text: //===- llvm/Support/YAMLTraits.h --------------------------------*- C++ -*-===//
  //
  // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  // See https://llvm.org/LICENSE.txt for license information.
  // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  //
  //===----------------------------------------------------------------------===//
  
  #ifndef LLVM_SUPPORT_YAMLTRAITS_H
  #define LLVM_SUPPORT_YAMLTRAITS_H
  
  #include "llvm/ADT/Optional.h"
  #include "llvm/ADT/SmallVector.h"
  #include "llvm/ADT/StringExtras.h"
  #include "llvm/ADT/StringMap.h"
  #include "llvm/ADT/StringRef.h"
  #include "llvm/ADT/Twine.h"
  #include "llvm/Support/AlignOf.h"
  #include "llvm/Support/Allocator.h"
  #include "llvm/Support/Endian.h"
  #include "llvm/Support/Regex.h"
  #include "llvm/Support/SourceMgr.h"
  #include "llvm/Support/YAMLParser.h"
  #include "llvm/Support/raw_ostream.h"
  #include <cassert>
  #include <cctype>
  #include <cstddef>
  #include <cstdint>
  #include <iterator>
  #include <map>
  #include <memory>
  #include <new>
  #include <string>
  #include <system_error>
  #include <type_traits>
  #include <vector>
  
  namespace llvm {
  namespace yaml {
  
  enum class NodeKind : uint8_t {
    Scalar,
    Map,
    Sequence,
  };
  
  struct EmptyContext {};
  
  /// This class should be specialized by any type that needs to be converted
  /// to/from a YAML mapping.  For example:
  ///
  ///     struct MappingTraits<MyStruct> {
  ///       static void mapping(IO &io, MyStruct &s) {
  ///         io.mapRequired("name", s.name);
  ///         io.mapRequired("size", s.size);
  ///         io.mapOptional("age",  s.age);
  ///       }
  ///     };
  template<class T>
  struct MappingTraits {
    // Must provide:
    // static void mapping(IO &io, T &fields);
    // Optionally may provide:
    // static StringRef validate(IO &io, T &fields);
    //
    // The optional flow flag will cause generated YAML to use a flow mapping
    // (e.g. { a: 0, b: 1 }):
    // static const bool flow = true;
  };
  
  /// This class is similar to MappingTraits<T> but allows you to pass in
  /// additional context for each map operation.  For example:
  ///
  ///     struct MappingContextTraits<MyStruct, MyContext> {
  ///       static void mapping(IO &io, MyStruct &s, MyContext &c) {
  ///         io.mapRequired("name", s.name);
  ///         io.mapRequired("size", s.size);
  ///         io.mapOptional("age",  s.age);
  ///         ++c.TimesMapped;
  ///       }
  ///     };
  template <class T, class Context> struct MappingContextTraits {
    // Must provide:
    // static void mapping(IO &io, T &fields, Context &Ctx);
    // Optionally may provide:
    // static StringRef validate(IO &io, T &fields, Context &Ctx);
    //
    // The optional flow flag will cause generated YAML to use a flow mapping
    // (e.g. { a: 0, b: 1 }):
    // static const bool flow = true;
  };
  
  /// This class should be specialized by any integral type that converts
  /// to/from a YAML scalar where there is a one-to-one mapping between
  /// in-memory values and a string in YAML.  For example:
  ///
  ///     struct ScalarEnumerationTraits<Colors> {
  ///         static void enumeration(IO &io, Colors &value) {
  ///           io.enumCase(value, "red",   cRed);
  ///           io.enumCase(value, "blue",  cBlue);
  ///           io.enumCase(value, "green", cGreen);
  ///         }
  ///       };
  template <typename T, typename Enable = void> struct ScalarEnumerationTraits {
    // Must provide:
    // static void enumeration(IO &io, T &value);
  };
  
  /// This class should be specialized by any integer type that is a union
  /// of bit values and the YAML representation is a flow sequence of
  /// strings.  For example:
  ///
  ///      struct ScalarBitSetTraits<MyFlags> {
  ///        static void bitset(IO &io, MyFlags &value) {
  ///          io.bitSetCase(value, "big",   flagBig);
  ///          io.bitSetCase(value, "flat",  flagFlat);
  ///          io.bitSetCase(value, "round", flagRound);
  ///        }
  ///      };
  template <typename T, typename Enable = void> struct ScalarBitSetTraits {
    // Must provide:
    // static void bitset(IO &io, T &value);
  };
  
  /// Describe which type of quotes should be used when quoting is necessary.
  /// Some non-printable characters need to be double-quoted, while some others
  /// are fine with simple-quoting, and some don't need any quoting.
  enum class QuotingType { None, Single, Double };
  
  /// This class should be specialized by type that requires custom conversion
  /// to/from a yaml scalar.  For example:
  ///
  ///    template<>
  ///    struct ScalarTraits<MyType> {
  ///      static void output(const MyType &val, void*, llvm::raw_ostream &out) {
  ///        // stream out custom formatting
  ///        out << llvm::format("%x", val);
  ///      }
  ///      static StringRef input(StringRef scalar, void*, MyType &value) {
  ///        // parse scalar and set `value`
  ///        // return empty string on success, or error string
  ///        return StringRef();
  ///      }
  ///      static QuotingType mustQuote(StringRef) { return QuotingType::Single; }
  ///    };
  template <typename T, typename Enable = void> struct ScalarTraits {
    // Must provide:
    //
    // Function to write the value as a string:
    // static void output(const T &value, void *ctxt, llvm::raw_ostream &out);
    //
    // Function to convert a string to a value.  Returns the empty
    // StringRef on success or an error string if string is malformed:
    // static StringRef input(StringRef scalar, void *ctxt, T &value);
    //
    // Function to determine if the value should be quoted.
    // static QuotingType mustQuote(StringRef);
  };
  
  /// This class should be specialized by type that requires custom conversion
  /// to/from a YAML literal block scalar. For example:
  ///
  ///    template <>
  ///    struct BlockScalarTraits<MyType> {
  ///      static void output(const MyType &Value, void*, llvm::raw_ostream &Out)
  ///      {
  ///        // stream out custom formatting
  ///        Out << Value;
  ///      }
  ///      static StringRef input(StringRef Scalar, void*, MyType &Value) {
  ///        // parse scalar and set `value`
  ///        // return empty string on success, or error string
  ///        return StringRef();
  ///      }
  ///    };
  template <typename T>
  struct BlockScalarTraits {
    // Must provide:
    //
    // Function to write the value as a string:
    // static void output(const T &Value, void *ctx, llvm::raw_ostream &Out);
    //
    // Function to convert a string to a value.  Returns the empty
    // StringRef on success or an error string if string is malformed:
    // static StringRef input(StringRef Scalar, void *ctxt, T &Value);
    //
    // Optional:
    // static StringRef inputTag(T &Val, std::string Tag)
    // static void outputTag(const T &Val, raw_ostream &Out)
  };
  
  /// This class should be specialized by type that requires custom conversion
  /// to/from a YAML scalar with optional tags. For example:
  ///
  ///    template <>
  ///    struct TaggedScalarTraits<MyType> {
  ///      static void output(const MyType &Value, void*, llvm::raw_ostream
  ///      &ScalarOut, llvm::raw_ostream &TagOut)
  ///      {
  ///        // stream out custom formatting including optional Tag
  ///        Out << Value;
  ///      }
  ///      static StringRef input(StringRef Scalar, StringRef Tag, void*, MyType
  ///      &Value) {
  ///        // parse scalar and set `value`
  ///        // return empty string on success, or error string
  ///        return StringRef();
  ///      }
  ///      static QuotingType mustQuote(const MyType &Value, StringRef) {
  ///        return QuotingType::Single;
  ///      }
  ///    };
  template <typename T> struct TaggedScalarTraits {
    // Must provide:
    //
    // Function to write the value and tag as strings:
    // static void output(const T &Value, void *ctx, llvm::raw_ostream &ScalarOut,
    // llvm::raw_ostream &TagOut);
    //
    // Function to convert a string to a value.  Returns the empty
    // StringRef on success or an error string if string is malformed:
    // static StringRef input(StringRef Scalar, StringRef Tag, void *ctxt, T
    // &Value);
    //
    // Function to determine if the value should be quoted.
    // static QuotingType mustQuote(const T &Value, StringRef Scalar);
  };
  
  /// This class should be specialized by any type that needs to be converted
  /// to/from a YAML sequence.  For example:
  ///
  ///    template<>
  ///    struct SequenceTraits<MyContainer> {
  ///      static size_t size(IO &io, MyContainer &seq) {
  ///        return seq.size();
  ///      }
  ///      static MyType& element(IO &, MyContainer &seq, size_t index) {
  ///        if ( index >= seq.size() )
  ///          seq.resize(index+1);
  ///        return seq[index];
  ///      }
  ///    };
  template<typename T, typename EnableIf = void>
  struct SequenceTraits {
    // Must provide:
    // static size_t size(IO &io, T &seq);
    // static T::value_type& element(IO &io, T &seq, size_t index);
    //
    // The following is option and will cause generated YAML to use
    // a flow sequence (e.g. [a,b,c]).
    // static const bool flow = true;
  };
  
  /// This class should be specialized by any type for which vectors of that
  /// type need to be converted to/from a YAML sequence.
  template<typename T, typename EnableIf = void>
  struct SequenceElementTraits {
    // Must provide:
    // static const bool flow;
  };
  
  /// This class should be specialized by any type that needs to be converted
  /// to/from a list of YAML documents.
  template<typename T>
  struct DocumentListTraits {
    // Must provide:
    // static size_t size(IO &io, T &seq);
    // static T::value_type& element(IO &io, T &seq, size_t index);
  };
  
  /// This class should be specialized by any type that needs to be converted
  /// to/from a YAML mapping in the case where the names of the keys are not known
  /// in advance, e.g. a string map.
  template <typename T>
  struct CustomMappingTraits {
    // static void inputOne(IO &io, StringRef key, T &elem);
    // static void output(IO &io, T &elem);
  };
  
  /// This class should be specialized by any type that can be represented as
  /// a scalar, map, or sequence, decided dynamically. For example:
  ///
  ///    typedef std::unique_ptr<MyBase> MyPoly;
  ///
  ///    template<>
  ///    struct PolymorphicTraits<MyPoly> {
  ///      static NodeKind getKind(const MyPoly &poly) {
  ///        return poly->getKind();
  ///      }
  ///      static MyScalar& getAsScalar(MyPoly &poly) {
  ///        if (!poly || !isa<MyScalar>(poly))
  ///          poly.reset(new MyScalar());
  ///        return *cast<MyScalar>(poly.get());
  ///      }
  ///      // ...
  ///    };
  template <typename T> struct PolymorphicTraits {
    // Must provide:
    // static NodeKind getKind(const T &poly);
    // static scalar_type &getAsScalar(T &poly);
    // static map_type &getAsMap(T &poly);
    // static sequence_type &getAsSequence(T &poly);
  };
  
  // Only used for better diagnostics of missing traits
  template <typename T>
  struct MissingTrait;
  
  // Test if ScalarEnumerationTraits<T> is defined on type T.
  template <class T>
  struct has_ScalarEnumerationTraits
  {
    using Signature_enumeration = void (*)(class IO&, T&);
  
    template <typename U>
    static char test(SameType<Signature_enumeration, &U::enumeration>*);
  
    template <typename U>
    static double test(...);
  
    static bool const value =
      (sizeof(test<ScalarEnumerationTraits<T>>(nullptr)) == 1);
  };
  
  // Test if ScalarBitSetTraits<T> is defined on type T.
  template <class T>
  struct has_ScalarBitSetTraits
  {
    using Signature_bitset = void (*)(class IO&, T&);
  
    template <typename U>
    static char test(SameType<Signature_bitset, &U::bitset>*);
  
    template <typename U>
    static double test(...);
  
    static bool const value = (sizeof(test<ScalarBitSetTraits<T>>(nullptr)) == 1);
  };
  
  // Test if ScalarTraits<T> is defined on type T.
  template <class T>
  struct has_ScalarTraits
  {
    using Signature_input = StringRef (*)(StringRef, void*, T&);
    using Signature_output = void (*)(const T&, void*, raw_ostream&);
    using Signature_mustQuote = QuotingType (*)(StringRef);
  
    template <typename U>
    static char test(SameType<Signature_input, &U::input> *,
                     SameType<Signature_output, &U::output> *,
                     SameType<Signature_mustQuote, &U::mustQuote> *);
  
    template <typename U>
    static double test(...);
  
    static bool const value =
        (sizeof(test<ScalarTraits<T>>(nullptr, nullptr, nullptr)) == 1);
  };
  
  // Test if BlockScalarTraits<T> is defined on type T.
  template <class T>
  struct has_BlockScalarTraits
  {
    using Signature_input = StringRef (*)(StringRef, void *, T &);
    using Signature_output = void (*)(const T &, void *, raw_ostream &);
  
    template <typename U>
    static char test(SameType<Signature_input, &U::input> *,
                     SameType<Signature_output, &U::output> *);
  
    template <typename U>
    static double test(...);
  
    static bool const value =
        (sizeof(test<BlockScalarTraits<T>>(nullptr, nullptr)) == 1);
  };
  
  // Test if TaggedScalarTraits<T> is defined on type T.
  template <class T> struct has_TaggedScalarTraits {
    using Signature_input = StringRef (*)(StringRef, StringRef, void *, T &);
    using Signature_output = void (*)(const T &, void *, raw_ostream &,
                                      raw_ostream &);
    using Signature_mustQuote = QuotingType (*)(const T &, StringRef);
  
    template <typename U>
    static char test(SameType<Signature_input, &U::input> *,
                     SameType<Signature_output, &U::output> *,
                     SameType<Signature_mustQuote, &U::mustQuote> *);
  
    template <typename U> static double test(...);
  
    static bool const value =
        (sizeof(test<TaggedScalarTraits<T>>(nullptr, nullptr, nullptr)) == 1);
  };
  
  // Test if MappingContextTraits<T> is defined on type T.
  template <class T, class Context> struct has_MappingTraits {
    using Signature_mapping = void (*)(class IO &, T &, Context &);
  
    template <typename U>
    static char test(SameType<Signature_mapping, &U::mapping>*);
  
    template <typename U>
    static double test(...);
  
    static bool const value =
        (sizeof(test<MappingContextTraits<T, Context>>(nullptr)) == 1);
  };
  
  // Test if MappingTraits<T> is defined on type T.
  template <class T> struct has_MappingTraits<T, EmptyContext> {
    using Signature_mapping = void (*)(class IO &, T &);
  
    template <typename U>
    static char test(SameType<Signature_mapping, &U::mapping> *);
  
    template <typename U> static double test(...);
  
    static bool const value = (sizeof(test<MappingTraits<T>>(nullptr)) == 1);
  };
  
  // Test if MappingContextTraits<T>::validate() is defined on type T.
  template <class T, class Context> struct has_MappingValidateTraits {
    using Signature_validate = StringRef (*)(class IO &, T &, Context &);
  
    template <typename U>
    static char test(SameType<Signature_validate, &U::validate>*);
  
    template <typename U>
    static double test(...);
  
    static bool const value =
        (sizeof(test<MappingContextTraits<T, Context>>(nullptr)) == 1);
  };
  
  // Test if MappingTraits<T>::validate() is defined on type T.
  template <class T> struct has_MappingValidateTraits<T, EmptyContext> {
    using Signature_validate = StringRef (*)(class IO &, T &);
  
    template <typename U>
    static char test(SameType<Signature_validate, &U::validate> *);
  
    template <typename U> static double test(...);
  
    static bool const value = (sizeof(test<MappingTraits<T>>(nullptr)) == 1);
  };
  
  // Test if SequenceTraits<T> is defined on type T.
  template <class T>
  struct has_SequenceMethodTraits
  {
    using Signature_size = size_t (*)(class IO&, T&);
  
    template <typename U>
    static char test(SameType<Signature_size, &U::size>*);
  
    template <typename U>
    static double test(...);
  
    static bool const value =  (sizeof(test<SequenceTraits<T>>(nullptr)) == 1);
  };
  
  // Test if CustomMappingTraits<T> is defined on type T.
  template <class T>
  struct has_CustomMappingTraits
  {
    using Signature_input = void (*)(IO &io, StringRef key, T &v);
  
    template <typename U>
    static char test(SameType<Signature_input, &U::inputOne>*);
  
    template <typename U>
    static double test(...);
  
    static bool const value =
        (sizeof(test<CustomMappingTraits<T>>(nullptr)) == 1);
  };
  
  // has_FlowTraits<int> will cause an error with some compilers because
  // it subclasses int.  Using this wrapper only instantiates the
  // real has_FlowTraits only if the template type is a class.
  template <typename T, bool Enabled = std::is_class<T>::value>
  class has_FlowTraits
  {
  public:
     static const bool value = false;
  };
  
  // Some older gcc compilers don't support straight forward tests
  // for members, so test for ambiguity cause by the base and derived
  // classes both defining the member.
  template <class T>
  struct has_FlowTraits<T, true>
  {
    struct Fallback { bool flow; };
    struct Derived : T, Fallback { };
  
    template<typename C>
    static char (&f(SameType<bool Fallback::*, &C::flow>*))[1];
  
    template<typename C>
    static char (&f(...))[2];
  
    static bool const value = sizeof(f<Derived>(nullptr)) == 2;
  };
  
  // Test if SequenceTraits<T> is defined on type T
  template<typename T>
  struct has_SequenceTraits : public std::integral_constant<bool,
                                        has_SequenceMethodTraits<T>::value > { };
  
  // Test if DocumentListTraits<T> is defined on type T
  template <class T>
  struct has_DocumentListTraits
  {
    using Signature_size = size_t (*)(class IO &, T &);
  
    template <typename U>
    static char test(SameType<Signature_size, &U::size>*);
  
    template <typename U>
    static double test(...);
  
    static bool const value = (sizeof(test<DocumentListTraits<T>>(nullptr))==1);
  };
  
  template <class T> struct has_PolymorphicTraits {
    using Signature_getKind = NodeKind (*)(const T &);
  
    template <typename U>
    static char test(SameType<Signature_getKind, &U::getKind> *);
  
    template <typename U> static double test(...);
  
    static bool const value = (sizeof(test<PolymorphicTraits<T>>(nullptr)) == 1);
  };
  
  inline bool isNumeric(StringRef S) {
    const static auto skipDigits = [](StringRef Input) {
      return Input.drop_front(
          std::min(Input.find_first_not_of("0123456789"), Input.size()));
    };
  
    // Make S.front() and S.drop_front().front() (if S.front() is [+-]) calls
    // safe.
    if (S.empty() || S.equals("+") || S.equals("-"))
      return false;
  
    if (S.equals(".nan") || S.equals(".NaN") || S.equals(".NAN"))
      return true;
  
    // Infinity and decimal numbers can be prefixed with sign.
    StringRef Tail = (S.front() == '-' || S.front() == '+') ? S.drop_front() : S;
  
    // Check for infinity first, because checking for hex and oct numbers is more
    // expensive.
    if (Tail.equals(".inf") || Tail.equals(".Inf") || Tail.equals(".INF"))
      return true;
  
    // Section 10.3.2 Tag Resolution
    // YAML 1.2 Specification prohibits Base 8 and Base 16 numbers prefixed with
    // [-+], so S should be used instead of Tail.
    if (S.startswith("0o"))
      return S.size() > 2 &&
             S.drop_front(2).find_first_not_of("01234567") == StringRef::npos;
  
    if (S.startswith("0x"))
      return S.size() > 2 && S.drop_front(2).find_first_not_of(
                                 "0123456789abcdefABCDEF") == StringRef::npos;
  
    // Parse float: [-+]? (\. [0-9]+ | [0-9]+ (\. [0-9]* )?) ([eE] [-+]? [0-9]+)?
    S = Tail;
  
    // Handle cases when the number starts with '.' and hence needs at least one
    // digit after dot (as opposed by number which has digits before the dot), but
    // doesn't have one.
    if (S.startswith(".") &&
        (S.equals(".") ||
         (S.size() > 1 && std::strchr("0123456789", S[1]) == nullptr)))
      return false;
  
    if (S.startswith("E") || S.startswith("e"))
      return false;
  
    enum ParseState {
      Default,
      FoundDot,
      FoundExponent,
    };
    ParseState State = Default;
  
    S = skipDigits(S);
  
    // Accept decimal integer.
    if (S.empty())
      return true;
  
    if (S.front() == '.') {
      State = FoundDot;
      S = S.drop_front();
    } else if (S.front() == 'e' || S.front() == 'E') {
      State = FoundExponent;
      S = S.drop_front();
    } else {
      return false;
    }
  
    if (State == FoundDot) {
      S = skipDigits(S);
      if (S.empty())
        return true;
  
      if (S.front() == 'e' || S.front() == 'E') {
        State = FoundExponent;
        S = S.drop_front();
      } else {
        return false;
      }
    }
  
    assert(State == FoundExponent && "Should have found exponent at this point.");
    if (S.empty())
      return false;
  
    if (S.front() == '+' || S.front() == '-') {
      S = S.drop_front();
      if (S.empty())
        return false;
    }
  
    return skipDigits(S).empty();
  }
  
  inline bool isNull(StringRef S) {
    return S.equals("null") || S.equals("Null") || S.equals("NULL") ||
           S.equals("~");
  }
  
  inline bool isBool(StringRef S) {
    return S.equals("true") || S.equals("True") || S.equals("TRUE") ||
           S.equals("false") || S.equals("False") || S.equals("FALSE");
  }
  
  // 5.1. Character Set
  // The allowed character range explicitly excludes the C0 control block #x0-#x1F
  // (except for TAB #x9, LF #xA, and CR #xD which are allowed), DEL #x7F, the C1
  // control block #x80-#x9F (except for NEL #x85 which is allowed), the surrogate
  // block #xD800-#xDFFF, #xFFFE, and #xFFFF.
  inline QuotingType needsQuotes(StringRef S) {
    if (S.empty())
      return QuotingType::Single;
  
    QuotingType MaxQuotingNeeded = QuotingType::None;
    if (isSpace(static_cast<unsigned char>(S.front())) ||
        isSpace(static_cast<unsigned char>(S.back())))
      MaxQuotingNeeded = QuotingType::Single;
    if (isNull(S))
      MaxQuotingNeeded = QuotingType::Single;
    if (isBool(S))
      MaxQuotingNeeded = QuotingType::Single;
    if (isNumeric(S))
      MaxQuotingNeeded = QuotingType::Single;
  
    // 7.3.3 Plain Style
    // Plain scalars must not begin with most indicators, as this would cause
    // ambiguity with other YAML constructs.
    static constexpr char Indicators[] = R"(-?:\,[]{}#&*!|>'"%@`)";
    if (S.find_first_of(Indicators) == 0)
      MaxQuotingNeeded = QuotingType::Single;
  
    for (unsigned char C : S) {
      // Alphanum is safe.
      if (isAlnum(C))
        continue;
  
      switch (C) {
      // Safe scalar characters.
      case '_':
      case '-':
      case '^':
      case '.':
      case ',':
      case ' ':
      // TAB (0x9) is allowed in unquoted strings.
      case 0x9:
        continue;
      // LF(0xA) and CR(0xD) may delimit values and so require at least single
      // quotes. LLVM YAML parser cannot handle single quoted multiline so use
      // double quoting to produce valid YAML.
      case 0xA:
      case 0xD:
        return QuotingType::Double;
      // DEL (0x7F) are excluded from the allowed character range.
      case 0x7F:
        return QuotingType::Double;
      // Forward slash is allowed to be unquoted, but we quote it anyway.  We have
      // many tests that use FileCheck against YAML output, and this output often
      // contains paths.  If we quote backslashes but not forward slashes then
      // paths will come out either quoted or unquoted depending on which platform
      // the test is run on, making FileCheck comparisons difficult.
      case '/':
      default: {
        // C0 control block (0x0 - 0x1F) is excluded from the allowed character
        // range.
        if (C <= 0x1F)
          return QuotingType::Double;
  
        // Always double quote UTF-8.
        if ((C & 0x80) != 0)
          return QuotingType::Double;
  
        // The character is not safe, at least simple quoting needed.
        MaxQuotingNeeded = QuotingType::Single;
      }
      }
    }
  
    return MaxQuotingNeeded;
  }
  
  template <typename T, typename Context>
  struct missingTraits
      : public std::integral_constant<bool,
                                      !has_ScalarEnumerationTraits<T>::value &&
                                          !has_ScalarBitSetTraits<T>::value &&
                                          !has_ScalarTraits<T>::value &&
                                          !has_BlockScalarTraits<T>::value &&
                                          !has_TaggedScalarTraits<T>::value &&
                                          !has_MappingTraits<T, Context>::value &&
                                          !has_SequenceTraits<T>::value &&
                                          !has_CustomMappingTraits<T>::value &&
                                          !has_DocumentListTraits<T>::value &&
                                          !has_PolymorphicTraits<T>::value> {};
  
  template <typename T, typename Context>
  struct validatedMappingTraits
      : public std::integral_constant<
            bool, has_MappingTraits<T, Context>::value &&
                      has_MappingValidateTraits<T, Context>::value> {};
  
  template <typename T, typename Context>
  struct unvalidatedMappingTraits
      : public std::integral_constant<
            bool, has_MappingTraits<T, Context>::value &&
                      !has_MappingValidateTraits<T, Context>::value> {};
  
  // Base class for Input and Output.
  class IO {
  public:
    IO(void *Ctxt = nullptr);
    virtual ~IO();
  
    virtual bool outputting() const = 0;
  
    virtual unsigned beginSequence() = 0;
    virtual bool preflightElement(unsigned, void *&) = 0;
    virtual void postflightElement(void*) = 0;
    virtual void endSequence() = 0;
    virtual bool canElideEmptySequence() = 0;
  
    virtual unsigned beginFlowSequence() = 0;
    virtual bool preflightFlowElement(unsigned, void *&) = 0;
    virtual void postflightFlowElement(void*) = 0;
    virtual void endFlowSequence() = 0;
  
    virtual bool mapTag(StringRef Tag, bool Default=false) = 0;
    virtual void beginMapping() = 0;
    virtual void endMapping() = 0;
    virtual bool preflightKey(const char*, bool, bool, bool &, void *&) = 0;
    virtual void postflightKey(void*) = 0;
    virtual std::vector<StringRef> keys() = 0;
  
    virtual void beginFlowMapping() = 0;
    virtual void endFlowMapping() = 0;
  
    virtual void beginEnumScalar() = 0;
    virtual bool matchEnumScalar(const char*, bool) = 0;
    virtual bool matchEnumFallback() = 0;
    virtual void endEnumScalar() = 0;
  
    virtual bool beginBitSetScalar(bool &) = 0;
    virtual bool bitSetMatch(const char*, bool) = 0;
    virtual void endBitSetScalar() = 0;
  
    virtual void scalarString(StringRef &, QuotingType) = 0;
    virtual void blockScalarString(StringRef &) = 0;
    virtual void scalarTag(std::string &) = 0;
  
    virtual NodeKind getNodeKind() = 0;
  
    virtual void setError(const Twine &) = 0;
  
    template <typename T>
    void enumCase(T &Val, const char* Str, const T ConstVal) {
      if ( matchEnumScalar(Str, outputting() && Val == ConstVal) ) {
        Val = ConstVal;
      }
    }
  
    // allow anonymous enum values to be used with LLVM_YAML_STRONG_TYPEDEF
    template <typename T>
    void enumCase(T &Val, const char* Str, const uint32_t ConstVal) {
      if ( matchEnumScalar(Str, outputting() && Val == static_cast<T>(ConstVal)) ) {
        Val = ConstVal;
      }
    }
  
    template <typename FBT, typename T>
    void enumFallback(T &Val) {
      if (matchEnumFallback()) {
        EmptyContext Context;
        // FIXME: Force integral conversion to allow strong typedefs to convert.
        FBT Res = static_cast<typename FBT::BaseType>(Val);
        yamlize(*this, Res, true, Context);
        Val = static_cast<T>(static_cast<typename FBT::BaseType>(Res));
      }
    }
  
    template <typename T>
    void bitSetCase(T &Val, const char* Str, const T ConstVal) {
      if ( bitSetMatch(Str, outputting() && (Val & ConstVal) == ConstVal) ) {
        Val = static_cast<T>(Val | ConstVal);
      }
    }
  
    // allow anonymous enum values to be used with LLVM_YAML_STRONG_TYPEDEF
    template <typename T>
    void bitSetCase(T &Val, const char* Str, const uint32_t ConstVal) {
      if ( bitSetMatch(Str, outputting() && (Val & ConstVal) == ConstVal) ) {
        Val = static_cast<T>(Val | ConstVal);
      }
    }
  
    template <typename T>
    void maskedBitSetCase(T &Val, const char *Str, T ConstVal, T Mask) {
      if (bitSetMatch(Str, outputting() && (Val & Mask) == ConstVal))
        Val = Val | ConstVal;
    }
  
    template <typename T>
    void maskedBitSetCase(T &Val, const char *Str, uint32_t ConstVal,
                          uint32_t Mask) {
      if (bitSetMatch(Str, outputting() && (Val & Mask) == ConstVal))
        Val = Val | ConstVal;
    }
  
    void *getContext() const;
    void setContext(void *);
  
    template <typename T> void mapRequired(const char *Key, T &Val) {
      EmptyContext Ctx;
      this->processKey(Key, Val, true, Ctx);
    }
  
    template <typename T, typename Context>
    void mapRequired(const char *Key, T &Val, Context &Ctx) {
      this->processKey(Key, Val, true, Ctx);
    }
  
    template <typename T> void mapOptional(const char *Key, T &Val) {
      EmptyContext Ctx;
      mapOptionalWithContext(Key, Val, Ctx);
    }
  
    template <typename T, typename DefaultT>
    void mapOptional(const char *Key, T &Val, const DefaultT &Default) {
      EmptyContext Ctx;
      mapOptionalWithContext(Key, Val, Default, Ctx);
    }
  
    template <typename T, typename Context>
    std::enable_if_t<has_SequenceTraits<T>::value, void>
    mapOptionalWithContext(const char *Key, T &Val, Context &Ctx) {
      // omit key/value instead of outputting empty sequence
      if (this->canElideEmptySequence() && !(Val.begin() != Val.end()))
        return;
      this->processKey(Key, Val, false, Ctx);
    }
  
    template <typename T, typename Context>
    void mapOptionalWithContext(const char *Key, Optional<T> &Val, Context &Ctx) {
      this->processKeyWithDefault(Key, Val, Optional<T>(), /*Required=*/false,
                                  Ctx);
    }
  
    template <typename T, typename Context>
    std::enable_if_t<!has_SequenceTraits<T>::value, void>
    mapOptionalWithContext(const char *Key, T &Val, Context &Ctx) {
      this->processKey(Key, Val, false, Ctx);
    }
  
    template <typename T, typename Context, typename DefaultT>
    void mapOptionalWithContext(const char *Key, T &Val, const DefaultT &Default,
                                Context &Ctx) {
      static_assert(std::is_convertible<DefaultT, T>::value,
                    "Default type must be implicitly convertible to value type!");
      this->processKeyWithDefault(Key, Val, static_cast<const T &>(Default),
                                  false, Ctx);
    }
  
  private:
    template <typename T, typename Context>
    void processKeyWithDefault(const char *Key, Optional<T> &Val,
                               const Optional<T> &DefaultValue, bool Required,
                               Context &Ctx) {
      assert(DefaultValue.hasValue() == false &&
             "Optional<T> shouldn't have a value!");
      void *SaveInfo;
      bool UseDefault = true;
      const bool sameAsDefault = outputting() && !Val.hasValue();
      if (!outputting() && !Val.hasValue())
        Val = T();
      if (Val.hasValue() &&
          this->preflightKey(Key, Required, sameAsDefault, UseDefault,
                             SaveInfo)) {
        yamlize(*this, Val.getValue(), Required, Ctx);
        this->postflightKey(SaveInfo);
      } else {
        if (UseDefault)
          Val = DefaultValue;
      }
    }
  
    template <typename T, typename Context>
    void processKeyWithDefault(const char *Key, T &Val, const T &DefaultValue,
                               bool Required, Context &Ctx) {
      void *SaveInfo;
      bool UseDefault;
      const bool sameAsDefault = outputting() && Val == DefaultValue;
      if ( this->preflightKey(Key, Required, sameAsDefault, UseDefault,
                                                                    SaveInfo) ) {
        yamlize(*this, Val, Required, Ctx);
        this->postflightKey(SaveInfo);
      }
      else {
        if ( UseDefault )
          Val = DefaultValue;
      }
    }
  
    template <typename T, typename Context>
    void processKey(const char *Key, T &Val, bool Required, Context &Ctx) {
      void *SaveInfo;
      bool UseDefault;
      if ( this->preflightKey(Key, Required, false, UseDefault, SaveInfo) ) {
        yamlize(*this, Val, Required, Ctx);
        this->postflightKey(SaveInfo);
      }
    }
  
  private:
    void *Ctxt;
  };
  
  namespace detail {
  
  template <typename T, typename Context>
  void doMapping(IO &io, T &Val, Context &Ctx) {
    MappingContextTraits<T, Context>::mapping(io, Val, Ctx);
  }
  
  template <typename T> void doMapping(IO &io, T &Val, EmptyContext &Ctx) {
    MappingTraits<T>::mapping(io, Val);
  }
  
  } // end namespace detail
  
  template <typename T>
  std::enable_if_t<has_ScalarEnumerationTraits<T>::value, void>
  yamlize(IO &io, T &Val, bool, EmptyContext &Ctx) {
    io.beginEnumScalar();
    ScalarEnumerationTraits<T>::enumeration(io, Val);
    io.endEnumScalar();
  }
  
  template <typename T>
  std::enable_if_t<has_ScalarBitSetTraits<T>::value, void>
  yamlize(IO &io, T &Val, bool, EmptyContext &Ctx) {
    bool DoClear;
    if ( io.beginBitSetScalar(DoClear) ) {
      if ( DoClear )
        Val = T();
      ScalarBitSetTraits<T>::bitset(io, Val);
      io.endBitSetScalar();
    }
  }
  
  template <typename T>
  std::enable_if_t<has_ScalarTraits<T>::value, void> yamlize(IO &io, T &Val, bool,
                                                             EmptyContext &Ctx) {
    if ( io.outputting() ) {
      std::string Storage;
      raw_string_ostream Buffer(Storage);
      ScalarTraits<T>::output(Val, io.getContext(), Buffer);
      StringRef Str = Buffer.str();
      io.scalarString(Str, ScalarTraits<T>::mustQuote(Str));
    }
    else {
      StringRef Str;
      io.scalarString(Str, ScalarTraits<T>::mustQuote(Str));
      StringRef Result = ScalarTraits<T>::input(Str, io.getContext(), Val);
      if ( !Result.empty() ) {
        io.setError(Twine(Result));
      }
    }
  }
  
  template <typename T>
  std::enable_if_t<has_BlockScalarTraits<T>::value, void>
  yamlize(IO &YamlIO, T &Val, bool, EmptyContext &Ctx) {
    if (YamlIO.outputting()) {
      std::string Storage;
      raw_string_ostream Buffer(Storage);
      BlockScalarTraits<T>::output(Val, YamlIO.getContext(), Buffer);
      StringRef Str = Buffer.str();
      YamlIO.blockScalarString(Str);
    } else {
      StringRef Str;
      YamlIO.blockScalarString(Str);
      StringRef Result =
          BlockScalarTraits<T>::input(Str, YamlIO.getContext(), Val);
      if (!Result.empty())
        YamlIO.setError(Twine(Result));
    }
  }
  
  template <typename T>
  std::enable_if_t<has_TaggedScalarTraits<T>::value, void>
  yamlize(IO &io, T &Val, bool, EmptyContext &Ctx) {
    if (io.outputting()) {
      std::string ScalarStorage, TagStorage;
      raw_string_ostream ScalarBuffer(ScalarStorage), TagBuffer(TagStorage);
      TaggedScalarTraits<T>::output(Val, io.getContext(), ScalarBuffer,
                                    TagBuffer);
      io.scalarTag(TagBuffer.str());
      StringRef ScalarStr = ScalarBuffer.str();
      io.scalarString(ScalarStr,
                      TaggedScalarTraits<T>::mustQuote(Val, ScalarStr));
    } else {
      std::string Tag;
      io.scalarTag(Tag);
      StringRef Str;
      io.scalarString(Str, QuotingType::None);
      StringRef Result =
          TaggedScalarTraits<T>::input(Str, Tag, io.getContext(), Val);
      if (!Result.empty()) {
        io.setError(Twine(Result));
      }
    }
  }
  
  template <typename T, typename Context>
  std::enable_if_t<validatedMappingTraits<T, Context>::value, void>
  yamlize(IO &io, T &Val, bool, Context &Ctx) {
    if (has_FlowTraits<MappingTraits<T>>::value)
      io.beginFlowMapping();
    else
      io.beginMapping();
    if (io.outputting()) {
      StringRef Err = MappingTraits<T>::validate(io, Val);
      if (!Err.empty()) {
        errs() << Err << "\n";
        assert(Err.empty() && "invalid struct trying to be written as yaml");
      }
    }
    detail::doMapping(io, Val, Ctx);
    if (!io.outputting()) {
      StringRef Err = MappingTraits<T>::validate(io, Val);
      if (!Err.empty())
        io.setError(Err);
    }
    if (has_FlowTraits<MappingTraits<T>>::value)
      io.endFlowMapping();
    else
      io.endMapping();
  }
  
  template <typename T, typename Context>
  std::enable_if_t<unvalidatedMappingTraits<T, Context>::value, void>
  yamlize(IO &io, T &Val, bool, Context &Ctx) {
    if (has_FlowTraits<MappingTraits<T>>::value) {
      io.beginFlowMapping();
      detail::doMapping(io, Val, Ctx);
      io.endFlowMapping();
    } else {
      io.beginMapping();
      detail::doMapping(io, Val, Ctx);
      io.endMapping();
    }
  }
  
  template <typename T>
  std::enable_if_t<has_CustomMappingTraits<T>::value, void>
  yamlize(IO &io, T &Val, bool, EmptyContext &Ctx) {
    if ( io.outputting() ) {
      io.beginMapping();
      CustomMappingTraits<T>::output(io, Val);
      io.endMapping();
    } else {
      io.beginMapping();
      for (StringRef key : io.keys())
        CustomMappingTraits<T>::inputOne(io, key, Val);
      io.endMapping();
    }
  }
  
  template <typename T>
  std::enable_if_t<has_PolymorphicTraits<T>::value, void>
  yamlize(IO &io, T &Val, bool, EmptyContext &Ctx) {
    switch (io.outputting() ? PolymorphicTraits<T>::getKind(Val)
                            : io.getNodeKind()) {
    case NodeKind::Scalar:
      return yamlize(io, PolymorphicTraits<T>::getAsScalar(Val), true, Ctx);
    case NodeKind::Map:
      return yamlize(io, PolymorphicTraits<T>::getAsMap(Val), true, Ctx);
    case NodeKind::Sequence:
      return yamlize(io, PolymorphicTraits<T>::getAsSequence(Val), true, Ctx);
    }
  }
  
  template <typename T>
  std::enable_if_t<missingTraits<T, EmptyContext>::value, void>
  yamlize(IO &io, T &Val, bool, EmptyContext &Ctx) {
    char missing_yaml_trait_for_type[sizeof(MissingTrait<T>)];
  }
  
  template <typename T, typename Context>
  std::enable_if_t<has_SequenceTraits<T>::value, void>
  yamlize(IO &io, T &Seq, bool, Context &Ctx) {
    if ( has_FlowTraits< SequenceTraits<T>>::value ) {
      unsigned incnt = io.beginFlowSequence();
      unsigned count = io.outputting() ? SequenceTraits<T>::size(io, Seq) : incnt;
      for(unsigned i=0; i < count; ++i) {
        void *SaveInfo;
        if ( io.preflightFlowElement(i, SaveInfo) ) {
          yamlize(io, SequenceTraits<T>::element(io, Seq, i), true, Ctx);
          io.postflightFlowElement(SaveInfo);
        }
      }
      io.endFlowSequence();
    }
    else {
      unsigned incnt = io.beginSequence();
      unsigned count = io.outputting() ? SequenceTraits<T>::size(io, Seq) : incnt;
      for(unsigned i=0; i < count; ++i) {
        void *SaveInfo;
        if ( io.preflightElement(i, SaveInfo) ) {
          yamlize(io, SequenceTraits<T>::element(io, Seq, i), true, Ctx);
          io.postflightElement(SaveInfo);
        }
      }
      io.endSequence();
    }
  }
  
  template<>
  struct ScalarTraits<bool> {
    static void output(const bool &, void* , raw_ostream &);
    static StringRef input(StringRef, void *, bool &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<StringRef> {
    static void output(const StringRef &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, StringRef &);
    static QuotingType mustQuote(StringRef S) { return needsQuotes(S); }
  };
  
  template<>
  struct ScalarTraits<std::string> {
    static void output(const std::string &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, std::string &);
    static QuotingType mustQuote(StringRef S) { return needsQuotes(S); }
  };
  
  template<>
  struct ScalarTraits<uint8_t> {
    static void output(const uint8_t &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, uint8_t &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<uint16_t> {
    static void output(const uint16_t &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, uint16_t &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<uint32_t> {
    static void output(const uint32_t &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, uint32_t &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<uint64_t> {
    static void output(const uint64_t &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, uint64_t &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<int8_t> {
    static void output(const int8_t &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, int8_t &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<int16_t> {
    static void output(const int16_t &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, int16_t &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<int32_t> {
    static void output(const int32_t &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, int32_t &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<int64_t> {
    static void output(const int64_t &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, int64_t &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<float> {
    static void output(const float &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, float &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<double> {
    static void output(const double &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, double &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  // For endian types, we use existing scalar Traits class for the underlying
  // type.  This way endian aware types are supported whenever the traits are
  // defined for the underlying type.
  template <typename value_type, support::endianness endian, size_t alignment>
  struct ScalarTraits<support::detail::packed_endian_specific_integral<
                          value_type, endian, alignment>,
                      std::enable_if_t<has_ScalarTraits<value_type>::value>> {
    using endian_type =
        support::detail::packed_endian_specific_integral<value_type, endian,
                                                         alignment>;
  
    static void output(const endian_type &E, void *Ctx, raw_ostream &Stream) {
      ScalarTraits<value_type>::output(static_cast<value_type>(E), Ctx, Stream);
    }
  
    static StringRef input(StringRef Str, void *Ctx, endian_type &E) {
      value_type V;
      auto R = ScalarTraits<value_type>::input(Str, Ctx, V);
      E = static_cast<endian_type>(V);
      return R;
    }
  
    static QuotingType mustQuote(StringRef Str) {
      return ScalarTraits<value_type>::mustQuote(Str);
    }
  };
  
  template <typename value_type, support::endianness endian, size_t alignment>
  struct ScalarEnumerationTraits<
      support::detail::packed_endian_specific_integral<value_type, endian,
                                                       alignment>,
      std::enable_if_t<has_ScalarEnumerationTraits<value_type>::value>> {
    using endian_type =
        support::detail::packed_endian_specific_integral<value_type, endian,
                                                         alignment>;
  
    static void enumeration(IO &io, endian_type &E) {
      value_type V = E;
      ScalarEnumerationTraits<value_type>::enumeration(io, V);
      E = V;
    }
  };
  
  template <typename value_type, support::endianness endian, size_t alignment>
  struct ScalarBitSetTraits<
      support::detail::packed_endian_specific_integral<value_type, endian,
                                                       alignment>,
      std::enable_if_t<has_ScalarBitSetTraits<value_type>::value>> {
    using endian_type =
        support::detail::packed_endian_specific_integral<value_type, endian,
                                                         alignment>;
    static void bitset(IO &io, endian_type &E) {
      value_type V = E;
      ScalarBitSetTraits<value_type>::bitset(io, V);
      E = V;
    }
  };
  
  // Utility for use within MappingTraits<>::mapping() method
  // to [de]normalize an object for use with YAML conversion.
  template <typename TNorm, typename TFinal>
  struct MappingNormalization {
    MappingNormalization(IO &i_o, TFinal &Obj)
        : io(i_o), BufPtr(nullptr), Result(Obj) {
      if ( io.outputting() ) {
        BufPtr = new (&Buffer) TNorm(io, Obj);
      }
      else {
        BufPtr = new (&Buffer) TNorm(io);
      }
    }
  
    ~MappingNormalization() {
      if ( ! io.outputting() ) {
        Result = BufPtr->denormalize(io);
      }
      BufPtr->~TNorm();
    }
  
    TNorm* operator->() { return BufPtr; }
  
  private:
    using Storage = AlignedCharArrayUnion<TNorm>;
  
    Storage       Buffer;
    IO           &io;
    TNorm        *BufPtr;
    TFinal       &Result;
  };
  
  // Utility for use within MappingTraits<>::mapping() method
  // to [de]normalize an object for use with YAML conversion.
  template <typename TNorm, typename TFinal>
  struct MappingNormalizationHeap {
    MappingNormalizationHeap(IO &i_o, TFinal &Obj, BumpPtrAllocator *allocator)
      : io(i_o), Result(Obj) {
      if ( io.outputting() ) {
        BufPtr = new (&Buffer) TNorm(io, Obj);
      }
      else if (allocator) {
        BufPtr = allocator->Allocate<TNorm>();
        new (BufPtr) TNorm(io);
      } else {
        BufPtr = new TNorm(io);
      }
    }
  
    ~MappingNormalizationHeap() {
      if ( io.outputting() ) {
        BufPtr->~TNorm();
      }
      else {
        Result = BufPtr->denormalize(io);
      }
    }
  
    TNorm* operator->() { return BufPtr; }
  
  private:
    using Storage = AlignedCharArrayUnion<TNorm>;
  
    Storage       Buffer;
    IO           &io;
    TNorm        *BufPtr = nullptr;
    TFinal       &Result;
  };
  
  ///
  /// The Input class is used to parse a yaml document into in-memory structs
  /// and vectors.
  ///
  /// It works by using YAMLParser to do a syntax parse of the entire yaml
  /// document, then the Input class builds a graph of HNodes which wraps
  /// each yaml Node.  The extra layer is buffering.  The low level yaml
  /// parser only lets you look at each node once.  The buffering layer lets
  /// you search and interate multiple times.  This is necessary because
  /// the mapRequired() method calls may not be in the same order
  /// as the keys in the document.
  ///
  class Input : public IO {
  public:
    // Construct a yaml Input object from a StringRef and optional
    // user-data. The DiagHandler can be specified to provide
    // alternative error reporting.
    Input(StringRef InputContent,
          void *Ctxt = nullptr,
          SourceMgr::DiagHandlerTy DiagHandler = nullptr,
          void *DiagHandlerCtxt = nullptr);
    Input(MemoryBufferRef Input,
          void *Ctxt = nullptr,
          SourceMgr::DiagHandlerTy DiagHandler = nullptr,
          void *DiagHandlerCtxt = nullptr);
    ~Input() override;
  
    // Check if there was an syntax or semantic error during parsing.
    std::error_code error();
  
  private:
    bool outputting() const override;
    bool mapTag(StringRef, bool) override;
    void beginMapping() override;
    void endMapping() override;
    bool preflightKey(const char *, bool, bool, bool &, void *&) override;
    void postflightKey(void *) override;
    std::vector<StringRef> keys() override;
    void beginFlowMapping() override;
    void endFlowMapping() override;
    unsigned beginSequence() override;
    void endSequence() override;
    bool preflightElement(unsigned index, void *&) override;
    void postflightElement(void *) override;
    unsigned beginFlowSequence() override;
    bool preflightFlowElement(unsigned , void *&) override;
    void postflightFlowElement(void *) override;
    void endFlowSequence() override;
    void beginEnumScalar() override;
    bool matchEnumScalar(const char*, bool) override;
    bool matchEnumFallback() override;
    void endEnumScalar() override;
    bool beginBitSetScalar(bool &) override;
    bool bitSetMatch(const char *, bool ) override;
    void endBitSetScalar() override;
    void scalarString(StringRef &, QuotingType) override;
    void blockScalarString(StringRef &) override;
    void scalarTag(std::string &) override;
    NodeKind getNodeKind() override;
    void setError(const Twine &message) override;
    bool canElideEmptySequence() override;
  
    class HNode {
      virtual void anchor();
  
    public:
      HNode(Node *n) : _node(n) { }
      virtual ~HNode() = default;
  
      static bool classof(const HNode *) { return true; }
  
      Node *_node;
    };
  
    class EmptyHNode : public HNode {
      void anchor() override;
  
    public:
      EmptyHNode(Node *n) : HNode(n) { }
  
      static bool classof(const HNode *n) { return NullNode::classof(n->_node); }
  
      static bool classof(const EmptyHNode *) { return true; }
    };
  
    class ScalarHNode : public HNode {
      void anchor() override;
  
    public:
      ScalarHNode(Node *n, StringRef s) : HNode(n), _value(s) { }
  
      StringRef value() const { return _value; }
  
      static bool classof(const HNode *n) {
        return ScalarNode::classof(n->_node) ||
               BlockScalarNode::classof(n->_node);
      }
  
      static bool classof(const ScalarHNode *) { return true; }
  
    protected:
      StringRef _value;
    };
  
    class MapHNode : public HNode {
      void anchor() override;
  
    public:
      MapHNode(Node *n) : HNode(n) { }
  
      static bool classof(const HNode *n) {
        return MappingNode::classof(n->_node);
      }
  
      static bool classof(const MapHNode *) { return true; }
  
      using NameToNode = StringMap<std::unique_ptr<HNode>>;
  
      NameToNode Mapping;
      SmallVector<std::string, 6> ValidKeys;
    };
  
    class SequenceHNode : public HNode {
      void anchor() override;
  
    public:
      SequenceHNode(Node *n) : HNode(n) { }
  
      static bool classof(const HNode *n) {
        return SequenceNode::classof(n->_node);
      }
  
      static bool classof(const SequenceHNode *) { return true; }
  
      std::vector<std::unique_ptr<HNode>> Entries;
    };
  
    std::unique_ptr<Input::HNode> createHNodes(Node *node);
    void setError(HNode *hnode, const Twine &message);
    void setError(Node *node, const Twine &message);
  
  public:
    // These are only used by operator>>. They could be private
    // if those templated things could be made friends.
    bool setCurrentDocument();
    bool nextDocument();
  
    /// Returns the current node that's being parsed by the YAML Parser.
    const Node *getCurrentNode() const;
  
  private:
    SourceMgr                           SrcMgr; // must be before Strm
    std::unique_ptr<llvm::yaml::Stream> Strm;
    std::unique_ptr<HNode>              TopNode;
    std::error_code                     EC;
    BumpPtrAllocator                    StringAllocator;
    document_iterator                   DocIterator;
    std::vector<bool>                   BitValuesUsed;
    HNode *CurrentNode = nullptr;
    bool                                ScalarMatchFound = false;
  };
  
  ///
  /// The Output class is used to generate a yaml document from in-memory structs
  /// and vectors.
  ///
  class Output : public IO {
  public:
    Output(raw_ostream &, void *Ctxt = nullptr, int WrapColumn = 70);
    ~Output() override;
  
    /// Set whether or not to output optional values which are equal
    /// to the default value.  By default, when outputting if you attempt
    /// to write a value that is equal to the default, the value gets ignored.
    /// Sometimes, it is useful to be able to see these in the resulting YAML
    /// anyway.
    void setWriteDefaultValues(bool Write) { WriteDefaultValues = Write; }
  
    bool outputting() const override;
    bool mapTag(StringRef, bool) override;
    void beginMapping() override;
    void endMapping() override;
    bool preflightKey(const char *key, bool, bool, bool &, void *&) override;
    void postflightKey(void *) override;
    std::vector<StringRef> keys() override;
    void beginFlowMapping() override;
    void endFlowMapping() override;
    unsigned beginSequence() override;
    void endSequence() override;
    bool preflightElement(unsigned, void *&) override;
    void postflightElement(void *) override;
    unsigned beginFlowSequence() override;
    bool preflightFlowElement(unsigned, void *&) override;
    void postflightFlowElement(void *) override;
    void endFlowSequence() override;
    void beginEnumScalar() override;
    bool matchEnumScalar(const char*, bool) override;
    bool matchEnumFallback() override;
    void endEnumScalar() override;
    bool beginBitSetScalar(bool &) override;
    bool bitSetMatch(const char *, bool ) override;
    void endBitSetScalar() override;
    void scalarString(StringRef &, QuotingType) override;
    void blockScalarString(StringRef &) override;
    void scalarTag(std::string &) override;
    NodeKind getNodeKind() override;
    void setError(const Twine &message) override;
    bool canElideEmptySequence() override;
  
    // These are only used by operator<<. They could be private
    // if that templated operator could be made a friend.
    void beginDocuments();
    bool preflightDocument(unsigned);
    void postflightDocument();
    void endDocuments();
  
  private:
    void output(StringRef s);
    void outputUpToEndOfLine(StringRef s);
    void newLineCheck();
    void outputNewLine();
    void paddedKey(StringRef key);
    void flowKey(StringRef Key);
  
    enum InState {
      inSeqFirstElement,
      inSeqOtherElement,
      inFlowSeqFirstElement,
      inFlowSeqOtherElement,
      inMapFirstKey,
      inMapOtherKey,
      inFlowMapFirstKey,
      inFlowMapOtherKey
    };
  
    static bool inSeqAnyElement(InState State);
    static bool inFlowSeqAnyElement(InState State);
    static bool inMapAnyKey(InState State);
    static bool inFlowMapAnyKey(InState State);
  
    raw_ostream &Out;
    int WrapColumn;
    SmallVector<InState, 8> StateStack;
    int Column = 0;
    int ColumnAtFlowStart = 0;
    int ColumnAtMapFlowStart = 0;
    bool NeedBitValueComma = false;
    bool NeedFlowSequenceComma = false;
    bool EnumerationMatchFound = false;
    bool WriteDefaultValues = false;
    StringRef Padding;
    StringRef PaddingBeforeContainer;
  };
  
  /// YAML I/O does conversion based on types. But often native data types
  /// are just a typedef of built in intergral types (e.g. int).  But the C++
  /// type matching system sees through the typedef and all the typedefed types
  /// look like a built in type. This will cause the generic YAML I/O conversion
  /// to be used. To provide better control over the YAML conversion, you can
  /// use this macro instead of typedef.  It will create a class with one field
  /// and automatic conversion operators to and from the base type.
  /// Based on BOOST_STRONG_TYPEDEF
  #define LLVM_YAML_STRONG_TYPEDEF(_base, _type)                                 \
      struct _type {                                                             \
          _type() = default;                                                     \
          _type(const _base v) : value(v) {}                                     \
          _type(const _type &v) = default;                                       \
          _type &operator=(const _type &rhs) = default;                          \
          _type &operator=(const _base &rhs) { value = rhs; return *this; }      \
          operator const _base & () const { return value; }                      \
          bool operator==(const _type &rhs) const { return value == rhs.value; } \
          bool operator==(const _base &rhs) const { return value == rhs; }       \
          bool operator<(const _type &rhs) const { return value < rhs.value; }   \
          _base value;                                                           \
          using BaseType = _base;                                                \
      };
  
  ///
  /// Use these types instead of uintXX_t in any mapping to have
  /// its yaml output formatted as hexadecimal.
  ///
  LLVM_YAML_STRONG_TYPEDEF(uint8_t, Hex8)
  LLVM_YAML_STRONG_TYPEDEF(uint16_t, Hex16)
  LLVM_YAML_STRONG_TYPEDEF(uint32_t, Hex32)
  LLVM_YAML_STRONG_TYPEDEF(uint64_t, Hex64)
  
  template<>
  struct ScalarTraits<Hex8> {
    static void output(const Hex8 &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, Hex8 &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<Hex16> {
    static void output(const Hex16 &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, Hex16 &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<Hex32> {
    static void output(const Hex32 &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, Hex32 &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  template<>
  struct ScalarTraits<Hex64> {
    static void output(const Hex64 &, void *, raw_ostream &);
    static StringRef input(StringRef, void *, Hex64 &);
    static QuotingType mustQuote(StringRef) { return QuotingType::None; }
  };
  
  // Define non-member operator>> so that Input can stream in a document list.
  template <typename T>
  inline std::enable_if_t<has_DocumentListTraits<T>::value, Input &>
  operator>>(Input &yin, T &docList) {
    int i = 0;
    EmptyContext Ctx;
    while ( yin.setCurrentDocument() ) {
      yamlize(yin, DocumentListTraits<T>::element(yin, docList, i), true, Ctx);
      if ( yin.error() )
        return yin;
      yin.nextDocument();
      ++i;
    }
    return yin;
  }
  
  // Define non-member operator>> so that Input can stream in a map as a document.
  template <typename T>
  inline std::enable_if_t<has_MappingTraits<T, EmptyContext>::value, Input &>
  operator>>(Input &yin, T &docMap) {
    EmptyContext Ctx;
    yin.setCurrentDocument();
    yamlize(yin, docMap, true, Ctx);
    return yin;
  }
  
  // Define non-member operator>> so that Input can stream in a sequence as
  // a document.
  template <typename T>
  inline std::enable_if_t<has_SequenceTraits<T>::value, Input &>
  operator>>(Input &yin, T &docSeq) {
    EmptyContext Ctx;
    if (yin.setCurrentDocument())
      yamlize(yin, docSeq, true, Ctx);
    return yin;
  }
  
  // Define non-member operator>> so that Input can stream in a block scalar.
  template <typename T>
  inline std::enable_if_t<has_BlockScalarTraits<T>::value, Input &>
  operator>>(Input &In, T &Val) {
    EmptyContext Ctx;
    if (In.setCurrentDocument())
      yamlize(In, Val, true, Ctx);
    return In;
  }
  
  // Define non-member operator>> so that Input can stream in a string map.
  template <typename T>
  inline std::enable_if_t<has_CustomMappingTraits<T>::value, Input &>
  operator>>(Input &In, T &Val) {
    EmptyContext Ctx;
    if (In.setCurrentDocument())
      yamlize(In, Val, true, Ctx);
    return In;
  }
  
  // Define non-member operator>> so that Input can stream in a polymorphic type.
  template <typename T>
  inline std::enable_if_t<has_PolymorphicTraits<T>::value, Input &>
  operator>>(Input &In, T &Val) {
    EmptyContext Ctx;
    if (In.setCurrentDocument())
      yamlize(In, Val, true, Ctx);
    return In;
  }
  
  // Provide better error message about types missing a trait specialization
  template <typename T>
  inline std::enable_if_t<missingTraits<T, EmptyContext>::value, Input &>
  operator>>(Input &yin, T &docSeq) {
    char missing_yaml_trait_for_type[sizeof(MissingTrait<T>)];
    return yin;
  }
  
  // Define non-member operator<< so that Output can stream out document list.
  template <typename T>
  inline std::enable_if_t<has_DocumentListTraits<T>::value, Output &>
  operator<<(Output &yout, T &docList) {
    EmptyContext Ctx;
    yout.beginDocuments();
    const size_t count = DocumentListTraits<T>::size(yout, docList);
    for(size_t i=0; i < count; ++i) {
      if ( yout.preflightDocument(i) ) {
        yamlize(yout, DocumentListTraits<T>::element(yout, docList, i), true,
                Ctx);
        yout.postflightDocument();
      }
    }
    yout.endDocuments();
    return yout;
  }
  
  // Define non-member operator<< so that Output can stream out a map.
  template <typename T>
  inline std::enable_if_t<has_MappingTraits<T, EmptyContext>::value, Output &>
  operator<<(Output &yout, T &map) {
    EmptyContext Ctx;
    yout.beginDocuments();
    if ( yout.preflightDocument(0) ) {
      yamlize(yout, map, true, Ctx);
      yout.postflightDocument();
    }
    yout.endDocuments();
    return yout;
  }
  
  // Define non-member operator<< so that Output can stream out a sequence.
  template <typename T>
  inline std::enable_if_t<has_SequenceTraits<T>::value, Output &>
  operator<<(Output &yout, T &seq) {
    EmptyContext Ctx;
    yout.beginDocuments();
    if ( yout.preflightDocument(0) ) {
      yamlize(yout, seq, true, Ctx);
      yout.postflightDocument();
    }
    yout.endDocuments();
    return yout;
  }
  
  // Define non-member operator<< so that Output can stream out a block scalar.
  template <typename T>
  inline std::enable_if_t<has_BlockScalarTraits<T>::value, Output &>
  operator<<(Output &Out, T &Val) {
    EmptyContext Ctx;
    Out.beginDocuments();
    if (Out.preflightDocument(0)) {
      yamlize(Out, Val, true, Ctx);
      Out.postflightDocument();
    }
    Out.endDocuments();
    return Out;
  }
  
  // Define non-member operator<< so that Output can stream out a string map.
  template <typename T>
  inline std::enable_if_t<has_CustomMappingTraits<T>::value, Output &>
  operator<<(Output &Out, T &Val) {
    EmptyContext Ctx;
    Out.beginDocuments();
    if (Out.preflightDocument(0)) {
      yamlize(Out, Val, true, Ctx);
      Out.postflightDocument();
    }
    Out.endDocuments();
    return Out;
  }
  
  // Define non-member operator<< so that Output can stream out a polymorphic
  // type.
  template <typename T>
  inline std::enable_if_t<has_PolymorphicTraits<T>::value, Output &>
  operator<<(Output &Out, T &Val) {
    EmptyContext Ctx;
    Out.beginDocuments();
    if (Out.preflightDocument(0)) {
      // FIXME: The parser does not support explicit documents terminated with a
      // plain scalar; the end-marker is included as part of the scalar token.
      assert(PolymorphicTraits<T>::getKind(Val) != NodeKind::Scalar && "plain scalar documents are not supported");
      yamlize(Out, Val, true, Ctx);
      Out.postflightDocument();
    }
    Out.endDocuments();
    return Out;
  }
  
  // Provide better error message about types missing a trait specialization
  template <typename T>
  inline std::enable_if_t<missingTraits<T, EmptyContext>::value, Output &>
  operator<<(Output &yout, T &seq) {
    char missing_yaml_trait_for_type[sizeof(MissingTrait<T>)];
    return yout;
  }
  
  template <bool B> struct IsFlowSequenceBase {};
  template <> struct IsFlowSequenceBase<true> { static const bool flow = true; };
  
  template <typename T, bool Flow>
  struct SequenceTraitsImpl : IsFlowSequenceBase<Flow> {
  private:
    using type = typename T::value_type;
  
  public:
    static size_t size(IO &io, T &seq) { return seq.size(); }
  
    static type &element(IO &io, T &seq, size_t index) {
      if (index >= seq.size())
        seq.resize(index + 1);
      return seq[index];
    }
  };
  
  // Simple helper to check an expression can be used as a bool-valued template
  // argument.
  template <bool> struct CheckIsBool { static const bool value = true; };
  
  // If T has SequenceElementTraits, then vector<T> and SmallVector<T, N> have
  // SequenceTraits that do the obvious thing.
  template <typename T>
  struct SequenceTraits<
      std::vector<T>,
      std::enable_if_t<CheckIsBool<SequenceElementTraits<T>::flow>::value>>
      : SequenceTraitsImpl<std::vector<T>, SequenceElementTraits<T>::flow> {};
  template <typename T, unsigned N>
  struct SequenceTraits<
      SmallVector<T, N>,
      std::enable_if_t<CheckIsBool<SequenceElementTraits<T>::flow>::value>>
      : SequenceTraitsImpl<SmallVector<T, N>, SequenceElementTraits<T>::flow> {};
  template <typename T>
  struct SequenceTraits<
      SmallVectorImpl<T>,
      std::enable_if_t<CheckIsBool<SequenceElementTraits<T>::flow>::value>>
      : SequenceTraitsImpl<SmallVectorImpl<T>, SequenceElementTraits<T>::flow> {};
  
  // Sequences of fundamental types use flow formatting.
  template <typename T>
  struct SequenceElementTraits<T,
                               std::enable_if_t<std::is_fundamental<T>::value>> {
    static const bool flow = true;
  };
  
  // Sequences of strings use block formatting.
  template<> struct SequenceElementTraits<std::string> {
    static const bool flow = false;
  };
  template<> struct SequenceElementTraits<StringRef> {
    static const bool flow = false;
  };
  template<> struct SequenceElementTraits<std::pair<std::string, std::string>> {
    static const bool flow = false;
  };
  
  /// Implementation of CustomMappingTraits for std::map<std::string, T>.
  template <typename T> struct StdMapStringCustomMappingTraitsImpl {
    using map_type = std::map<std::string, T>;
  
    static void inputOne(IO &io, StringRef key, map_type &v) {
      io.mapRequired(key.str().c_str(), v[std::string(key)]);
    }
  
    static void output(IO &io, map_type &v) {
      for (auto &p : v)
        io.mapRequired(p.first.c_str(), p.second);
    }
  };
  
  } // end namespace yaml
  } // end namespace llvm
  
  #define LLVM_YAML_IS_SEQUENCE_VECTOR_IMPL(TYPE, FLOW)                          \
    namespace llvm {                                                             \
    namespace yaml {                                                             \
    static_assert(                                                               \
        !std::is_fundamental<TYPE>::value &&                                     \
        !std::is_same<TYPE, std::string>::value &&                               \
        !std::is_same<TYPE, llvm::StringRef>::value,                             \
        "only use LLVM_YAML_IS_SEQUENCE_VECTOR for types you control");          \
    template <> struct SequenceElementTraits<TYPE> {                             \
      static const bool flow = FLOW;                                             \
    };                                                                           \
    }                                                                            \
    }
  
  /// Utility for declaring that a std::vector of a particular type
  /// should be considered a YAML sequence.
  #define LLVM_YAML_IS_SEQUENCE_VECTOR(type)                                     \
    LLVM_YAML_IS_SEQUENCE_VECTOR_IMPL(type, false)
  
  /// Utility for declaring that a std::vector of a particular type
  /// should be considered a YAML flow sequence.
  #define LLVM_YAML_IS_FLOW_SEQUENCE_VECTOR(type)                                \
    LLVM_YAML_IS_SEQUENCE_VECTOR_IMPL(type, true)
  
  #define LLVM_YAML_DECLARE_MAPPING_TRAITS(Type)                                 \
    namespace llvm {                                                             \
    namespace yaml {                                                             \
    template <> struct MappingTraits<Type> {                                     \
      static void mapping(IO &IO, Type &Obj);                                    \
    };                                                                           \
    }                                                                            \
    }
  
  #define LLVM_YAML_DECLARE_ENUM_TRAITS(Type)                                    \
    namespace llvm {                                                             \
    namespace yaml {                                                             \
    template <> struct ScalarEnumerationTraits<Type> {                           \
      static void enumeration(IO &io, Type &Value);                              \
    };                                                                           \
    }                                                                            \
    }
  
  #define LLVM_YAML_DECLARE_BITSET_TRAITS(Type)                                  \
    namespace llvm {                                                             \
    namespace yaml {                                                             \
    template <> struct ScalarBitSetTraits<Type> {                                \
      static void bitset(IO &IO, Type &Options);                                 \
    };                                                                           \
    }                                                                            \
    }
  
  #define LLVM_YAML_DECLARE_SCALAR_TRAITS(Type, MustQuote)                       \
    namespace llvm {                                                             \
    namespace yaml {                                                             \
    template <> struct ScalarTraits<Type> {                                      \
      static void output(const Type &Value, void *ctx, raw_ostream &Out);        \
      static StringRef input(StringRef Scalar, void *ctxt, Type &Value);         \
      static QuotingType mustQuote(StringRef) { return MustQuote; }              \
    };                                                                           \
    }                                                                            \
    }
  
  /// Utility for declaring that a std::vector of a particular type
  /// should be considered a YAML document list.
  #define LLVM_YAML_IS_DOCUMENT_LIST_VECTOR(_type)                               \
    namespace llvm {                                                             \
    namespace yaml {                                                             \
    template <unsigned N>                                                        \
    struct DocumentListTraits<SmallVector<_type, N>>                             \
        : public SequenceTraitsImpl<SmallVector<_type, N>, false> {};            \
    template <>                                                                  \
    struct DocumentListTraits<std::vector<_type>>                                \
        : public SequenceTraitsImpl<std::vector<_type>, false> {};               \
    }                                                                            \
    }
  
  /// Utility for declaring that std::map<std::string, _type> should be considered
  /// a YAML map.
  #define LLVM_YAML_IS_STRING_MAP(_type)                                         \
    namespace llvm {                                                             \
    namespace yaml {                                                             \
    template <>                                                                  \
    struct CustomMappingTraits<std::map<std::string, _type>>                     \
        : public StdMapStringCustomMappingTraitsImpl<_type> {};                  \
    }                                                                            \
    }
  
  LLVM_YAML_IS_FLOW_SEQUENCE_VECTOR(llvm::yaml::Hex64)
  LLVM_YAML_IS_FLOW_SEQUENCE_VECTOR(llvm::yaml::Hex32)
  LLVM_YAML_IS_FLOW_SEQUENCE_VECTOR(llvm::yaml::Hex16)
  LLVM_YAML_IS_FLOW_SEQUENCE_VECTOR(llvm::yaml::Hex8)
  
  #endif // LLVM_SUPPORT_YAMLTRAITS_H
  