Add 'third_party/abseil_cpp/' from commit '768eb2ca2857342673fcd462792ce04b8bac3fa3' r/781

git-subtree-dir: third_party/abseil_cpp
git-subtree-mainline: ffb2ae54beb5796cd408fbe15d2d2da09ff37adf
git-subtree-split: 768eb2ca2857342673fcd462792ce04b8bac3fa3

1 files changed, 745 insertions, 0 deletions

diff --git a/third_party/abseil_cpp/absl/flags/internal/flag.h b/third_party/abseil_cpp/absl/flags/internal/flag.h
new file mode 100644
index 000000000000..e374ecde3d95
--- /dev/null
+++ b/third_party/abseil_cpp/absl/flags/internal/flag.h
@@ -0,0 +1,745 @@
+//
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_FLAGS_INTERNAL_FLAG_H_
+#define ABSL_FLAGS_INTERNAL_FLAG_H_
+
+#include <stdint.h>
+
+#include <atomic>
+#include <cstring>
+#include <memory>
+#include <string>
+#include <type_traits>
+#include <typeinfo>
+
+#include "absl/base/call_once.h"
+#include "absl/base/config.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/flags/config.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/internal/registry.h"
+#include "absl/flags/marshalling.h"
+#include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/synchronization/mutex.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// Forward declaration of absl::Flag<T> public API.
+namespace flags_internal {
+template <typename T>
+class Flag;
+}  // namespace flags_internal
+
+#if defined(_MSC_VER) && !defined(__clang__)
+template <typename T>
+class Flag;
+#else
+template <typename T>
+using Flag = flags_internal::Flag<T>;
+#endif
+
+template <typename T>
+ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag);
+
+template <typename T>
+void SetFlag(absl::Flag<T>* flag, const T& v);
+
+template <typename T, typename V>
+void SetFlag(absl::Flag<T>* flag, const V& v);
+
+namespace flags_internal {
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag value type operations, eg., parsing, copying, etc. are provided
+// by function specific to that type with a signature matching FlagOpFn.
+
+enum class FlagOp {
+  kAlloc,
+  kDelete,
+  kCopy,
+  kCopyConstruct,
+  kSizeof,
+  kFastTypeId,
+  kRuntimeTypeId,
+  kParse,
+  kUnparse,
+  kValueOffset,
+};
+using FlagOpFn = void* (*)(FlagOp, const void*, void*, void*);
+
+// Forward declaration for Flag value specific operations.
+template <typename T>
+void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3);
+
+// Allocate aligned memory for a flag value.
+inline void* Alloc(FlagOpFn op) {
+  return op(FlagOp::kAlloc, nullptr, nullptr, nullptr);
+}
+// Deletes memory interpreting obj as flag value type pointer.
+inline void Delete(FlagOpFn op, void* obj) {
+  op(FlagOp::kDelete, nullptr, obj, nullptr);
+}
+// Copies src to dst interpreting as flag value type pointers.
+inline void Copy(FlagOpFn op, const void* src, void* dst) {
+  op(FlagOp::kCopy, src, dst, nullptr);
+}
+// Construct a copy of flag value in a location pointed by dst
+// based on src - pointer to the flag's value.
+inline void CopyConstruct(FlagOpFn op, const void* src, void* dst) {
+  op(FlagOp::kCopyConstruct, src, dst, nullptr);
+}
+// Makes a copy of flag value pointed by obj.
+inline void* Clone(FlagOpFn op, const void* obj) {
+  void* res = flags_internal::Alloc(op);
+  flags_internal::CopyConstruct(op, obj, res);
+  return res;
+}
+// Returns true if parsing of input text is successfull.
+inline bool Parse(FlagOpFn op, absl::string_view text, void* dst,
+                  std::string* error) {
+  return op(FlagOp::kParse, &text, dst, error) != nullptr;
+}
+// Returns string representing supplied value.
+inline std::string Unparse(FlagOpFn op, const void* val) {
+  std::string result;
+  op(FlagOp::kUnparse, val, &result, nullptr);
+  return result;
+}
+// Returns size of flag value type.
+inline size_t Sizeof(FlagOpFn op) {
+  // This sequence of casts reverses the sequence from
+  // `flags_internal::FlagOps()`
+  return static_cast<size_t>(reinterpret_cast<intptr_t>(
+      op(FlagOp::kSizeof, nullptr, nullptr, nullptr)));
+}
+// Returns fast type id coresponding to the value type.
+inline FlagFastTypeId FastTypeId(FlagOpFn op) {
+  return reinterpret_cast<FlagFastTypeId>(
+      op(FlagOp::kFastTypeId, nullptr, nullptr, nullptr));
+}
+// Returns fast type id coresponding to the value type.
+inline const std::type_info* RuntimeTypeId(FlagOpFn op) {
+  return reinterpret_cast<const std::type_info*>(
+      op(FlagOp::kRuntimeTypeId, nullptr, nullptr, nullptr));
+}
+// Returns offset of the field value_ from the field impl_ inside of
+// absl::Flag<T> data. Given FlagImpl pointer p you can get the
+// location of the corresponding value as:
+//      reinterpret_cast<char*>(p) + ValueOffset().
+inline ptrdiff_t ValueOffset(FlagOpFn op) {
+  // This sequence of casts reverses the sequence from
+  // `flags_internal::FlagOps()`
+  return static_cast<ptrdiff_t>(reinterpret_cast<intptr_t>(
+      op(FlagOp::kValueOffset, nullptr, nullptr, nullptr)));
+}
+
+// Returns an address of RTTI's typeid(T).
+template <typename T>
+inline const std::type_info* GenRuntimeTypeId() {
+#if defined(ABSL_FLAGS_INTERNAL_HAS_RTTI)
+  return &typeid(T);
+#else
+  return nullptr;
+#endif
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag help auxiliary structs.
+
+// This is help argument for absl::Flag encapsulating the string literal pointer
+// or pointer to function generating it as well as enum descriminating two
+// cases.
+using HelpGenFunc = std::string (*)();
+
+union FlagHelpMsg {
+  constexpr explicit FlagHelpMsg(const char* help_msg) : literal(help_msg) {}
+  constexpr explicit FlagHelpMsg(HelpGenFunc help_gen) : gen_func(help_gen) {}
+
+  const char* literal;
+  HelpGenFunc gen_func;
+};
+
+enum class FlagHelpKind : uint8_t { kLiteral = 0, kGenFunc = 1 };
+
+struct FlagHelpArg {
+  FlagHelpMsg source;
+  FlagHelpKind kind;
+};
+
+extern const char kStrippedFlagHelp[];
+
+// HelpConstexprWrap is used by struct AbslFlagHelpGenFor##name generated by
+// ABSL_FLAG macro. It is only used to silence the compiler in the case where
+// help message expression is not constexpr and does not have type const char*.
+// If help message expression is indeed constexpr const char* HelpConstexprWrap
+// is just a trivial identity function.
+template <typename T>
+const char* HelpConstexprWrap(const T&) {
+  return nullptr;
+}
+constexpr const char* HelpConstexprWrap(const char* p) { return p; }
+constexpr const char* HelpConstexprWrap(char* p) { return p; }
+
+// These two HelpArg overloads allows us to select at compile time one of two
+// way to pass Help argument to absl::Flag. We'll be passing
+// AbslFlagHelpGenFor##name as T and integer 0 as a single argument to prefer
+// first overload if possible. If T::Const is evaluatable on constexpr
+// context (see non template int parameter below) we'll choose first overload.
+// In this case the help message expression is immediately evaluated and is used
+// to construct the absl::Flag. No additionl code is generated by ABSL_FLAG.
+// Otherwise SFINAE kicks in and first overload is dropped from the
+// consideration, in which case the second overload will be used. The second
+// overload does not attempt to evaluate the help message expression
+// immediately and instead delays the evaluation by returing the function
+// pointer (&T::NonConst) genering the help message when necessary. This is
+// evaluatable in constexpr context, but the cost is an extra function being
+// generated in the ABSL_FLAG code.
+template <typename T, int = (T::Const(), 1)>
+constexpr FlagHelpArg HelpArg(int) {
+  return {FlagHelpMsg(T::Const()), FlagHelpKind::kLiteral};
+}
+
+template <typename T>
+constexpr FlagHelpArg HelpArg(char) {
+  return {FlagHelpMsg(&T::NonConst), FlagHelpKind::kGenFunc};
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag default value auxiliary structs.
+
+// Signature for the function generating the initial flag value (usually
+// based on default value supplied in flag's definition)
+using FlagDfltGenFunc = void (*)(void*);
+
+union FlagDefaultSrc {
+  constexpr explicit FlagDefaultSrc(FlagDfltGenFunc gen_func_arg)
+      : gen_func(gen_func_arg) {}
+
+#define ABSL_FLAGS_INTERNAL_DFLT_FOR_TYPE(T, name) \
+  T name##_value;                                  \
+  constexpr explicit FlagDefaultSrc(T value) : name##_value(value) {}  // NOLINT
+  ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(ABSL_FLAGS_INTERNAL_DFLT_FOR_TYPE)
+#undef ABSL_FLAGS_INTERNAL_DFLT_FOR_TYPE
+
+  void* dynamic_value;
+  FlagDfltGenFunc gen_func;
+};
+
+enum class FlagDefaultKind : uint8_t {
+  kDynamicValue = 0,
+  kGenFunc = 1,
+  kOneWord = 2  // for default values UP to one word in size
+};
+
+struct FlagDefaultArg {
+  FlagDefaultSrc source;
+  FlagDefaultKind kind;
+};
+
+// This struct and corresponding overload to InitDefaultValue are used to
+// facilitate usage of {} as default value in ABSL_FLAG macro.
+// TODO(rogeeff): Fix handling types with explicit constructors.
+struct EmptyBraces {};
+
+template <typename T>
+constexpr T InitDefaultValue(T t) {
+  return t;
+}
+
+template <typename T>
+constexpr T InitDefaultValue(EmptyBraces) {
+  return T{};
+}
+
+template <typename ValueT, typename GenT,
+          typename std::enable_if<std::is_integral<ValueT>::value, int>::type =
+              (GenT{}, 0)>
+constexpr FlagDefaultArg DefaultArg(int) {
+  return {FlagDefaultSrc(GenT{}.value), FlagDefaultKind::kOneWord};
+}
+
+template <typename ValueT, typename GenT>
+constexpr FlagDefaultArg DefaultArg(char) {
+  return {FlagDefaultSrc(&GenT::Gen), FlagDefaultKind::kGenFunc};
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag current value auxiliary structs.
+
+constexpr int64_t UninitializedFlagValue() { return 0xababababababababll; }
+
+template <typename T>
+using FlagUseOneWordStorage = std::integral_constant<
+    bool, absl::type_traits_internal::is_trivially_copyable<T>::value &&
+              (sizeof(T) <= 8)>;
+
+#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
+// Clang does not always produce cmpxchg16b instruction when alignment of a 16
+// bytes type is not 16.
+struct alignas(16) AlignedTwoWords {
+  int64_t first;
+  int64_t second;
+
+  bool IsInitialized() const {
+    return first != flags_internal::UninitializedFlagValue();
+  }
+};
+
+template <typename T>
+using FlagUseTwoWordsStorage = std::integral_constant<
+    bool, absl::type_traits_internal::is_trivially_copyable<T>::value &&
+              (sizeof(T) > 8) && (sizeof(T) <= 16)>;
+#else
+// This is actually unused and only here to avoid ifdefs in other palces.
+struct AlignedTwoWords {
+  constexpr AlignedTwoWords() noexcept : dummy() {}
+  constexpr AlignedTwoWords(int64_t, int64_t) noexcept : dummy() {}
+  char dummy;
+
+  bool IsInitialized() const {
+    std::abort();
+    return true;
+  }
+};
+
+// This trait should be type dependent, otherwise SFINAE below will fail
+template <typename T>
+using FlagUseTwoWordsStorage =
+    std::integral_constant<bool, sizeof(T) != sizeof(T)>;
+#endif
+
+template <typename T>
+using FlagUseBufferStorage =
+    std::integral_constant<bool, !FlagUseOneWordStorage<T>::value &&
+                                     !FlagUseTwoWordsStorage<T>::value>;
+
+enum class FlagValueStorageKind : uint8_t {
+  kAlignedBuffer = 0,
+  kOneWordAtomic = 1,
+  kTwoWordsAtomic = 2
+};
+
+template <typename T>
+static constexpr FlagValueStorageKind StorageKind() {
+  return FlagUseBufferStorage<T>::value
+             ? FlagValueStorageKind::kAlignedBuffer
+             : FlagUseOneWordStorage<T>::value
+                   ? FlagValueStorageKind::kOneWordAtomic
+                   : FlagValueStorageKind::kTwoWordsAtomic;
+}
+
+struct FlagOneWordValue {
+  constexpr FlagOneWordValue() : value(UninitializedFlagValue()) {}
+
+  std::atomic<int64_t> value;
+};
+
+struct FlagTwoWordsValue {
+  constexpr FlagTwoWordsValue()
+      : value(AlignedTwoWords{UninitializedFlagValue(), 0}) {}
+
+  std::atomic<AlignedTwoWords> value;
+};
+
+template <typename T,
+          FlagValueStorageKind Kind = flags_internal::StorageKind<T>()>
+struct FlagValue;
+
+template <typename T>
+struct FlagValue<T, FlagValueStorageKind::kAlignedBuffer> {
+  bool Get(T*) const { return false; }
+
+  alignas(T) char value[sizeof(T)];
+};
+
+template <typename T>
+struct FlagValue<T, FlagValueStorageKind::kOneWordAtomic> : FlagOneWordValue {
+  bool Get(T* dst) const {
+    int64_t one_word_val = value.load(std::memory_order_acquire);
+    if (ABSL_PREDICT_FALSE(one_word_val == UninitializedFlagValue())) {
+      return false;
+    }
+    std::memcpy(dst, static_cast<const void*>(&one_word_val), sizeof(T));
+    return true;
+  }
+};
+
+template <typename T>
+struct FlagValue<T, FlagValueStorageKind::kTwoWordsAtomic> : FlagTwoWordsValue {
+  bool Get(T* dst) const {
+    AlignedTwoWords two_words_val = value.load(std::memory_order_acquire);
+    if (ABSL_PREDICT_FALSE(!two_words_val.IsInitialized())) {
+      return false;
+    }
+    std::memcpy(dst, static_cast<const void*>(&two_words_val), sizeof(T));
+    return true;
+  }
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag callback auxiliary structs.
+
+// Signature for the mutation callback used by watched Flags
+// The callback is noexcept.
+// TODO(rogeeff): add noexcept after C++17 support is added.
+using FlagCallbackFunc = void (*)();
+
+struct FlagCallback {
+  FlagCallbackFunc func;
+  absl::Mutex guard;  // Guard for concurrent callback invocations.
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag implementation, which does not depend on flag value type.
+// The class encapsulates the Flag's data and access to it.
+
+struct DynValueDeleter {
+  explicit DynValueDeleter(FlagOpFn op_arg = nullptr);
+  void operator()(void* ptr) const;
+
+  FlagOpFn op;
+};
+
+class FlagState;
+
+class FlagImpl final : public flags_internal::CommandLineFlag {
+ public:
+  constexpr FlagImpl(const char* name, const char* filename, FlagOpFn op,
+                     FlagHelpArg help, FlagValueStorageKind value_kind,
+                     FlagDefaultArg default_arg)
+      : name_(name),
+        filename_(filename),
+        op_(op),
+        help_(help.source),
+        help_source_kind_(static_cast<uint8_t>(help.kind)),
+        value_storage_kind_(static_cast<uint8_t>(value_kind)),
+        def_kind_(static_cast<uint8_t>(default_arg.kind)),
+        modified_(false),
+        on_command_line_(false),
+        counter_(0),
+        callback_(nullptr),
+        default_value_(default_arg.source),
+        data_guard_{} {}
+
+  // Constant access methods
+  void Read(void* dst) const override ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+  // Mutating access methods
+  void Write(const void* src) ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+  // Interfaces to operate on callbacks.
+  void SetCallback(const FlagCallbackFunc mutation_callback)
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  void InvokeCallback() const ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+
+  // Used in read/write operations to validate source/target has correct type.
+  // For example if flag is declared as absl::Flag<int> FLAGS_foo, a call to
+  // absl::GetFlag(FLAGS_foo) validates that the type of FLAGS_foo is indeed
+  // int. To do that we pass the "assumed" type id (which is deduced from type
+  // int) as an argument `type_id`, which is in turn is validated against the
+  // type id stored in flag object by flag definition statement.
+  void AssertValidType(FlagFastTypeId type_id,
+                       const std::type_info* (*gen_rtti)()) const;
+
+ private:
+  template <typename T>
+  friend class Flag;
+  friend class FlagState;
+
+  // Ensures that `data_guard_` is initialized and returns it.
+  absl::Mutex* DataGuard() const ABSL_LOCK_RETURNED((absl::Mutex*)&data_guard_);
+  // Returns heap allocated value of type T initialized with default value.
+  std::unique_ptr<void, DynValueDeleter> MakeInitValue() const
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+  // Flag initialization called via absl::call_once.
+  void Init();
+
+  // Offset value access methods. One per storage kind. These methods to not
+  // respect const correctness, so be very carefull using them.
+
+  // This is a shared helper routine which encapsulates most of the magic. Since
+  // it is only used inside the three routines below, which are defined in
+  // flag.cc, we can define it in that file as well.
+  template <typename StorageT>
+  StorageT* OffsetValue() const;
+  // This is an accessor for a value stored in an aligned buffer storage.
+  // Returns a mutable pointer to the start of a buffer.
+  void* AlignedBufferValue() const;
+  // This is an accessor for a value stored as one word atomic. Returns a
+  // mutable reference to an atomic value.
+  std::atomic<int64_t>& OneWordValue() const;
+  // This is an accessor for a value stored as two words atomic. Returns a
+  // mutable reference to an atomic value.
+  std::atomic<AlignedTwoWords>& TwoWordsValue() const;
+
+  // Attempts to parse supplied `value` string. If parsing is successful,
+  // returns new value. Otherwise returns nullptr.
+  std::unique_ptr<void, DynValueDeleter> TryParse(absl::string_view value,
+                                                  std::string* err) const
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+  // Stores the flag value based on the pointer to the source.
+  void StoreValue(const void* src) ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+
+  FlagHelpKind HelpSourceKind() const {
+    return static_cast<FlagHelpKind>(help_source_kind_);
+  }
+  FlagValueStorageKind ValueStorageKind() const {
+    return static_cast<FlagValueStorageKind>(value_storage_kind_);
+  }
+  FlagDefaultKind DefaultKind() const
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard()) {
+    return static_cast<FlagDefaultKind>(def_kind_);
+  }
+
+  // CommandLineFlag interface implementation
+  absl::string_view Name() const override;
+  std::string Filename() const override;
+  std::string Help() const override;
+  FlagFastTypeId TypeId() const override;
+  bool IsSpecifiedOnCommandLine() const override
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  std::string DefaultValue() const override ABSL_LOCKS_EXCLUDED(*DataGuard());
+  std::string CurrentValue() const override ABSL_LOCKS_EXCLUDED(*DataGuard());
+  bool ValidateInputValue(absl::string_view value) const override
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  void CheckDefaultValueParsingRoundtrip() const override
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+  // Interfaces to save and restore flags to/from persistent state.
+  // Returns current flag state or nullptr if flag does not support
+  // saving and restoring a state.
+  std::unique_ptr<FlagStateInterface> SaveState() override
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+  // Restores the flag state to the supplied state object. If there is
+  // nothing to restore returns false. Otherwise returns true.
+  bool RestoreState(const FlagState& flag_state)
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+  bool ParseFrom(absl::string_view value, FlagSettingMode set_mode,
+                 ValueSource source, std::string* error) override
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+  // Immutable flag's state.
+
+  // Flags name passed to ABSL_FLAG as second arg.
+  const char* const name_;
+  // The file name where ABSL_FLAG resides.
+  const char* const filename_;
+  // Type-specific operations "vtable".
+  const FlagOpFn op_;
+  // Help message literal or function to generate it.
+  const FlagHelpMsg help_;
+  // Indicates if help message was supplied as literal or generator func.
+  const uint8_t help_source_kind_ : 1;
+  // Kind of storage this flag is using for the flag's value.
+  const uint8_t value_storage_kind_ : 2;
+
+  uint8_t : 0;  // The bytes containing the const bitfields must not be
+                // shared with bytes containing the mutable bitfields.
+
+  // Mutable flag's state (guarded by `data_guard_`).
+
+  // def_kind_ is not guard by DataGuard() since it is accessed in Init without
+  // locks.
+  uint8_t def_kind_ : 2;
+  // Has this flag's value been modified?
+  bool modified_ : 1 ABSL_GUARDED_BY(*DataGuard());
+  // Has this flag been specified on command line.
+  bool on_command_line_ : 1 ABSL_GUARDED_BY(*DataGuard());
+
+  // Unique tag for absl::call_once call to initialize this flag.
+  absl::once_flag init_control_;
+
+  // Mutation counter
+  int64_t counter_ ABSL_GUARDED_BY(*DataGuard());
+  // Optional flag's callback and absl::Mutex to guard the invocations.
+  FlagCallback* callback_ ABSL_GUARDED_BY(*DataGuard());
+  // Either a pointer to the function generating the default value based on the
+  // value specified in ABSL_FLAG or pointer to the dynamically set default
+  // value via SetCommandLineOptionWithMode. def_kind_ is used to distinguish
+  // these two cases.
+  FlagDefaultSrc default_value_;
+
+  // This is reserved space for an absl::Mutex to guard flag data. It will be
+  // initialized in FlagImpl::Init via placement new.
+  // We can't use "absl::Mutex data_guard_", since this class is not literal.
+  // We do not want to use "absl::Mutex* data_guard_", since this would require
+  // heap allocation during initialization, which is both slows program startup
+  // and can fail. Using reserved space + placement new allows us to avoid both
+  // problems.
+  alignas(absl::Mutex) mutable char data_guard_[sizeof(absl::Mutex)];
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// The Flag object parameterized by the flag's value type. This class implements
+// flag reflection handle interface.
+
+template <typename T>
+class Flag {
+ public:
+  constexpr Flag(const char* name, const char* filename, FlagHelpArg help,
+                 const FlagDefaultArg default_arg)
+      : impl_(name, filename, &FlagOps<T>, help,
+              flags_internal::StorageKind<T>(), default_arg),
+        value_() {}
+
+  // CommandLineFlag interface
+  absl::string_view Name() const { return impl_.Name(); }
+  std::string Filename() const { return impl_.Filename(); }
+  std::string Help() const { return impl_.Help(); }
+  // Do not use. To be removed.
+  bool IsSpecifiedOnCommandLine() const {
+    return impl_.IsSpecifiedOnCommandLine();
+  }
+  std::string DefaultValue() const { return impl_.DefaultValue(); }
+  std::string CurrentValue() const { return impl_.CurrentValue(); }
+
+ private:
+  template <typename U, bool do_register>
+  friend class FlagRegistrar;
+
+#if !defined(_MSC_VER) || defined(__clang__)
+  template <typename U>
+  friend U absl::GetFlag(const flags_internal::Flag<U>& flag);
+  template <typename U>
+  friend void absl::SetFlag(flags_internal::Flag<U>* flag, const U& v);
+  template <typename U, typename V>
+  friend void absl::SetFlag(flags_internal::Flag<U>* flag, const V& v);
+#else
+  template <typename U>
+  friend class absl::Flag;
+#endif
+
+  T Get() const {
+    // See implementation notes in CommandLineFlag::Get().
+    union U {
+      T value;
+      U() {}
+      ~U() { value.~T(); }
+    };
+    U u;
+
+#if !defined(NDEBUG)
+    impl_.AssertValidType(base_internal::FastTypeId<T>(), &GenRuntimeTypeId<T>);
+#endif
+
+    if (!value_.Get(&u.value)) impl_.Read(&u.value);
+    return std::move(u.value);
+  }
+  void Set(const T& v) {
+    impl_.AssertValidType(base_internal::FastTypeId<T>(), &GenRuntimeTypeId<T>);
+    impl_.Write(&v);
+  }
+
+  // Flag's data
+  // The implementation depends on value_ field to be placed exactly after the
+  // impl_ field, so that impl_ can figure out the offset to the value and
+  // access it.
+  FlagImpl impl_;
+  FlagValue<T> value_;
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Implementation of Flag value specific operations routine.
+template <typename T>
+void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3) {
+  switch (op) {
+    case FlagOp::kAlloc: {
+      std::allocator<T> alloc;
+      return std::allocator_traits<std::allocator<T>>::allocate(alloc, 1);
+    }
+    case FlagOp::kDelete: {
+      T* p = static_cast<T*>(v2);
+      p->~T();
+      std::allocator<T> alloc;
+      std::allocator_traits<std::allocator<T>>::deallocate(alloc, p, 1);
+      return nullptr;
+    }
+    case FlagOp::kCopy:
+      *static_cast<T*>(v2) = *static_cast<const T*>(v1);
+      return nullptr;
+    case FlagOp::kCopyConstruct:
+      new (v2) T(*static_cast<const T*>(v1));
+      return nullptr;
+    case FlagOp::kSizeof:
+      return reinterpret_cast<void*>(static_cast<uintptr_t>(sizeof(T)));
+    case FlagOp::kFastTypeId:
+      return const_cast<void*>(base_internal::FastTypeId<T>());
+    case FlagOp::kRuntimeTypeId:
+      return const_cast<std::type_info*>(GenRuntimeTypeId<T>());
+    case FlagOp::kParse: {
+      // Initialize the temporary instance of type T based on current value in
+      // destination (which is going to be flag's default value).
+      T temp(*static_cast<T*>(v2));
+      if (!absl::ParseFlag<T>(*static_cast<const absl::string_view*>(v1), &temp,
+                              static_cast<std::string*>(v3))) {
+        return nullptr;
+      }
+      *static_cast<T*>(v2) = std::move(temp);
+      return v2;
+    }
+    case FlagOp::kUnparse:
+      *static_cast<std::string*>(v2) =
+          absl::UnparseFlag<T>(*static_cast<const T*>(v1));
+      return nullptr;
+    case FlagOp::kValueOffset: {
+      // Round sizeof(FlagImp) to a multiple of alignof(FlagValue<T>) to get the
+      // offset of the data.
+      ptrdiff_t round_to = alignof(FlagValue<T>);
+      ptrdiff_t offset =
+          (sizeof(FlagImpl) + round_to - 1) / round_to * round_to;
+      return reinterpret_cast<void*>(offset);
+    }
+  }
+  return nullptr;
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// This class facilitates Flag object registration and tail expression-based
+// flag definition, for example:
+// ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher);
+struct FlagRegistrarEmpty {};
+template <typename T, bool do_register>
+class FlagRegistrar {
+ public:
+  explicit FlagRegistrar(Flag<T>* flag) : flag_(flag) {
+    if (do_register) flags_internal::RegisterCommandLineFlag(&flag_->impl_);
+  }
+
+  FlagRegistrar OnUpdate(FlagCallbackFunc cb) && {
+    flag_->impl_.SetCallback(cb);
+    return *this;
+  }
+
+  // Make the registrar "die" gracefully as an empty struct on a line where
+  // registration happens. Registrar objects are intended to live only as
+  // temporary.
+  operator FlagRegistrarEmpty() const { return {}; }  // NOLINT
+
+ private:
+  Flag<T>* flag_;  // Flag being registered (not owned).
+};
+
+}  // namespace flags_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FLAGS_INTERNAL_FLAG_H_