// Copyright 2022 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_TYPES_EXPECTED_H_
#define BASE_TYPES_EXPECTED_H_

#include <concepts>
#include <string_view>
#include <type_traits>
#include <utility>
#include <variant>

#include "base/check.h"
#include "base/strings/strcat.h"
#include "base/strings/to_string.h"
#include "base/types/expected_internal.h"  // IWYU pragma: export

// Class template `expected<T, E>` is a vocabulary type which contains an
// expected value of type `T`, or an error `E`. The class skews towards behaving
// like a `T`, because its intended use is when the expected type is contained.
// When something unexpected occurs, more typing is required. When all is good,
// code mostly looks as if a `T` were being handled.
//
// Class template `expected<T, E>` contains either:
// * A value of type `T`, the expected value type; or
// * A value of type `E`, an error type used when an unexpected outcome
// occurred.
//
// The interface can be queried as to whether the underlying value is the
// expected value (of type `T`) or an unexpected value (of type `E`). The
// interface and the rational are based on `std::optional`. We consider
// `expected<T, E>` as a supplement to `optional<T>`, expressing why an expected
// value isn’t contained in the object.
//
// Example Usage:
//
// Before:
//   bool ParseInt32(std::string_view input,
//                   int32_t* output,
//                   ParseIntError* error);
//   ...
//
//   int32_t output;
//   ParseIntError error;
//   if (ParseInt32("...". &output, &error)) {
//     // process `output`
//   } else {
//     // process `error`
//   }
//
// After:
//
//   base::expected<int32_t, ParseIntError> ParseInt32(std::string_view input);
//   ...
//
//   if (auto parsed = ParseInt32("..."); parsed.has_value()) {
//     // process `parsed.value()`
//   } else {
//     // process `parsed.error()`
//   }
//
// For even less boilerplate, see expected_macros.h.
//
// Note that there are various transformation member functions. To avoid having
// to puzzle through the standard-ese on their documentation, here's a quick
// reference table, assuming a source `expected<T, E> ex;` and types U and G
// convertible from T and E, respectively:
//
//                        Return type     Val when ex = t  Val when ex = e
//                        -----------     ---------------  ---------------
// ex.value_or(t2)        T               t                t2
// ex.and_then(f)         expected<U, E>  f(t)             unexpected(e)
// ex.transform(f)        expected<U, E>  expected(f(t))   unexpected(e)
// ex.or_else(f)          expected<T, G>  expected(t)      f(e)
// ex.transform_error(f)  expected<T, G>  expected(t)      unexpected(f(e))
//
// References:
// * https://wg21.link/P0323
// * https://eel.is/c++draft/expected
namespace base {

// Note: base::unexpected and base::expected are C++17 compatible backports of
// C++23's std::unexpected and std::expected. They differ from the standard in
// the following ways:
//
// * Not all member functions can be used in a constexpr context. This is due to
//   limitations in both the C++17 language and the Abseil library used for the
//   implementation.
// * Since Chromium does not support exceptions, there is no bad_expected_access
//   exception and the program will just terminate when the exception would have
//   been thrown. Furthermore, all member functions are marked noexcept.
// * C++23 allows an implicit conversion from U to expected<T, E> if U is
//   implicitly convertible to T; the Chromium version only allows an implicit
//   conversion if U is implicitly convertible to T *and* U is *not* implicitly
//   convertible to E, to guard against bug-prone patterns such as:
//     // creates an expected value containing true, not an unexpected value
//     // containing 123L.
//     expected<bool, long> e = 123L;
// * Because of the above restriction, the Chromium version also introduces
//   `base::ok` as a complement to `base::unexpected` to simplify returning
//   success values when the implicit conversion above is disallowed.
// * Calling operator* or operator-> on an unexpected value results in program
//   termination, and not UB.
// * There is no operator bool due to bug-prone usage when the value type is
//   convertible to bool, see e.g. https://abseil.io/tips/141.
// * Moving out of an expected object will put it into a moved-from state.
//   Trying to use it before re-initializing it will result in program
//   termination.
// * The expected<void> specialization is done via a defaulted boolean template
//   parameter, due to the lack of requires clauses in C++17.
// * Since equality operators can not be defaulted in C++17, equality and
//   inequality operators are specified explicitly.
// * base::expected implements the monadic interface proposal
//   (https://wg21.link/P2505), which is currently only on track for C++26.

// Class template used as a type hint for constructing a `base::expected`
// containing a value (i.e. success). Useful when implicit conversion
// construction of `base::expected` is disallowed, e.g. due to ambiguity.
// Example usage:
//
//   base::expected<std::string, std::string> RunOp() {
//     std::string error;
//     std::string result = RunOpImpl(..., &error);
//     if (!error.empty()) {
//       return base::unexpected(std::move(error));
//
//     // The C++23 std::expected proposal allows this to be simply written as
//     //   return result;
//     //
//     // However, the Chromium version disallows this if E implicitly converts
//     // to T, so without base::ok(), this would have to be written as:
//     //   return base::expected<std::string, std::string>(std::move(result));
//
//     return base::ok(std::move(result));
//   }
template <typename T>
class ok final {
 public:
  template <typename U = T>
    requires(internal::IsOkValueConstruction<T, U>)
  constexpr explicit ok(U&& val) noexcept : value_(std::forward<U>(val)) {}

  template <typename... Args>
  constexpr explicit ok(std::in_place_t, Args&&... args) noexcept
      : value_(std::forward<Args>(args)...) {}

  template <typename U, typename... Args>
  constexpr ok(std::in_place_t,
               std::initializer_list<U> il,
               Args&&... args) noexcept
      : value_(il, std::forward<Args>(args)...) {}

  constexpr T& value() & noexcept { return value_; }
  constexpr const T& value() const& noexcept { return value_; }
  constexpr T&& value() && noexcept { return std::move(value()); }
  constexpr const T&& value() const&& noexcept { return std::move(value()); }

  constexpr void swap(ok& other) noexcept {
    using std::swap;
    swap(value(), other.value());
  }

  friend constexpr void swap(ok& x, ok& y) noexcept { x.swap(y); }

  std::string ToString() const {
    return StrCat({"ok(", base::ToString(value()), ")"});
  }

 private:
  T value_;
};

template <typename T>
  requires(std::is_void_v<T>)
class ok<T> final {
 public:
  constexpr ok() noexcept = default;

  std::string ToString() const { return "ok()"; }
};

template <typename T, typename U>
constexpr bool operator==(const ok<T>& lhs, const ok<U>& rhs) noexcept {
  if constexpr (std::is_void_v<T> && std::is_void_v<U>) {
    return true;
  } else if constexpr (std::is_void_v<T> || std::is_void_v<U>) {
    return false;
  } else {
    return lhs.value() == rhs.value();
  }
}

template <typename T>
ok(T) -> ok<T>;

ok() -> ok<void>;

// [expected.un.object], class template unexpected
// https://eel.is/c++draft/expected#un.object
template <typename E>
class unexpected final {
 public:
  // [expected.un.ctor] Constructors
  template <typename Err = E>
    requires(internal::IsUnexpectedValueConstruction<E, Err>)
  constexpr explicit unexpected(Err&& err) noexcept
      : error_(std::forward<Err>(err)) {}

  template <typename... Args>
  constexpr explicit unexpected(std::in_place_t, Args&&... args) noexcept
      : error_(std::forward<Args>(args)...) {}

  template <typename U, typename... Args>
  constexpr unexpected(std::in_place_t,
                       std::initializer_list<U> il,
                       Args&&... args) noexcept
      : error_(il, std::forward<Args>(args)...) {}

  // [expected.un.obs] Observers
  constexpr E& error() & noexcept { return error_; }
  constexpr const E& error() const& noexcept { return error_; }
  constexpr E&& error() && noexcept { return std::move(error()); }
  constexpr const E&& error() const&& noexcept { return std::move(error()); }

  // [expected.un.swap] Swap
  constexpr void swap(unexpected& other) noexcept {
    using std::swap;
    swap(error(), other.error());
  }

  friend constexpr void swap(unexpected& x, unexpected& y) noexcept {
    x.swap(y);
  }

  // Deviation from the Standard: stringification support.
  //
  // If we move to `std::unexpected` someday, we would need to either forego
  // nice formatted output or move to `std::format` or similar, which can have
  // customized output for STL types.
  std::string ToString() const noexcept {
    return StrCat({"Unexpected(", base::ToString(error()), ")"});
  }

 private:
  E error_;
};

// [expected.un.eq] Equality operator
template <typename E, typename G>
constexpr bool operator==(const unexpected<E>& lhs,
                          const unexpected<G>& rhs) noexcept {
  return lhs.error() == rhs.error();
}

template <typename E, typename G>
constexpr bool operator!=(const unexpected<E>& lhs,
                          const unexpected<G>& rhs) noexcept {
  return !(lhs == rhs);
}

template <typename E>
unexpected(E) -> unexpected<E>;

// [expected.expected], class template expected
// https://eel.is/c++draft/expected#expected
template <typename T, typename E>
class [[nodiscard, gsl::Owner]] expected final {
  // Note: A partial specialization for void value types follows below.
  static_assert(!std::is_void_v<T>, "Error: T must not be void");

 public:
  using value_type = T;
  using error_type = E;
  using unexpected_type = unexpected<E>;

  // Alias template to explicitly opt into the std::pointer_traits machinery.
  // See e.g. https://en.cppreference.com/w/cpp/memory/pointer_traits#Notes
  template <typename U>
  using rebind = expected<U, E>;

  template <typename U, typename G>
  friend class expected;

  // [expected.object.ctor], constructors
  constexpr expected() noexcept = default;

  // Converting copy and move constructors. These constructors are explicit if
  // either the value or error type is not implicitly convertible from `rhs`'s
  // corresponding type.
  template <typename U, typename G>
    requires(internal::IsValidConversion<T, E, const U&, const G&>)
  explicit(!std::convertible_to<const U&, T> ||
           !std::convertible_to<const G&, E>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(const expected<U, G>& rhs) noexcept
      : impl_(rhs.impl_) {}

  template <typename U, typename G>
    requires(internal::IsValidConversion<T, E, U, G>)
  explicit(!std::convertible_to<U, T> || !std::convertible_to<G, E>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(expected<U, G>&& rhs) noexcept
      : impl_(std::move(rhs.impl_)) {}

  // Deviation from the Standard, which allows implicit conversions as long as U
  // is implicitly convertible to T: Chromium additionally requires that U is
  // not implicitly convertible to E.
  template <typename U = std::remove_cv_t<T>>
    requires(internal::IsValidValueConstruction<T, E, U>)
  explicit(!std::convertible_to<U, T> || std::convertible_to<U, E>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(U&& v) noexcept
      : impl_(kValTag, std::forward<U>(v)) {}

  template <typename U>
    requires(std::constructible_from<T, const U&>)
  explicit(!std::convertible_to<const U&, T>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(const ok<U>& o) noexcept
      : impl_(kValTag, o.value()) {}

  template <typename U>
    requires(std::constructible_from<T, U>)
  explicit(!std::convertible_to<U, T>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(ok<U>&& o) noexcept
      : impl_(kValTag, std::move(o.value())) {}

  template <typename G>
    requires(std::constructible_from<E, const G&>)
  explicit(!std::convertible_to<const G&, E>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(const unexpected<G>& e) noexcept
      : impl_(kErrTag, e.error()) {}

  template <typename G>
    requires(std::constructible_from<E, G>)
  explicit(!std::convertible_to<G, E>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(unexpected<G>&& e) noexcept
      : impl_(kErrTag, std::move(e.error())) {}

  template <typename... Args>
  constexpr explicit expected(std::in_place_t, Args&&... args) noexcept
      : impl_(kValTag, std::forward<Args>(args)...) {}

  template <typename U, typename... Args>
  constexpr expected(std::in_place_t,
                     std::initializer_list<U> il,
                     Args&&... args) noexcept
      : impl_(kValTag, il, std::forward<Args>(args)...) {}

  template <typename... Args>
  constexpr explicit expected(unexpect_t, Args&&... args) noexcept
      : impl_(kErrTag, std::forward<Args>(args)...) {}

  template <typename U, typename... Args>
  constexpr expected(unexpect_t,
                     std::initializer_list<U> il,
                     Args&&... args) noexcept
      : impl_(kErrTag, il, std::forward<Args>(args)...) {}

  // [expected.object.assign], assignment
  template <typename U = std::remove_cv_t<T>>
    requires(internal::IsValueAssignment<T, E, U>)
  constexpr expected& operator=(U&& v) noexcept {
    emplace(std::forward<U>(v));
    return *this;
  }

  template <typename U>
  constexpr expected& operator=(const ok<U>& o) noexcept {
    emplace(o.value());
    return *this;
  }

  template <typename U>
  constexpr expected& operator=(ok<U>&& o) noexcept {
    emplace(std::move(o.value()));
    return *this;
  }

  template <typename G>
  constexpr expected& operator=(const unexpected<G>& e) noexcept {
    impl_.emplace_error(e.error());
    return *this;
  }

  template <typename G>
  constexpr expected& operator=(unexpected<G>&& e) noexcept {
    impl_.emplace_error(std::move(e.error()));
    return *this;
  }

  template <typename... Args>
  constexpr T& emplace(Args&&... args) noexcept {
    return impl_.emplace_value(std::forward<Args>(args)...);
  }

  template <typename U, typename... Args>
  constexpr T& emplace(std::initializer_list<U> il, Args&&... args) noexcept {
    return impl_.emplace_value(il, std::forward<Args>(args)...);
  }

  // [expected.object.swap], swap
  constexpr void swap(expected& rhs) noexcept { impl_.swap(rhs.impl_); }
  friend constexpr void swap(expected& x, expected& y) noexcept { x.swap(y); }

  // [expected.object.obs], observers
  constexpr T* operator->() noexcept { return std::addressof(value()); }
  constexpr const T* operator->() const noexcept {
    return std::addressof(value());
  }

  constexpr T& operator*() & noexcept { return value(); }
  constexpr const T& operator*() const& noexcept { return value(); }
  constexpr T&& operator*() && noexcept { return std::move(value()); }
  constexpr const T&& operator*() const&& noexcept {
    return std::move(value());
  }

  // Note: Deviation from the Standard: No operator bool due to bug-prone
  // patterns when the value type is convertible to bool, see e.g.
  // https://abseil.io/tips/141.
  constexpr bool has_value() const noexcept { return impl_.has_value(); }

  constexpr T& value() & noexcept { return impl_.value(); }
  constexpr const T& value() const& noexcept { return impl_.value(); }
  constexpr T&& value() && noexcept { return std::move(value()); }
  constexpr const T&& value() const&& noexcept { return std::move(value()); }

  constexpr E& error() & noexcept { return impl_.error(); }
  constexpr const E& error() const& noexcept { return impl_.error(); }
  constexpr E&& error() && noexcept { return std::move(error()); }
  constexpr const E&& error() const&& noexcept { return std::move(error()); }

  template <typename U = std::remove_cv_t<T>>
  constexpr T value_or(U&& v) const& noexcept {
    static_assert(std::copy_constructible<T>,
                  "expected<T, E>::value_or: T must be copy constructible");
    static_assert(std::convertible_to<U&&, T>,
                  "expected<T, E>::value_or: U must be convertible to T");
    return has_value() ? value() : static_cast<T>(std::forward<U>(v));
  }

  template <typename U = std::remove_cv_t<T>>
  constexpr T value_or(U&& v) && noexcept {
    static_assert(std::move_constructible<T>,
                  "expected<T, E>::value_or: T must be move constructible");
    static_assert(std::convertible_to<U&&, T>,
                  "expected<T, E>::value_or: U must be convertible to T");
    return has_value() ? std::move(value())
                       : static_cast<T>(std::forward<U>(v));
  }

  template <typename G = E>
  constexpr E error_or(G&& e) const& noexcept {
    static_assert(std::copy_constructible<E>,
                  "expected<T, E>::error_or: E must be copy constructible");
    static_assert(std::convertible_to<G&&, E>,
                  "expected<T, E>::error_or: G must be convertible to E");
    return has_value() ? static_cast<E>(std::forward<G>(e)) : error();
  }

  template <typename G = E>
  constexpr E error_or(G&& e) && noexcept {
    static_assert(std::move_constructible<E>,
                  "expected<T, E>::error_or: E must be move constructible");
    static_assert(std::convertible_to<G&&, E>,
                  "expected<T, E>::error_or: G must be convertible to E");
    return has_value() ? static_cast<E>(std::forward<G>(e))
                       : std::move(error());
  }

  // [expected.object.monadic], monadic operations
  //
  // This section implements the monadic interface consisting of `and_then`,
  // `or_else`, `transform` and `transform_error`.

  // `and_then`: This methods accepts a callable `f` that is invoked with
  // `value()` in case `has_value()` is true.
  //
  // `f`'s return type is required to be a (cvref-qualified) specialization of
  // base::expected with a matching error_type, i.e. it needs to be of the form
  // base::expected<U, E> cv ref for some U.
  //
  // If `has_value()` is false, this is effectively a no-op, and the function
  // returns base::expected<U, E>(unexpect, std::forward<E>(error()));
  //
  // `and_then` is overloaded for all possible forms of const and ref
  // qualifiers.
  template <typename F>
    requires(std::copy_constructible<E>)
  constexpr auto and_then(F&& f) & noexcept {
    return internal::AndThen(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::copy_constructible<E>)
  constexpr auto and_then(F&& f) const& noexcept {
    return internal::AndThen(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<E>)
  constexpr auto and_then(F&& f) && noexcept {
    return internal::AndThen(std::move(*this), std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<E>)
  constexpr auto and_then(F&& f) const&& noexcept {
    return internal::AndThen(std::move(*this), std::forward<F>(f));
  }

  // `or_else`: This methods accepts a callable `f` that is invoked with
  // `error()` in case `has_value()` is false.
  //
  // `f`'s return type is required to be a (cvref-qualified) specialization of
  // base::expected with a matching value_type, i.e. it needs to be of the form
  // base::expected<T, G> cv ref for some G.
  //
  // If `has_value()` is true, this is effectively a no-op, and the function
  // returns base::expected<T, G>(std::forward<T>(value()));
  //
  // `or_else` is overloaded for all possible forms of const and ref
  // qualifiers.
  template <typename F>
    requires(std::copy_constructible<T>)
  constexpr auto or_else(F&& f) & noexcept {
    return internal::OrElse(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::copy_constructible<T>)
  constexpr auto or_else(F&& f) const& noexcept {
    return internal::OrElse(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<T>)
  constexpr auto or_else(F&& f) && noexcept {
    return internal::OrElse(std::move(*this), std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<T>)
  constexpr auto or_else(F&& f) const&& noexcept {
    return internal::OrElse(std::move(*this), std::forward<F>(f));
  }

  // `transform`: This methods accepts a callable `f` that is invoked with
  // `value()` in case `has_value()` is true.
  //
  // `f`'s return type U needs to be a valid value_type for expected, i.e. any
  // type for which `remove_cv_t` is either void, or a complete non-array object
  // type that is not `std::in_place_t`, `base::unexpect_t`, or a
  // specialization of `base::ok` or `base::unexpected`.
  //
  // Returns an instance of base::expected<remove_cv_t<U>, E> that is
  // constructed with f(value()) if there is a value, or unexpected(error())
  // otherwise.
  //
  // `transform` is overloaded for all possible forms of const and ref
  // qualifiers.
  template <typename F>
    requires(std::copy_constructible<E>)
  constexpr auto transform(F&& f) & noexcept {
    return internal::Transform(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::copy_constructible<E>)
  constexpr auto transform(F&& f) const& noexcept {
    return internal::Transform(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<E>)
  constexpr auto transform(F&& f) && noexcept {
    return internal::Transform(std::move(*this), std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<E>)
  constexpr auto transform(F&& f) const&& noexcept {
    return internal::Transform(std::move(*this), std::forward<F>(f));
  }

  // `transform_error`: This methods accepts a callable `f` that is invoked with
  // `error()` in case `has_value()` is false.
  //
  // `f`'s return type G needs to be a valid error_type for expected, i.e. any
  // type for which `remove_cv_t` is a complete non-array object type that is
  // not `std::in_place_t`, `base::unexpect_t`, or a specialization of
  // `base::ok` or `base::unexpected`.
  //
  // Returns an instance of base::expected<T, remove_cv_t<G>> that is
  // constructed with unexpected(f(error())) if there is no value, or value()
  // otherwise.
  //
  // `transform_error` is overloaded for all possible forms of const and ref
  // qualifiers.
  template <typename F>
    requires(std::copy_constructible<T>)
  constexpr auto transform_error(F&& f) & noexcept {
    return internal::TransformError(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::copy_constructible<T>)
  constexpr auto transform_error(F&& f) const& noexcept {
    return internal::TransformError(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<T>)
  constexpr auto transform_error(F&& f) && noexcept {
    return internal::TransformError(std::move(*this), std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<T>)
  constexpr auto transform_error(F&& f) const&& noexcept {
    return internal::TransformError(std::move(*this), std::forward<F>(f));
  }

  // Deviation from the Standard: stringification support.
  //
  // If we move to `std::expected` someday, we would need to either forego nice
  // formatted output or move to `std::format` or similar, which can have
  // customized output for STL types.
  std::string ToString() const {
    return has_value() ? StrCat({"Expected(", base::ToString(value()), ")"})
                       : StrCat({"Unexpected(", base::ToString(error()), ")"});
  }

 private:
  using Impl = internal::ExpectedImpl<T, E>;
  static constexpr auto kValTag = Impl::kValTag;
  static constexpr auto kErrTag = Impl::kErrTag;

  Impl impl_;
};

// [expected.void], partial specialization of expected for void types
template <typename T, typename E>
  requires(std::is_void_v<T>)
class [[nodiscard]] expected<T, E> final {
  // Note: A partial specialization for non-void value types can be found above.
  static_assert(std::is_void_v<T>, "Error: T must be void");

 public:
  using value_type = T;
  using error_type = E;
  using unexpected_type = unexpected<E>;

  // Alias template to explicitly opt into the std::pointer_traits machinery.
  // See e.g. https://en.cppreference.com/w/cpp/memory/pointer_traits#Notes
  template <typename U>
  using rebind = expected<U, E>;

  template <typename U, typename G>
  friend class expected;

  // [expected.void.ctor], constructors
  constexpr expected() noexcept = default;

  // Converting copy and move constructors. These constructors are explicit if
  // the error type is not implicitly convertible from `rhs`'s error type.
  template <typename U, typename G>
    requires(internal::IsValidVoidConversion<E, U, const G&>)
  explicit(!std::convertible_to<const G&, E>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(const expected<U, G>& rhs) noexcept
      : impl_(rhs.impl_) {}

  template <typename U, typename G>
    requires(internal::IsValidVoidConversion<E, U, G>)
  explicit(!std::convertible_to<G, E>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(expected<U, G>&& rhs) noexcept
      : impl_(std::move(rhs.impl_)) {}

  // NOLINTNEXTLINE(google-explicit-constructor)
  constexpr /* implicit */ expected(base::ok<T>) noexcept {}

  template <typename G>
    requires(std::constructible_from<E, const G&>)
  explicit(!std::convertible_to<const G&, E>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(const unexpected<G>& e) noexcept
      : impl_(kErrTag, e.error()) {}

  template <typename G>
    requires(std::constructible_from<E, G>)
  explicit(!std::convertible_to<G, E>)
      // NOLINTNEXTLINE(google-explicit-constructor)
      constexpr expected(unexpected<G>&& e) noexcept
      : impl_(kErrTag, std::move(e.error())) {}

  constexpr explicit expected(std::in_place_t) noexcept {}

  template <typename... Args>
  constexpr explicit expected(unexpect_t, Args&&... args) noexcept
      : impl_(kErrTag, std::forward<Args>(args)...) {}

  template <typename U, typename... Args>
  constexpr expected(unexpect_t,
                     std::initializer_list<U> il,
                     Args&&... args) noexcept
      : impl_(kErrTag, il, std::forward<Args>(args)...) {}

  // [expected.void.assign], assignment
  template <typename G>
  constexpr expected& operator=(const unexpected<G>& e) noexcept {
    impl_.emplace_error(e.error());
    return *this;
  }

  template <typename G>
  constexpr expected& operator=(unexpected<G>&& e) noexcept {
    impl_.emplace_error(std::move(e.error()));
    return *this;
  }

  constexpr void emplace() noexcept { impl_.emplace_value(); }

  // [expected.void.swap], swap
  constexpr void swap(expected& rhs) noexcept { impl_.swap(rhs.impl_); }
  friend constexpr void swap(expected& x, expected& y) noexcept { x.swap(y); }

  // [expected.void.obs], observers
  // Note: Deviation from the Standard: No operator bool due to consistency with
  // non-void expected types.
  constexpr bool has_value() const noexcept { return impl_.has_value(); }

  constexpr void operator*() const { CHECK(has_value()); }
  constexpr void value() const { CHECK(has_value()); }

  constexpr E& error() & { return impl_.error(); }
  constexpr const E& error() const& { return impl_.error(); }
  constexpr E&& error() && { return std::move(error()); }
  constexpr const E&& error() const&& { return std::move(error()); }

  template <typename G>
  constexpr E error_or(G&& e) const& noexcept {
    static_assert(std::copy_constructible<E>,
                  "expected<T, E>::error_or: E must be copy constructible");
    static_assert(std::convertible_to<G&&, E>,
                  "expected<T, E>::error_or: G must be convertible to E");
    return has_value() ? static_cast<E>(std::forward<G>(e)) : error();
  }

  template <typename G>
  constexpr E error_or(G&& e) && noexcept {
    static_assert(std::move_constructible<E>,
                  "expected<T, E>::error_or: E must be move constructible");
    static_assert(std::convertible_to<G&&, E>,
                  "expected<T, E>::error_or: G must be convertible to E");
    return has_value() ? static_cast<E>(std::forward<G>(e))
                       : std::move(error());
  }

  // [expected.void.monadic], monadic operations
  //
  // This section implements the monadic interface consisting of `and_then`,
  // `or_else`, `transform` and `transform_error`. In contrast to the non void
  // specialization it is mandated that the callables for `and_then` and
  // `transform`don't take any arguments.

  // `and_then`: This methods accepts a callable `f` that is invoked with
  // no arguments in case `has_value()` is true.
  //
  // `f`'s return type is required to be a (cvref-qualified) specialization of
  // base::expected with a matching error_type, i.e. it needs to be of the form
  // base::expected<U, E> cv ref for some U.
  //
  // If `has_value()` is false, this is effectively a no-op, and the function
  // returns base::expected<U, E>(unexpect, std::forward<E>(error()));
  //
  // `and_then` is overloaded for all possible forms of const and ref
  // qualifiers.
  template <typename F>
    requires(std::copy_constructible<E>)
  constexpr auto and_then(F&& f) & noexcept {
    return internal::AndThen(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::copy_constructible<E>)
  constexpr auto and_then(F&& f) const& noexcept {
    return internal::AndThen(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<E>)
  constexpr auto and_then(F&& f) && noexcept {
    return internal::AndThen(std::move(*this), std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<E>)
  constexpr auto and_then(F&& f) const&& noexcept {
    return internal::AndThen(std::move(*this), std::forward<F>(f));
  }

  // `or_else`: This methods accepts a callable `f` that is invoked with
  // `error()` in case `has_value()` is false.
  //
  // `f`'s return type is required to be a (cvref-qualified) specialization of
  // base::expected with a matching value_type, i.e. it needs to be cv void.
  //
  // If `has_value()` is true, this is effectively a no-op, and the function
  // returns base::expected<cv void, G>().
  //
  // `or_else` is overloaded for all possible forms of const and ref
  // qualifiers.
  template <typename F>
  constexpr auto or_else(F&& f) & noexcept {
    return internal::OrElse(*this, std::forward<F>(f));
  }

  template <typename F>
  constexpr auto or_else(F&& f) const& noexcept {
    return internal::OrElse(*this, std::forward<F>(f));
  }

  template <typename F>
  constexpr auto or_else(F&& f) && noexcept {
    return internal::OrElse(std::move(*this), std::forward<F>(f));
  }

  template <typename F>
  constexpr auto or_else(F&& f) const&& noexcept {
    return internal::OrElse(std::move(*this), std::forward<F>(f));
  }

  // `transform`: This methods accepts a callable `f` that is invoked with no
  // arguments in case `has_value()` is true.
  //
  // `f`'s return type U needs to be a valid value_type for expected, i.e. any
  // type for which `remove_cv_t` is either void, or a complete non-array object
  // type that is not `std::in_place_t`, `base::unexpect_t`, or a
  // specialization of `base::ok` or `base::unexpected`.
  //
  // Returns an instance of base::expected<remove_cv_t<U>, E> that is
  // constructed with f() if has_value() is true, or unexpected(error())
  // otherwise.
  //
  // `transform` is overloaded for all possible forms of const and ref
  // qualifiers.
  template <typename F>
    requires(std::copy_constructible<E>)
  constexpr auto transform(F&& f) & noexcept {
    return internal::Transform(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::copy_constructible<E>)
  constexpr auto transform(F&& f) const& noexcept {
    return internal::Transform(*this, std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<E>)
  constexpr auto transform(F&& f) && noexcept {
    return internal::Transform(std::move(*this), std::forward<F>(f));
  }

  template <typename F>
    requires(std::move_constructible<E>)
  constexpr auto transform(F&& f) const&& noexcept {
    return internal::Transform(std::move(*this), std::forward<F>(f));
  }

  // `transform_error`: This methods accepts a callable `f` that is invoked with
  // `error()` in case `has_value()` is false.
  //
  // `f`'s return type G needs to be a valid error_type for expected, i.e. any
  // type for which `remove_cv_t` is a complete non-array object type that is
  // not `std::in_place_t`, `base::unexpect_t`, or a specialization of
  // `base::ok` or `base::unexpected`.
  //
  // Returns an instance of base::expected<cv void, remove_cv_t<G>> that is
  // constructed with unexpected(f(error())) if there is no value, or default
  // constructed otherwise.
  //
  // `transform_error` is overloaded for all possible forms of const and ref
  // qualifiers.
  template <typename F>
  constexpr auto transform_error(F&& f) & noexcept {
    return internal::TransformError(*this, std::forward<F>(f));
  }

  template <typename F>
  constexpr auto transform_error(F&& f) const& noexcept {
    return internal::TransformError(*this, std::forward<F>(f));
  }

  template <typename F>
  constexpr auto transform_error(F&& f) && noexcept {
    return internal::TransformError(std::move(*this), std::forward<F>(f));
  }

  template <typename F>
  constexpr auto transform_error(F&& f) const&& noexcept {
    return internal::TransformError(std::move(*this), std::forward<F>(f));
  }

  // Deviation from the Standard: stringification support.
  //
  // If we move to `std::expected` someday, we would need to either forego nice
  // formatted output or move to `std::format` or similar, which can have
  // customized output for STL types.
  std::string ToString() const {
    return has_value() ? "Expected()"
                       : StrCat({"Unexpected(", base::ToString(error()), ")"});
  }

 private:
  // Note: Since we can't store void types we use std::monostate instead.
  using Impl = internal::ExpectedImpl<std::monostate, E>;
  static constexpr auto kErrTag = Impl::kErrTag;

  Impl impl_;
};

// [expected.object.eq], equality operators
// [expected.void.eq], equality operators
template <typename T, typename E, typename U, typename G>
constexpr bool operator==(const expected<T, E>& x,
                          const expected<U, G>& y) noexcept {
  if (x.has_value() != y.has_value()) {
    return false;
  }

  if (x.has_value()) {
    // Values for expected void types always compare equal.
    if constexpr (std::is_void_v<T> && std::is_void_v<U>) {
      return true;
    } else {
      return x.value() == y.value();
    }
  }

  return x.error() == y.error();
}

template <typename T, typename E, typename U>
  requires(!std::is_void_v<T>)
constexpr bool operator==(const expected<T, E>& x, const U& v) noexcept {
  return x.has_value() && x.value() == v;
}

template <typename T, typename E, typename U>
constexpr bool operator==(const expected<T, E>& x, const ok<U>& o) noexcept {
  if constexpr (std::is_void_v<T> && std::is_void_v<U>) {
    return x.has_value();
  } else if constexpr (std::is_void_v<T> || std::is_void_v<U>) {
    return false;
  } else {
    return x.has_value() && x.value() == o.value();
  }
}

template <typename T, typename E, typename G>
constexpr bool operator==(const expected<T, E>& x,
                          const unexpected<G>& e) noexcept {
  return !x.has_value() && x.error() == e.error();
}

}  // namespace base

#endif  // BASE_TYPES_EXPECTED_H_