Index: include/llvm/Support/JSON.h
===================================================================
--- /dev/null
+++ include/llvm/Support/JSON.h
@@ -0,0 +1,548 @@
+//===--- 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/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.
+/// The string keys may be owned or references.
+class Object : public std::map<ObjectKey, Value> {
+public:
+  explicit Object() {}
+  // We use a custom struct for list-init, because pair forces extra copies.
+  struct KV;
+  explicit Object(std::initializer_list<KV> Properties);
+
+  // Allow [] 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(const ObjectKey &K);
+  const Value *get(const ObjectKey &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(const ObjectKey &K) const;
+  llvm::Optional<bool> getBoolean(const ObjectKey &K) const;
+  llvm::Optional<double> getNumber(const ObjectKey &K) const;
+  llvm::Optional<int64_t> getInteger(const ObjectKey &K) const;
+  llvm::Optional<llvm::StringRef> getString(const ObjectKey &K) const;
+  const json::Object *getObject(const ObjectKey &K) const;
+  json::Object *getObject(const ObjectKey &K);
+  const json::Array *getArray(const ObjectKey &K) const;
+  json::Array *getArray(const ObjectKey &K);
+};
+
+/// An Array is a JSON array, which contains heterogeneous JSON values.
+class Array : public std::vector<Value> {
+public:
+  explicit Array() {}
+  explicit Array(std::initializer_list<Value> Elements);
+  template <typename Collection> explicit Array(const Collection &C) {
+    for (const auto &V : C)
+      emplace_back(V);
+  }
+
+  // Typed accessors return None/nullptr if the element has the wrong type.
+  llvm::Optional<std::nullptr_t> getNull(size_t I) const;
+  llvm::Optional<bool> getBoolean(size_t I) const;
+  llvm::Optional<double> getNumber(size_t I) const;
+  llvm::Optional<int64_t> getInteger(size_t I) const;
+  llvm::Optional<llvm::StringRef> getString(size_t I) const;
+  const Object *getObject(size_t I) const;
+  Object *getObject(size_t I);
+  const Array *getArray(size_t I) const;
+  Array *getArray(size_t I);
+};
+
+/// 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.asString()
+///     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->asObject())
+///     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::integral_constant<
+          bool, !std::is_same<T, bool>::value>::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> asNull() const {
+    if (LLVM_LIKELY(Type == T_Null))
+      return nullptr;
+    return llvm::None;
+  }
+  llvm::Optional<bool> asBoolean() const {
+    if (LLVM_LIKELY(Type == T_Boolean))
+      return as<bool>();
+    return llvm::None;
+  }
+  llvm::Optional<double> asNumber() const {
+    if (LLVM_LIKELY(Type == T_Number))
+      return as<double>();
+    return llvm::None;
+  }
+  llvm::Optional<int64_t> asInteger() const {
+    if (LLVM_LIKELY(Type == T_Number)) {
+      double D = as<double>();
+      if (LLVM_LIKELY(std::modf(D, &D) == 0 &&
+                      D >= std::numeric_limits<int64_t>::min() &&
+                      D <= std::numeric_limits<int64_t>::max()))
+        return D;
+    }
+    return llvm::None;
+  }
+  llvm::Optional<llvm::StringRef> asString() 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 *asObject() const {
+    return LLVM_LIKELY(Type == T_Object) ? &as<json::Object>() : nullptr;
+  }
+  json::Object *asObject() {
+    return LLVM_LIKELY(Type == T_Object) ? &as<json::Object>() : nullptr;
+  }
+  const json::Array *asArray() const {
+    return LLVM_LIKELY(Type == T_Array) ? &as<json::Array>() : nullptr;
+  }
+  json::Array *asArray() {
+    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 (&as<T>()) 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,
+    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); }
+
+/// 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; }
+
+  friend bool operator<(const ObjectKey &L, const ObjectKey &R) {
+    return L.Data < R.Data;
+  }
+
+private:
+  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);
+}
+
+struct Object::KV {
+  ObjectKey K;
+  Value V;
+};
+
+inline Object::Object(std::initializer_list<KV> Properties) {
+  for (const auto &P : Properties)
+    emplace(P.K, nullptr).first->second.moveFrom(std::move(P.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.asString()) {
+    Out = *S;
+    return true;
+  }
+  return false;
+}
+inline bool fromJSON(const Value &E, int &Out) {
+  if (auto S = E.asInteger()) {
+    Out = *S;
+    return true;
+  }
+  return false;
+}
+inline bool fromJSON(const Value &E, double &Out) {
+  if (auto S = E.asNumber()) {
+    Out = *S;
+    return true;
+  }
+  return false;
+}
+inline bool fromJSON(const Value &E, bool &Out) {
+  if (auto S = E.asBoolean()) {
+    Out = *S;
+    return true;
+  }
+  return false;
+}
+template <typename T> bool fromJSON(const Value &E, llvm::Optional<T> &Out) {
+  if (E.asNull()) {
+    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.asArray()) {
+    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.asObject()) {
+    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.asObject()) {}
+
+  /// 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: lib/Support/CMakeLists.txt
===================================================================
--- lib/Support/CMakeLists.txt
+++ lib/Support/CMakeLists.txt
@@ -80,6 +80,7 @@
   IntEqClasses.cpp
   IntervalMap.cpp
   JamCRC.cpp
+  JSON.cpp
   KnownBits.cpp
   LEB128.cpp
   LineIterator.cpp
Index: lib/Support/JSON.cpp
===================================================================
--- /dev/null
+++ lib/Support/JSON.cpp
@@ -0,0 +1,640 @@
+//=== 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 emplace(K, Value(nullptr)).first->second;
+}
+Value &Object::operator[](ObjectKey &&K) {
+  return emplace(std::move(K), Value(nullptr)).first->second;
+}
+Value *Object::get(const ObjectKey &K) {
+  auto I = find(K);
+  if (I == end())
+    return nullptr;
+  return &I->second;
+}
+const Value *Object::get(const ObjectKey &K) const {
+  auto I = find(K);
+  if (I == end())
+    return nullptr;
+  return &I->second;
+}
+llvm::Optional<std::nullptr_t> Object::getNull(const ObjectKey &K) const {
+  if (auto *V = get(K))
+    return V->asNull();
+  return llvm::None;
+}
+llvm::Optional<bool> Object::getBoolean(const ObjectKey &K) const {
+  if (auto *V = get(K))
+    return V->asBoolean();
+  return llvm::None;
+}
+llvm::Optional<double> Object::getNumber(const ObjectKey &K) const {
+  if (auto *V = get(K))
+    return V->asNumber();
+  return llvm::None;
+}
+llvm::Optional<int64_t> Object::getInteger(const ObjectKey &K) const {
+  if (auto *V = get(K))
+    return V->asInteger();
+  return llvm::None;
+}
+llvm::Optional<llvm::StringRef> Object::getString(const ObjectKey &K) const {
+  if (auto *V = get(K))
+    return V->asString();
+  return llvm::None;
+}
+const json::Object *Object::getObject(const ObjectKey &K) const {
+  if (auto *V = get(K))
+    return V->asObject();
+  return nullptr;
+}
+json::Object *Object::getObject(const ObjectKey &K) {
+  if (auto *V = get(K))
+    return V->asObject();
+  return nullptr;
+}
+const json::Array *Object::getArray(const ObjectKey &K) const {
+  if (auto *V = get(K))
+    return V->asArray();
+  return nullptr;
+}
+json::Array *Object::getArray(const ObjectKey &K) {
+  if (auto *V = get(K))
+    return V->asArray();
+  return nullptr;
+}
+
+Array::Array(std::initializer_list<Value> Elements) {
+  reserve(Elements.size());
+  for (const Value &V : Elements) {
+    emplace_back(nullptr);
+    back().moveFrom(std::move(V));
+  }
+};
+llvm::Optional<std::nullptr_t> Array::getNull(size_t I) const {
+  return (*this)[I].asNull();
+}
+llvm::Optional<bool> Array::getBoolean(size_t I) const {
+  return (*this)[I].asBoolean();
+}
+llvm::Optional<double> Array::getNumber(size_t I) const {
+  return (*this)[I].asNumber();
+}
+llvm::Optional<int64_t> Array::getInteger(size_t I) const {
+  return (*this)[I].asInteger();
+}
+llvm::Optional<llvm::StringRef> Array::getString(size_t I) const {
+  return (*this)[I].asString();
+}
+const Object *Array::getObject(size_t I) const { return (*this)[I].asObject(); }
+Object *Array::getObject(size_t I) { return (*this)[I].asObject(); }
+const Array *Array::getArray(size_t I) const { return (*this)[I].asArray(); }
+Array *Array::getArray(size_t I) { return (*this)[I].asArray(); }
+
+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.asNull() == *R.asNull();
+    case Value::Boolean:
+      return *L.asBoolean() == *R.asBoolean();
+    case Value::Number:
+      return *L.asNumber() == *R.asNumber();
+    case Value::String:
+      return *L.asString() == *R.asString();
+    case Value::Array:
+      return *L.asArray() == *R.asArray();
+    case Value::Object:
+      return *L.asObject() == *R.asObject();
+  }
+  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.asArray();
+        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.asObject();
+        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;
+
+} // 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 : 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: unittests/Support/CMakeLists.txt
===================================================================
--- unittests/Support/CMakeLists.txt
+++ unittests/Support/CMakeLists.txt
@@ -30,6 +30,7 @@
   FormatVariadicTest.cpp
   GlobPatternTest.cpp
   Host.cpp
+  JSONTest.cpp
   LEB128Test.cpp
   LineIteratorTest.cpp
   LockFileManagerTest.cpp
Index: unittests/Support/JSONTest.cpp
===================================================================
--- /dev/null
+++ unittests/Support/JSONTest.cpp
@@ -0,0 +1,291 @@
+//===-- 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->asObject();
+  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->getBoolean(1), llvm::Optional<bool>(true));
+  ASSERT_TRUE(A->getArray(4));
+  EXPECT_EQ(*A->getArray(4), (Array{1, 2, 3}));
+  EXPECT_EQ(A->getArray(4)->getInteger(1), llvm::Optional<int64_t>(2));
+  int I = 0;
+  for (Value &E : *A) {
+    if (I++ == 5) {
+      ASSERT_TRUE(E.asObject());
+      EXPECT_EQ(E.asObject()->getString("time"),
+                llvm::Optional<llvm::StringRef>("arrow"));
+    } else
+      EXPECT_FALSE(E.asObject());
+  }
+}
+
+// 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