EQ(R"({"A":{"B":{"X":"Y"}}})",
+            s(Object{{"A", Object{{"B", Object{{"X", "Y"}}}}}}));
+}
+
+TEST(JSONTest, StringOwnership) {
+  char X[] = "Hello";
+  Value Alias = static_cast<const char *>(X);
+  X[1] = 'a';
+  EXPECT_EQ(R"("Hallo")", s(Alias));
+
+  std::string Y = "Hello";
+  Value Copy = Y;
+  Y[1] = 'a';
+  EXPECT_EQ(R"("Hello")", s(Copy));
+}
+
+TEST(JSONTest, CanonicalOutput) {
+  // Objects are sorted (but arrays aren't)!
+  EXPECT_EQ(R"({"a":1,"b":2,"c":3})", s(Object{{"a", 1}, {"c", 3}, {"b", 2}}));
+  EXPECT_EQ(R"(["a","c","b"])", s({"a", "c", "b"}));
+  EXPECT_EQ("3", s(3.0));
+}
+
+TEST(JSONTest, Escaping) {
+  std::string test = {
+      0,                    // Strings may contain nulls.
+      '\b',   '\f',         // Have mnemonics, but we escape numerically.
+      '\r',   '\n',   '\t', // Escaped with mnemonics.
+      'S',    '\"',   '\\', // Printable ASCII characters.
+      '\x7f',               // Delete is not escaped.
+      '\xce', '\x94',       // Non-ASCII UTF-8 is not escaped.
+  };
+
+  std::string teststring = R"("\u0000\u0008\u000c\r\n\tS\"\\)"
+                           "\x7f\xCE\x94\"";
+
+  EXPECT_EQ(teststring, s(test));
+
+  EXPECT_EQ(R"({"object keys are\nescaped":true})",
+            s(Object{{"object keys are\nescaped", true}}));
+}
+
+TEST(JSONTest, PrettyPrinting) {
+  const char str[] = R"({
+  "empty_array": [],
+  "empty_object": {},
+  "full_array": [
+    1,
+    null
+  ],
+  "full_object": {
+    "nested_array": [
+      {
+        "property": "value"
+      }
+    ]
+  }
+})";
+
+  EXPECT_EQ(str, sp(Object{
+                     {"empty_object", Object{}},
+                     {"empty_array", {}},
+                     {"full_array", {1, nullptr}},
+                     {"full_object",
+                      Object{
+                          {"nested_array",
+                           {Object{
+                               {"property", "value"},
+                           }}},
+                      }},
+                 }));
+}
+
+TEST(JSONTest, Parse) {
+  auto Compare = [](llvm::StringRef S, Value Expected) {
+    if (auto E = parse(S)) {
+      // Compare both string forms and with operator==, in case we have bugs.
+      EXPECT_EQ(*E, Expected);
+      EXPECT_EQ(sp(*E), sp(Expected));
+    } else {
+      handleAllErrors(E.takeError(), [S](const llvm::ErrorInfoBase &E) {
+        FAIL() << "Failed to parse JSON >>> " << S << " <<<: " << E.message();
+      });
+    }
+  };
+
+  Compare(R"(true)", true);
+  Compare(R"(false)", false);
+  Compare(R"(null)", nullptr);
+
+  Compare(R"(42)", 42);
+  Compare(R"(2.5)", 2.5);
+  Compare(R"(2e50)", 2e50);
+  Compare(R"(1.2e3456789)", std::numeric_limits<double>::infinity());
+
+  Compare(R"("foo")", "foo");
+  Compare(R"("\"\\\b\f\n\r\t")", "\"\\\b\f\n\r\t");
+  Compare(R"("\u0000")", llvm::StringRef("\0", 1));
+  Compare("\"\x7f\"", "\x7f");
+  Compare(R"("\ud801\udc37")", u8"\U00010437"); // UTF16 surrogate pair escape.
+  Compare("\"\xE2\x82\xAC\xF0\x9D\x84\x9E\"", u8"\u20ac\U0001d11e"); // UTF8
+  Compare(
+      R"("LoneLeading=\ud801, LoneTrailing=\udc01, LeadingLeadingTrailing=\ud801\ud801\udc37")",
+      u8"LoneLeading=\ufffd, LoneTrailing=\ufffd, "
+      u8"LeadingLeadingTrailing=\ufffd\U00010437"); // Invalid unicode.
+
+  Compare(R"({"":0,"":0})", Object{{"", 0}});
+  Compare(R"({"obj":{},"arr":[]})", Object{{"obj", Object{}}, {"arr", {}}});
+  Compare(R"({"\n":{"\u0000":[[[[]]]]}})",
+          Object{{"\n", Object{
+                            {llvm::StringRef("\0", 1), {{{{}}}}},
+                        }}});
+  Compare("\r[\n\t] ", {});
+}
+
+TEST(JSONTest, ParseErrors) {
+  auto ExpectErr = [](llvm::StringRef Msg, llvm::StringRef S) {
+    if (auto E = parse(S)) {
+      // Compare both string forms and with operator==, in case we have bugs.
+      FAIL() << "Parsed JSON >>> " << S << " <<< but wanted error: " << Msg;
+    } else {
+      handleAllErrors(E.takeError(), [S, Msg](const llvm::ErrorInfoBase &E) {
+        EXPECT_THAT(E.message(), testing::HasSubstr(Msg)) << S;
+      });
+    }
+  };
+  ExpectErr("Unexpected EOF", "");
+  ExpectErr("Unexpected EOF", "[");
+  ExpectErr("Text after end of document", "[][]");
+  ExpectErr("Invalid JSON value (false?)", "fuzzy");
+  ExpectErr("Expected , or ]", "[2?]");
+  ExpectErr("Expected object key", "{a:2}");
+  ExpectErr("Expected : after object key", R"({"a",2})");
+  ExpectErr("Expected , or } after object property", R"({"a":2 "b":3})");
+  ExpectErr("Invalid JSON value", R"([&%!])");
+  ExpectErr("Invalid JSON value (number?)", "1e1.0");
+  ExpectErr("Unterminated string", R"("abc\"def)");
+  ExpectErr("Control character in string", "\"abc\ndef\"");
+  ExpectErr("Invalid escape sequence", R"("\030")");
+  ExpectErr("Invalid \\u escape sequence", R"("\usuck")");
+  ExpectErr("[3:3, byte=19]", R"({
+  "valid": 1,
+  invalid: 2
+})");
+}
+
+TEST(JSONTest, Inspection) {
+  llvm::Expected<Value> Doc = parse(R"(
+    {
+      "null": null,
+      "boolean": false,
+      "number": 2.78,
+      "string": "json",
+      "array": [null, true, 3.14, "hello", [1,2,3], {"time": "arrow"}],
+      "object": {"fruit": "banana"}
+    }
+  )");
+  EXPECT_TRUE(!!Doc);
+
+  Object *O = Doc->getAsObject();
+  ASSERT_TRUE(O);
+
+  EXPECT_FALSE(O->getNull("missing"));
+  EXPECT_FALSE(O->getNull("boolean"));
+  EXPECT_TRUE(O->getNull("null"));
+
+  EXPECT_EQ(O->getNumber("number"), llvm::Optional<double>(2.78));
+  EXPECT_FALSE(O->getInteger("number"));
+  EXPECT_EQ(O->getString("string"), llvm::Optional<llvm::StringRef>("json"));
+  ASSERT_FALSE(O->getObject("missing"));
+  ASSERT_FALSE(O->getObject("array"));
+  ASSERT_TRUE(O->getObject("object"));
+  EXPECT_EQ(*O->getObject("object"), (Object{{"fruit", "banana"}}));
+
+  Array *A = O->getArray("array");
+  ASSERT_TRUE(A);
+  EXPECT_EQ((*A)[1].getAsBoolean(), llvm::Optional<bool>(true));
+  ASSERT_TRUE((*A)[4].getAsArray());
+  EXPECT_EQ(*(*A)[4].getAsArray(), (Array{1, 2, 3}));
+  EXPECT_EQ((*(*A)[4].getAsArray())[1].getAsInteger(),
+            llvm::Optional<int64_t>(2));
+  int I = 0;
+  for (Value &E : *A) {
+    if (I++ == 5) {
+      ASSERT_TRUE(E.getAsObject());
+      EXPECT_EQ(E.getAsObject()->getString("time"),
+                llvm::Optional<llvm::StringRef>("arrow"));
+    } else
+      EXPECT_FALSE(E.getAsObject());
+  }
+}
+
+// Sample struct with typical JSON-mapping rules.
+struct CustomStruct {
+  CustomStruct() : B(false) {}
+  CustomStruct(std::string S, llvm::Optional<int> I, bool B)
+      : S(S), I(I), B(B) {}
+  std::string S;
+  llvm::Optional<int> I;
+  bool B;
+};
+inline bool operator==(const CustomStruct &L, const CustomStruct &R) {
+  return L.S == R.S && L.I == R.I && L.B == R.B;
+}
+inline llvm::raw_ostream &operator<<(llvm::raw_ostream &OS,
+                                     const CustomStruct &S) {
+  return OS << "(" << S.S << ", " << (S.I ? std::to_string(*S.I) : "None")
+            << ", " << S.B << ")";
+}
+bool fromJSON(const Value &E, CustomStruct &R) {
+  ObjectMapper O(E);
+  if (!O || !O.map("str", R.S) || !O.map("int", R.I))
+    return false;
+  O.map("bool", R.B);
+  return true;
+}
+
+TEST(JSONTest, Deserialize) {
+  std::map<std::string, std::vector<CustomStruct>> R;
+  CustomStruct ExpectedStruct = {"foo", 42, true};
+  std::map<std::string, std::vector<CustomStruct>> Expected;
+  Value J = Object{
+      {"foo",
+       Array{
+           Object{
+               {"str", "foo"},
+               {"int", 42},
+               {"bool", true},
+               {"unknown", "ignored"},
+           },
+           Object{{"str", "bar"}},
+           Object{
+               {"str", "baz"}, {"bool", "string"}, // OK, deserialize ignores.
+           },
+       }}};
+  Expected["foo"] = {
+      CustomStruct("foo", 42, true),
+      CustomStruct("bar", llvm::None, false),
+      CustomStruct("baz", llvm::None, false),
+  };
+  ASSERT_TRUE(fromJSON(J, R));
+  EXPECT_EQ(R, Expected);
+
+  CustomStruct V;
+  EXPECT_FALSE(fromJSON(nullptr, V)) << "Not an object " << V;
+  EXPECT_FALSE(fromJSON(Object{}, V)) << "Missing required field " << V;
+  EXPECT_FALSE(fromJSON(Object{{"str", 1}}, V)) << "Wrong type " << V;
+  // Optional<T> must parse as the correct type if present.
+  EXPECT_FALSE(fromJSON(Object{{"str", 1}, {"int", "string"}}, V))
+      << "Wrong type for Optional<T> " << V;
+}
+
+} // namespace
+} // namespace json
+} // namespace llvm