[clangd] Add parsing and value inspection to JSONExpr.

Summary:
This will replace the places where we're using YAMLParser to parse JSON now:
  - the new marshalling code (T::parse()) should handle fewer cases and require
    fewer explicit casts
  - we'll early-reject invalid JSON that YAMLParser accepts
  - we'll be able to fix protocol-parsing bugs caused by the fact that YAML can
    only parse forward

I plan to do the conversion as soon as this lands, but I don't want it in one
patch as the protocol.cpp changes are conflict-prone.

Reviewers: ioeric

Subscribers: ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D40182

llvm-svn: 318774

clang-tools-extra/clangd/JSONExpr.cpp

@@ -22,10 +22,10 @@ void Expr::copyFrom(const Expr &M) {
     create<std::string>(M.as<std::string>());
     break;
   case T_Object:
-    create<Object>(M.as<Object>());
+    create<ObjectExpr>(M.as<ObjectExpr>());
     break;
   case T_Array:
-    create<Array>(M.as<Array>());
+    create<ArrayExpr>(M.as<ArrayExpr>());
     break;
   }
 }
@@ -46,11 +46,11 @@ void Expr::moveFrom(const Expr &&M) {
     M.Type = T_Null;
     break;
   case T_Object:
-    create<Object>(std::move(M.as<Object>()));
+    create<ObjectExpr>(std::move(M.as<ObjectExpr>()));
     M.Type = T_Null;
     break;
   case T_Array:
-    create<Array>(std::move(M.as<Array>()));
+    create<ArrayExpr>(std::move(M.as<ArrayExpr>()));
     M.Type = T_Null;
     break;
   }
@@ -69,14 +69,318 @@ void Expr::destroy() {
     as<std::string>().~basic_string();
     break;
   case T_Object:
-    as<Object>().~Object();
+    as<ObjectExpr>().~ObjectExpr();
     break;
   case T_Array:
-    as<Array>().~Array();
+    as<ArrayExpr>().~ArrayExpr();
     break;
   }
 }
 
+namespace {
+// Simple recursive-descent JSON parser.
+class Parser {
+public:
+  Parser(StringRef JSON)
+      : Start(JSON.begin()), P(JSON.begin()), End(JSON.end()) {}
+
+  bool parseExpr(Expr &Out);
+
+  bool assertEnd() {
+    eatWhitespace();
+    if (P == End)
+      return true;
+    return parseError("Text after end of document");
+  }
+
+  Error takeError() {
+    assert(Error);
+    return std::move(*Error);
+  }
+
+private:
+  void eatWhitespace() {
+    while (P != End && (*P == ' ' || *P == '\r' || *P == '\n' || *P == '\t'))
+      ++P;
+  }
+
+  // On invalid syntax, parseX() functions return false and and set Error.
+  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 == '.';
+  }
+  static void encodeUtf8(uint32_t Rune, std::string &Out);
+
+  Optional<Error> Error;
+  const char *Start, *P, *End;
+};
+
+bool Parser::parseExpr(Expr &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 bareword");
+  case 't':
+    Out = true;
+    return (next() == 'r' && next() == 'u' && next() == 'e') ||
+           parseError("Invalid bareword");
+  case 'f':
+    Out = false;
+    return (next() == 'a' && next() == 'l' && next() == 's' && next() == 'e') ||
+           parseError("Invalid bareword");
+  case '"': {
+    std::string S;
+    if (parseString(S)) {
+      Out = std::move(S);
+      return true;
+    }
+    return false;
+  }
+  case '[': {
+    Out = json::ary{};
+    json::ary &A = *Out.array();
+    eatWhitespace();
+    if (peek() == ']') {
+      ++P;
+      return true;
+    }
+    for (;;) {
+      A.emplace_back(nullptr);
+      if (!parseExpr(A.back()))
+        return false;
+      eatWhitespace();
+      switch (next()) {
+      case ',':
+        eatWhitespace();
+        continue;
+      case ']':
+        return true;
+      default:
+        return parseError("Expected , or ] after array element");
+      }
+    }
+  }
+  case '{': {
+    Out = json::obj{};
+    json::obj &O = *Out.object();
+    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 (!parseExpr(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("Expected 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 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;
+}
+
+void Parser::encodeUtf8(uint32_t Rune, std::string &Out) {
+  if (Rune <= 0x7F) {
+    Out.push_back(Rune & 0x7F);
+  } else if (Rune <= 0x7FF) {
+    uint8_t FirstByte = 0xC0 | ((Rune & 0x7C0) >> 6);
+    uint8_t SecondByte = 0x80 | (Rune & 0x3F);
+    Out.push_back(FirstByte);
+    Out.push_back(SecondByte);
+  } else if (Rune <= 0xFFFF) {
+    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 <= 0x10FFFF) {
+    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 \uNNNN escape sequence, the \u have already been consumed.
+// May parse multiple escapes in the presence of surrogate pairs.
+bool Parser::parseUnicode(std::string &Out) {
+  // Note that invalid unicode is not a JSON error. It gets replaced by U+FFFD.
+  auto Invalid = [&] { Out.append(/* UTF-8 */ {'\xef', '\xbf', '\xbd'}); };
+  auto Parse4Hex = [this](uint16_t &Out) {
+    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;
+  if (!Parse4Hex(First))
+    return false;
+
+  // We loop to allow proper surrogate-pair error handling.
+  while (true) {
+    if (LLVM_LIKELY(First < 0xD800 || First >= 0xE000)) { // BMP.
+      encodeUtf8(First, Out);
+      return true;
+    }
+
+    if (First >= 0xDC00) {
+      Invalid(); // Lone trailing surrogate.
+      return true;
+    }
+
+    // We have a leading surrogate, and need a trailing one.
+    // Don't advance P: a lone surrogate is valid JSON (but invalid unicode)
+    if (P + 2 > End || *P != '\\' || *(P + 1) != 'u') {
+      Invalid(); // Lone leading not followed by \u...
+      return true;
+    }
+    P += 2;
+    uint16_t Second;
+    if (!Parse4Hex(Second))
+      return false;
+    if (Second < 0xDC00 && Second >= 0xE000) {
+      Invalid();      // Leading surrogate not followed by trailing.
+      First = Second; // Second escape still needs to be processed.
+      continue;
+    }
+
+    // Valid surrogate pair.
+    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;
+    }
+  }
+  Error.emplace(
+      llvm::make_unique<ParseError>(Msg, Line, P - StartOfLine, P - Start));
+  return false;
+}
+} // namespace
+
+Expected<Expr> parse(StringRef JSON) {
+  Parser P(JSON);
+  json::Expr E = nullptr;
+  if (P.parseExpr(E))
+    if (P.assertEnd())
+      return std::move(E);
+  return P.takeError();
+}
+char ParseError::ID = 0;
+
 } // namespace json
 } // namespace clangd
 } // namespace clang
@@ -144,7 +448,7 @@ void clang::clangd::json::Expr::print(raw_ostream &OS,
     bool Comma = false;
     OS << '{';
     I(Indent);
-    for (const auto &P : as<Expr::Object>()) {
+    for (const auto &P : as<Expr::ObjectExpr>()) {
       if (Comma)
         OS << ',';
       Comma = true;
@@ -164,7 +468,7 @@ void clang::clangd::json::Expr::print(raw_ostream &OS,
     bool Comma = false;
     OS << '[';
     I(Indent);
-    for (const auto &E : as<Expr::Array>()) {
+    for (const auto &E : as<Expr::ArrayExpr>()) {
       if (Comma)
         OS << ',';
       Comma = true;
@@ -187,6 +491,25 @@ llvm::raw_ostream &operator<<(raw_ostream &OS, const Expr &E) {
   E.print(OS, [](IndenterAction A) { /*ignore*/ });
   return OS;
 }
+
+bool operator==(const Expr &L, const Expr &R) {
+  if (L.kind() != R.kind())
+    return false;
+  switch (L.kind()) {
+  case Expr::Null:
+    return L.null() == R.null();
+  case Expr::Boolean:
+    return L.boolean() == R.boolean();
+  case Expr::Number:
+    return L.boolean() == R.boolean();
+  case Expr::String:
+    return L.string() == R.string();
+  case Expr::Array:
+    return *L.array() == *R.array();
+  case Expr::Object:
+    return *L.object() == *R.object();
+  }
+}
 } // namespace json
 } // namespace clangd
 } // namespace clang

clang-tools-extra/clangd/JSONExpr.h

@@ -1,4 +1,4 @@
-//===--- JSONExpr.h - composable JSON expressions ---------------*- C++ -*-===//
+//===--- JSONExpr.h - JSON expressions, parsing and serialization - C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
@@ -7,6 +7,8 @@
 //
 //===---------------------------------------------------------------------===//
 
+// FIXME: rename to JSON.h now that the scope is wider?
+
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_JSON_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_JSON_H
 
@@ -14,6 +16,7 @@
 
 #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"
 
@@ -21,10 +24,12 @@ namespace clang {
 namespace clangd {
 namespace json {
 
-// An Expr is an opaque temporary JSON structure used to compose documents.
+// An Expr is an JSON value of unknown type.
 // They can be copied, but should generally be moved.
 //
-// You can implicitly construct literals from:
+// === Composing expressions ===
+//
+// You can implicitly construct Exprs from:
 //   - strings: std::string, SmallString, formatv, StringRef, char*
 //              (char*, and StringRef are references, not copies!)
 //   - numbers
@@ -39,25 +44,62 @@ namespace json {
 // These can be list-initialized, or used to build up collections in a loop.
 // json::ary(Collection) converts all items in a collection to Exprs.
 //
+// === Inspecting expressions ===
+//
+// Each Expr is one of the JSON kinds:
+//   null    (nullptr_t)
+//   boolean (bool)
+//   number  (double)
+//   string  (StringRef)
+//   array   (json::ary)
+//   object  (json::obj)
+//
+// The kind can be queried directly, or implicitly via the typed accessors:
+//   if (Optional<StringRef> S = E.string())
+//     assert(E.kind() == Expr::String);
+//
+// Array and Object also have typed indexing accessors for easy traversal:
+//   Expected<Expr> E = parse(R"( {"options": {"font": "sans-serif"}} )");
+//   if (json::obj* O = E->object())
+//     if (json::obj* Opts = O->object("options"))
+//       if (Optional<StringRef> Font = Opts->string("font"))
+//         assert(Opts->at("font").kind() == Expr::String);
+//
+// === Serialization ===
+//
 // Exprs can be serialized to JSON:
 //   1) raw_ostream << Expr                    // Basic formatting.
 //   2) raw_ostream << formatv("{0}", Expr)    // Basic formatting.
 //   3) raw_ostream << formatv("{0:2}", Expr)  // Pretty-print with indent 2.
+//
+// And parsed:
+//   Expected<Expr> E = json::parse("[1, 2, null]");
+//   assert(E && E->kind() == Expr::Array);
 class Expr {
 public:
-  class Object;
+  enum Kind {
+    Null,
+    Boolean,
+    Number,
+    String,
+    Array,
+    Object,
+  };
+  class ObjectExpr;
   class ObjectKey;
-  class Array;
+  class ArrayExpr;
 
   // It would be nice to have Expr() be null. But that would make {} null too...
   Expr(const Expr &M) { copyFrom(M); }
   Expr(Expr &&M) { moveFrom(std::move(M)); }
   // "cheating" move-constructor for moving from initializer_list.
   Expr(const Expr &&M) { moveFrom(std::move(M)); }
-  Expr(std::initializer_list<Expr> Elements) : Expr(Array(Elements)) {}
+  Expr(std::initializer_list<Expr> Elements) : Expr(ArrayExpr(Elements)) {}
-  Expr(Array &&Elements) : Type(T_Array) { create<Array>(std::move(Elements)); }
+  Expr(ArrayExpr &&Elements) : Type(T_Array) {
-  Expr(Object &&Properties) : Type(T_Object) {
+    create<ArrayExpr>(std::move(Elements));
-    create<Object>(std::move(Properties));
+  }
+  Expr(ObjectExpr &&Properties) : Type(T_Object) {
+    create<ObjectExpr>(std::move(Properties));
   }
   // Strings: types with value semantics.
   Expr(std::string &&V) : Type(T_String) { create<std::string>(std::move(V)); }
@@ -104,6 +146,60 @@ public:
   }
   ~Expr() { 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;
+    }
+  }
+
+  // Typed accessors return None/nullptr if the Expr is not of this type.
+  llvm::Optional<std::nullptr_t> null() const {
+    if (LLVM_LIKELY(Type == T_Null))
+      return nullptr;
+    return llvm::None;
+  }
+  llvm::Optional<bool> boolean() const {
+    if (LLVM_LIKELY(Type == T_Null))
+      return as<bool>();
+    return llvm::None;
+  }
+  llvm::Optional<double> number() const {
+    if (LLVM_LIKELY(Type == T_Number))
+      return as<double>();
+    return llvm::None;
+  }
+  llvm::Optional<llvm::StringRef> string() 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 ObjectExpr *object() const {
+    return LLVM_LIKELY(Type == T_Object) ? &as<ObjectExpr>() : nullptr;
+  }
+  ObjectExpr *object() {
+    return LLVM_LIKELY(Type == T_Object) ? &as<ObjectExpr>() : nullptr;
+  }
+  const ArrayExpr *array() const {
+    return LLVM_LIKELY(Type == T_Array) ? &as<ArrayExpr>() : nullptr;
+  }
+  ArrayExpr *array() {
+    return LLVM_LIKELY(Type == T_Array) ? &as<ArrayExpr>() : nullptr;
+  }
+
   friend llvm::raw_ostream &operator<<(llvm::raw_ostream &, const Expr &);
 
 private:
@@ -137,10 +233,8 @@ private:
   mutable ExprType Type;
 
 public:
-  // ObjectKey is a used to capture keys in Expr::Objects. It's like Expr but:
+  // ObjectKey is a used to capture keys in Expr::ObjectExpr. Like Expr but:
   //   - only strings are allowed
-  //   - it's copyable (for std::map)
-  //   - we're slightly more eager to copy, to allow efficient key compares
   //   - it's optimized for the string literal case (Owned == nullptr)
   class ObjectKey {
   public:
@@ -183,12 +277,12 @@ public:
     llvm::StringRef Data;
   };
 
-  class Object : public std::map<ObjectKey, Expr> {
+  class ObjectExpr : public std::map<ObjectKey, Expr> {
   public:
-    explicit Object() {}
+    explicit ObjectExpr() {}
     // Use a custom struct for list-init, because pair forces extra copies.
     struct KV;
-    explicit Object(std::initializer_list<KV> Properties);
+    explicit ObjectExpr(std::initializer_list<KV> Properties);
 
     // Allow [] as if Expr was default-constructible as null.
     Expr &operator[](const ObjectKey &K) {
@@ -199,15 +293,15 @@ public:
     }
   };
 
-  class Array : public std::vector<Expr> {
+  class ArrayExpr : public std::vector<Expr> {
   public:
-    explicit Array() {}
+    explicit ArrayExpr() {}
-    explicit Array(std::initializer_list<Expr> Elements) {
+    explicit ArrayExpr(std::initializer_list<Expr> Elements) {
       reserve(Elements.size());
       for (const Expr &V : Elements)
         emplace_back(std::move(V));
     };
-    template <typename Collection> explicit Array(const Collection &C) {
+    template <typename Collection> explicit ArrayExpr(const Collection &C) {
       for (const auto &V : C)
         emplace_back(V);
     }
@@ -215,23 +309,50 @@ public:
 
 private:
   mutable llvm::AlignedCharArrayUnion<bool, double, llvm::StringRef,
-                                      std::string, Array, Object>
+                                      std::string, ArrayExpr, ObjectExpr>
       Union;
 };
 
-struct Expr::Object::KV {
+bool operator==(const Expr &, const Expr &);
+inline bool operator!=(const Expr &L, const Expr &R) { return !(L == R); }
+inline bool operator==(const Expr::ObjectKey &L, const Expr::ObjectKey &R) {
+  return llvm::StringRef(L) == llvm::StringRef(R);
+}
+inline bool operator!=(const Expr::ObjectKey &L, const Expr::ObjectKey &R) {
+  return !(L == R);
+}
+
+struct Expr::ObjectExpr::KV {
   ObjectKey K;
   Expr V;
 };
 
-inline Expr::Object::Object(std::initializer_list<KV> Properties) {
+inline Expr::ObjectExpr::ObjectExpr(std::initializer_list<KV> Properties) {
   for (const auto &P : Properties)
     emplace(std::move(P.K), std::move(P.V));
 }
 
 // Give Expr::{Object,Array} more convenient names for literal use.
-using obj = Expr::Object;
+using obj = Expr::ObjectExpr;
-using ary = Expr::Array;
+using ary = Expr::ArrayExpr;
+
+llvm::Expected<Expr> 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
 } // namespace clangd

clang-tools-extra/unittests/clangd/JSONExprTests.cpp

@@ -15,6 +15,9 @@
 namespace clang {
 namespace clangd {
 namespace json {
+void PrintTo(const Expr &E, std::ostream *OS) {
+  llvm::raw_os_ostream(*OS) << llvm::formatv("{0:2}", E);
+}
 namespace {
 
 std::string s(const Expr &E) { return llvm::formatv("{0}", E).str(); }
@@ -108,6 +111,77 @@ TEST(JSONExprTests, PrettyPrinting) {
                  }));
 }
 
+TEST(JSONTest, Parse) {
+  auto Compare = [](llvm::StringRef S, Expr 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)", 1.0 / 0.0);
+
+  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")", "\U00010437"); // UTF16 surrogate pair escape.
+  Compare("\"\xE2\x82\xAC\xF0\x9D\x84\x9E\"", "\u20ac\U0001d11e"); // UTF8
+  Compare(R"("\ud801")", "\ufffd"); // Invalid codepoint.
+
+  Compare(R"({"":0,"":0})", obj{{"", 0}});
+  Compare(R"({"obj":{},"arr":[]})", obj{{"obj", obj{}}, {"arr", {}}});
+  Compare(R"({"\n":{"\u0000":[[[[]]]]}})",
+          obj{{"\n", obj{
+                         {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("Text after end of document", "[][]");
+  ExpectErr("Invalid bareword", "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("Expected JSON value", R"([&%!])");
+  ExpectErr("Invalid 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
+})");
+}
+
 } // namespace
 } // namespace json
 } // namespace clangd