diplomat_runtime.hpp

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#ifndef ICU4X_DIPLOMAT_RUNTIME_CPP_H
#define ICU4X_DIPLOMAT_RUNTIME_CPP_H
 
#include <optional>
#include <string>
#include <string_view>
#include <type_traits>
#include <variant>
#include <vector>
#include <cstdint>
#include <functional>
#include <memory>
#include <limits>
 
 
#if __cplusplus >= 202002L
#include <span>
#else
#include <array>
#endif
 
#ifndef DIPLOMAT_LIFETIME_BOUND
#if defined(__has_cpp_attribute)
#if __has_cpp_attribute(msvc::lifetimebound)
#define DIPLOMAT_LIFETIME_BOUND [[msvc::lifetimebound]]
#elif __has_cpp_attribute(clang::lifetimebound)
#define DIPLOMAT_LIFETIME_BOUND [[clang::lifetimebound]]
#endif
#endif
#endif
 
#ifndef DIPLOMAT_LIFETIME_BOUND
#define DIPLOMAT_LIFETIME_BOUND
#endif
 
namespace icu4x {
namespace diplomat {
 
namespace capi {
extern "C" {
 
static_assert(sizeof(char) == sizeof(uint8_t), "your architecture's `char` is not 8 bits");
static_assert(sizeof(char16_t) == sizeof(uint16_t), "your architecture's `char16_t` is not 16 bits");
static_assert(sizeof(char32_t) == sizeof(uint32_t), "your architecture's `char32_t` is not 32 bits");
 
typedef struct DiplomatWrite {
    void* context;
    char* buf;
    size_t len;
    size_t cap;
    bool grow_failed;
    void (*flush)(struct DiplomatWrite*);
    bool (*grow)(struct DiplomatWrite*, size_t);
} DiplomatWrite;
 
bool diplomat_is_str(const char* buf, size_t len);
 
#define MAKE_SLICES(name, c_ty) \
    typedef struct Diplomat##name##View { \
        const c_ty* data; \
        size_t len; \
    } Diplomat##name##View; \
    typedef struct Diplomat##name##ViewMut { \
        c_ty* data; \
        size_t len; \
    } Diplomat##name##ViewMut; \
    typedef struct Diplomat##name##Array { \
        const c_ty* data; \
        size_t len; \
    } Diplomat##name##Array;
 
#define MAKE_SLICES_AND_OPTIONS(name, c_ty) \
    MAKE_SLICES(name, c_ty) \
    typedef struct Option##name {union { c_ty ok; }; bool is_ok; } Option##name; \
    typedef struct Option##name##View {union { Diplomat##name##View ok; }; bool is_ok; } Option##name##View; \
    typedef struct Option##name##ViewMut {union { Diplomat##name##ViewMut ok; }; bool is_ok; } Option##name##ViewMut; \
    typedef struct Option##name##Array {union { Diplomat##name##Array ok; }; bool is_ok; } Option##name##Array; \
 
MAKE_SLICES_AND_OPTIONS(I8, int8_t)
MAKE_SLICES_AND_OPTIONS(U8, uint8_t)
MAKE_SLICES_AND_OPTIONS(I16, int16_t)
MAKE_SLICES_AND_OPTIONS(U16, uint16_t)
MAKE_SLICES_AND_OPTIONS(I32, int32_t)
MAKE_SLICES_AND_OPTIONS(U32, uint32_t)
MAKE_SLICES_AND_OPTIONS(I64, int64_t)
MAKE_SLICES_AND_OPTIONS(U64, uint64_t)
MAKE_SLICES_AND_OPTIONS(Isize, intptr_t)
MAKE_SLICES_AND_OPTIONS(Usize, size_t)
MAKE_SLICES_AND_OPTIONS(F32, float)
MAKE_SLICES_AND_OPTIONS(F64, double)
MAKE_SLICES_AND_OPTIONS(Bool, bool)
MAKE_SLICES_AND_OPTIONS(Char, char32_t)
MAKE_SLICES_AND_OPTIONS(String, char)
MAKE_SLICES_AND_OPTIONS(String16, char16_t)
MAKE_SLICES_AND_OPTIONS(Strings, DiplomatStringView)
MAKE_SLICES_AND_OPTIONS(Strings16, DiplomatString16View)
 
} // extern "C"
} // namespace capi
 
extern "C" inline void _flush(capi::DiplomatWrite* w) {
  std::string* string = reinterpret_cast<std::string*>(w->context);
  string->resize(w->len);
}
 
extern "C" inline bool _grow_impl(capi::DiplomatWrite* w, uintptr_t requested) {
  std::string* string = reinterpret_cast<std::string*>(w->context);
  string->resize(requested);
  w->cap = string->length();
  w->buf = &(*string)[0];
  return true;
}
 
extern "C" inline bool _grow(capi::DiplomatWrite* w, size_t requested) {
  return _grow_impl(w, static_cast<uintptr_t>(requested));
}
 
inline capi::DiplomatWrite WriteFromString(std::string& string) {
  capi::DiplomatWrite w;
  w.context = &string;
  w.buf = &string[0];
  w.len = string.length();
  w.cap = string.length();
  // Will never become true, as _grow is infallible.
  w.grow_failed = false;
  w.flush = _flush;
  w.grow = _grow;
  return w;
}
 
extern "C" inline void _flush_u16string(capi::DiplomatWrite* w) {
  std::u16string* string = reinterpret_cast<std::u16string*>(w->context);
  string->resize(w->len / sizeof(char16_t));
}
 
extern "C" inline bool _grow_u16string_impl(capi::DiplomatWrite* w, uintptr_t requested) {
  std::u16string* string = reinterpret_cast<std::u16string*>(w->context);
  size_t requested_elements = (requested + sizeof(char16_t) - 1) / sizeof(char16_t);
  string->resize(requested_elements);
  w->cap = string->length() * sizeof(char16_t);
  w->buf = reinterpret_cast<char*>(&(*string)[0]);
  return true;
}
 
extern "C" inline bool _grow_u16string(capi::DiplomatWrite* w, size_t requested) {
  return _grow_u16string_impl(w, static_cast<uintptr_t>(requested));
}
 
inline capi::DiplomatWrite WriteFromString16(std::u16string& string) {
  capi::DiplomatWrite w;
  w.context = &string;
  w.buf = reinterpret_cast<char*>(&string[0]);
  w.len = string.length() * sizeof(char16_t);
  w.cap = string.length() * sizeof(char16_t);
  // Will never become true, as _grow_u16string is infallible.
  w.grow_failed = false;
  w.flush = _flush_u16string;
  w.grow = _grow_u16string;
  return w;
}
 
template<typename T>
inline void _flush_vector(capi::DiplomatWrite* w) {
  std::vector<T>* vec = reinterpret_cast<std::vector<T>*>(w->context);
  vec->resize(w->len / sizeof(T));
}
 
template<typename T>
inline bool _grow_vector_impl(capi::DiplomatWrite* w, uintptr_t requested) {
  std::vector<T>* vec = reinterpret_cast<std::vector<T>*>(w->context);
  size_t requested_elements = (requested + sizeof(T) - 1) / sizeof(T);
  vec->resize(requested_elements);
  w->cap = vec->size() * sizeof(T);
  w->buf = reinterpret_cast<char*>(vec->data());
  return true;
}
 
template<typename T>
inline bool _grow_vector(capi::DiplomatWrite* w, size_t requested) {
  return _grow_vector_impl<T>(w, static_cast<uintptr_t>(requested));
}
 
template<typename T>
inline capi::DiplomatWrite WriteFromVector(std::vector<T>& vec) {
  capi::DiplomatWrite w;
  w.context = &vec;
  w.buf = reinterpret_cast<char*>(vec.data());
  w.len = vec.size() * sizeof(T);
  w.cap = vec.size() * sizeof(T);
  // Will never become true, as _grow_vector is infallible.
  w.grow_failed = false;
  w.flush = _flush_vector<T>;
  w.grow = _grow_vector<T>;
  return w;
}
 
// This "trait" allows one to use _write() methods to efficiently
// write to a custom string type. To do this you need to write a specialized
// `WriteTrait<YourType>` (see WriteTrait<std::string> below)
// that is capable of constructing a DiplomatWrite, which can wrap
// your string type with appropriate resize/flush functionality.
template<typename T> struct WriteTrait {
  // Fill in this method on a specialization to implement this trait
  // static inline capi::DiplomatWrite Construct(T& t);
};
 
template<> struct WriteTrait<std::string> {
  static inline capi::DiplomatWrite Construct(std::string& t) {
    return diplomat::WriteFromString(t);
  }
};
 
template<> struct WriteTrait<std::u16string> {
  static inline capi::DiplomatWrite Construct(std::u16string& t) {
    return diplomat::WriteFromString16(t);
  }
};
 
template<typename T> struct WriteTrait<std::vector<T>> {
  static inline capi::DiplomatWrite Construct(std::vector<T>& t) {
    return diplomat::WriteFromVector(t);
  }
};
 
template<class T> struct Ok {
  T inner;
 
  // Move constructor always allowed
  Ok(T&& i): inner(std::forward<T>(i)) {}
 
  //  copy constructor allowed only for trivially copyable types
  template<typename X = T, typename = typename std::enable_if<std::is_trivially_copyable<X>::value>::type>
  Ok(const T& i) : inner(i) {}
 
  Ok() = default;
  Ok(Ok&&) noexcept = default;
  Ok(const Ok &) = default;
  Ok& operator=(const Ok&) = default;
  Ok& operator=(Ok&&) noexcept = default;
};
 
 
template<class T> struct Err {
  T inner;
 
  // Move constructor always allowed
  Err(T&& i): inner(std::forward<T>(i)) {}
 
  //  copy constructor allowed only for trivially copyable types
  template<typename X = T, typename = typename std::enable_if<std::is_trivially_copyable<X>::value>::type>
  Err(const T& i) : inner(i) {}
 
  Err() = default;
  Err(Err&&) noexcept = default;
  Err(const Err &) = default;
  Err& operator=(const Err&) = default;
  Err& operator=(Err&&) noexcept = default;
};
 
template<class T, class E>
class result {
protected:
    std::variant<Ok<T>, Err<E>> val;
public:
 
  result(Ok<T>&& v): val(std::move(v)) {}
  result(Err<E>&& v): val(std::move(v)) {}
  result() = default;
  result(const result &) = default;
  result& operator=(const result&) = default;
  result& operator=(result&&) noexcept = default;
  result(result &&) noexcept = default;
  ~result() = default;
  bool is_ok() const {
    return std::holds_alternative<Ok<T>>(this->val);
  }
  bool is_err() const {
    return std::holds_alternative<Err<E>>(this->val);
  }
 
  template<typename U = T, typename std::enable_if_t<!std::is_reference_v<U>, std::nullptr_t> = nullptr>
  std::optional<T> ok() && {
    if (!this->is_ok()) {
      return std::nullopt;
    }
    return std::make_optional(std::move(std::get<Ok<T>>(std::move(this->val)).inner));
  }
 
  template<typename U = E, typename std::enable_if_t<!std::is_reference_v<U>, std::nullptr_t> = nullptr>
  std::optional<E> err() && {
    if (!this->is_err()) {
      return std::nullopt;
    }
    return std::make_optional(std::move(std::get<Err<E>>(std::move(this->val)).inner));
  }
 
  // std::optional does not work with reference types directly, so wrap them if present
  template<typename U = T, typename std::enable_if_t<std::is_reference_v<U>, std::nullptr_t> = nullptr>
  std::optional<std::reference_wrapper<std::remove_reference_t<T>>> ok() && {
    if (!this->is_ok()) {
      return std::nullopt;
    }
    return std::make_optional(std::reference_wrapper(std::forward<T>(std::get<Ok<T>>(std::move(this->val)).inner)));
  }
 
  template<typename U = E, typename std::enable_if_t<std::is_reference_v<U>, std::nullptr_t> = nullptr>
  std::optional<std::reference_wrapper<std::remove_reference_t<E>>> err() && {
    if (!this->is_err()) {
      return std::nullopt;
    }
    return std::make_optional(std::reference_wrapper(std::forward<E>(std::get<Err<E>>(std::move(this->val)).inner)));
  }
 
  void set_ok(T&& t) {
    this->val = Ok<T>(std::move(t));
  }
 
  void set_err(E&& e) {
    this->val = Err<E>(std::move(e));
  }
 
  template<typename T2>
  result<T2, E> replace_ok(T2&& t) {
    if (this->is_err()) {
      return result<T2, E>(Err<E>(std::get<Err<E>>(std::move(this->val))));
    } else {
      return result<T2, E>(Ok<T2>(std::move(t)));
    }
  }
};
 
class Utf8Error {};
 
// Use custom std::span on C++17, otherwise use std::span
#if __cplusplus >= 202002L
 
constexpr std::size_t dynamic_extent = std::dynamic_extent;
template<class T, std::size_t E = dynamic_extent> using span = std::span<T, E>;
 
#else // __cplusplus < 202002L
 
// C++-17-compatible-ish std::span
constexpr size_t dynamic_extent = std::numeric_limits<std::size_t>::max();
template <class T, std::size_t Extent = dynamic_extent>
class span {
public:
  constexpr span(T *data = nullptr, size_t size = Extent)
    : data_(data), size_(size) {}
 
  constexpr span(const span<T> &o)
    : data_(o.data_), size_(o.size_) {}
  template <size_t N>
  constexpr span(std::array<typename std::remove_const_t<T>, N> &arr)
    : data_(const_cast<T *>(arr.data())), size_(N) {}
 
  constexpr T* data() const noexcept {
    return this->data_;
  }
  constexpr size_t size() const noexcept {
    return this->size_;
  }
 
  constexpr T *begin() const noexcept { return data(); }
  constexpr T *end() const noexcept { return data() + size(); }
 
  void operator=(span<T> o) {
    data_ = o.data_;
    size_ = o.size_;
  }
 
private:
  T* data_;
  size_t size_;
};
 
#endif // __cplusplus >= 202002L
 
// An ABI stable std::basic_string_view equivalent for the case of string
// views in slices
template <class CharT, class Traits = std::char_traits<CharT>>
class basic_string_view_for_slice {
public:
  using std_string_view           = std::basic_string_view<CharT, Traits>;
  using traits_type               = typename std_string_view::traits_type;
  using value_type                = typename std_string_view::value_type;
  using pointer                   = typename std_string_view::pointer;
  using const_pointer             = typename std_string_view::const_pointer;
  using size_type                 = typename std_string_view::size_type;
  using difference_type           = typename std_string_view::difference_type;
 
  constexpr basic_string_view_for_slice() noexcept
    : basic_string_view_for_slice{std_string_view{}} {}
 
  constexpr basic_string_view_for_slice(const basic_string_view_for_slice& other) noexcept = default;
 
  constexpr basic_string_view_for_slice(const const_pointer s, const size_type count)
    : basic_string_view_for_slice{std_string_view{s, count}} {}
 
  constexpr basic_string_view_for_slice(const const_pointer s)
    : basic_string_view_for_slice{std_string_view{s}} {}
 
  constexpr basic_string_view_for_slice& operator=(const basic_string_view_for_slice& view) noexcept = default;
 
  constexpr basic_string_view_for_slice(const std_string_view& s) noexcept
    : data_{s.data(), s.size()} {}
 
  constexpr basic_string_view_for_slice& operator=(const std_string_view& s) noexcept {
    data_ = {s.data(), s.size()};
    return *this;
  }
 
  constexpr operator std_string_view() const noexcept { return {data(), size()}; }
  constexpr std_string_view as_sv() const noexcept { return *this; }
 
  constexpr const_pointer data() const noexcept { return data_.data; }
  constexpr size_type size() const noexcept { return data_.len; }
 
private:
  using capi_type =
    std::conditional_t<std::is_same_v<value_type, char>,
      capi::DiplomatStringView,
    std::conditional_t<std::is_same_v<value_type, char16_t>,
      capi::DiplomatString16View,
      void>>;
 
  static_assert(!std::is_void_v<capi_type>,
    "ABI compatible string_views are only supported for char and char16_t");
 
  capi_type data_;
};
 
// We only implement these specialisations as diplomat doesn't provide c abi
// types for others
using string_view_for_slice = basic_string_view_for_slice<char>;
using u16string_view_for_slice = basic_string_view_for_slice<char16_t>;
 
using string_view_span = span<const string_view_for_slice>;
using u16string_view_span = span<const u16string_view_for_slice>;
 
// Interop between std::function & our C Callback wrapper type
 
template <typename T, typename = void>
struct as_ffi {
  using type = T;
};
 
template <typename T>
struct as_ffi<T, std::void_t<decltype(std::declval<std::remove_pointer_t<T>>().AsFFI())>> {
  using type = decltype(std::declval<std::remove_pointer_t<T>>().AsFFI());
};
 
template <typename T>
struct as_ffi<std::unique_ptr<T>> {
  using type = decltype(std::declval<T>().AsFFI());
};
 
template<typename T>
using as_ffi_t = typename as_ffi<T>::type;
 
template<typename T>
using replace_string_view_t = std::conditional_t<std::is_same_v<T, std::string_view>, capi::DiplomatStringView, T>;
 
template<typename T, typename = void>
struct diplomat_c_span_convert {
  using type = T;
};
 
#define MAKE_SLICE_CONVERTERS(name, c_ty) \
  template<typename T> \
  struct diplomat_c_span_convert<T, std::enable_if_t<std::is_same_v<T, span<const c_ty>>>> { \
    using type = diplomat::capi::Diplomat##name##View; \
  }; \
  template<typename T> \
  struct diplomat_c_span_convert<T, std::enable_if_t<std::is_same_v<T, span<c_ty>>>> { \
    using type = diplomat::capi::Diplomat##name##ViewMut; \
  }; \
 
#if !defined(__sun) || !defined(_CHAR_IS_SIGNED)
// int8_t and char are the same type on Solaris. Guard this definition to avoid
// conflicts.
MAKE_SLICE_CONVERTERS(I8, int8_t)
#endif
MAKE_SLICE_CONVERTERS(U8, uint8_t)
MAKE_SLICE_CONVERTERS(I16, int16_t)
MAKE_SLICE_CONVERTERS(U16, uint16_t)
MAKE_SLICE_CONVERTERS(I32, int32_t)
MAKE_SLICE_CONVERTERS(U32, uint32_t)
MAKE_SLICE_CONVERTERS(I64, int64_t)
MAKE_SLICE_CONVERTERS(U64, uint64_t)
MAKE_SLICE_CONVERTERS(F32, float)
MAKE_SLICE_CONVERTERS(F64, double)
MAKE_SLICE_CONVERTERS(Bool, bool)
MAKE_SLICE_CONVERTERS(Char, char32_t)
MAKE_SLICE_CONVERTERS(String, char)
MAKE_SLICE_CONVERTERS(String16, char16_t)
 
template<typename T>
using diplomat_c_span_convert_t = typename diplomat_c_span_convert<T>::type;
 
template<typename T>
struct is_unique_ptr {
  static constexpr bool value = false;
  using type = T;
};
 
template<typename T>
struct is_unique_ptr<std::unique_ptr<T>> {
  static constexpr bool value = true;
  using type = T;
};
 
template<typename T>
constexpr bool is_unique_ptr_v = is_unique_ptr<T>::value;
 
template<typename T>
struct is_optional {
  static constexpr bool value = false;
};
 
template<typename T>
struct is_optional<std::optional<T>> {
  static constexpr bool value = true;
};
 
template<typename T>
constexpr bool is_optional_v = is_optional<T>::value;
 
template<typename T>
using replace_fn_t = diplomat_c_span_convert_t<replace_string_view_t<as_ffi_t<T>>>;
 
namespace fn_trait_helpers {
  // For a given T, creates a function that take in the C ABI version & return the C++ type.
  template<typename T>
  T replace(replace_fn_t<T> val) {
    if constexpr(std::is_same_v<T, std::string_view>)   {
        return std::string_view{val.data, val.len};
    } else if constexpr (!std::is_same_v<T, diplomat_c_span_convert_t<T>>) {
      return T{ val.data, val.len };
    } else if constexpr (!std::is_same_v<T, as_ffi_t<T>>) {
      if constexpr (std::is_lvalue_reference_v<T>) {
        return *std::remove_reference_t<T>::FromFFI(val);
      } else if constexpr (is_unique_ptr_v<T>) {
        return T(is_unique_ptr<T>::type::FromFFI(val));
      }
      else {
        return T::FromFFI(val);
      }
    }
    else {
        return val;
    }
  }
 
  template<typename T>
  replace_fn_t<T> replace_ret(T val) {
    if constexpr(std::is_same_v<T, std::string_view>)   {
        return {val.data(), val.size()};
    } else if constexpr (!std::is_same_v<T, diplomat_c_span_convert_t<T>>) {
      // Can we convert straight away to our slice type, or (in the case of ABI compatible structs), do we have to do a reinterpret cast?
      if constexpr(std::is_same_v<decltype(std::declval<T>().data()), decltype(replace_fn_t<T>::data)>) {
        return replace_fn_t<T> { val.data(), val.size() };
      } else {
        return replace_fn_t<T> { reinterpret_cast<decltype(replace_fn_t<T>::data)>(val.data()), val.size() };
      }
    } else if constexpr(!std::is_same_v<T, as_ffi_t<T>>) {
      return val.AsFFI();
    } else {
      return val;
    }
  }
 
  template<typename TOut, typename T>
  TOut replace_optional_ret(std::optional<T> optional) {
    constexpr bool has_ok = !std::is_same_v<T, std::monostate>;
 
    bool is_ok = optional.has_value();
 
    TOut out;
    out.is_ok = is_ok;
 
    if constexpr(has_ok) {
      if (is_ok) {
        out.ok = replace_ret<T>(optional.value());
      }
    }
    return out;
  }
 
  template<typename T, typename E, typename TOut>
  TOut replace_result(diplomat::result<T, E> res) {
      auto is_ok = res.is_ok();
 
      constexpr bool has_ok = !std::is_same_v<T, std::monostate>;
      constexpr bool has_err = !std::is_same_v<E, std::monostate>;
 
      TOut out;
      out.is_ok = is_ok;
 
      if constexpr (has_ok) {
        if (is_ok) {
          auto val = std::move(res).ok().value();
          if constexpr (is_optional_v<T>) {
            out.ok = replace_optional_ret<decltype(out.ok)>(val);
          } else {
            out.ok = replace_ret<T>(val);
          }
        }
      }
 
      if constexpr(has_err) {
        if (!is_ok) {
          auto val = std::move(res).err().value();
          if constexpr(is_optional_v<E>) {
            out.err = replace_optional_ret<decltype(out.err)>(val);
          } else {
            out.err = replace_ret<E>(val);
          }
        }
      }
 
      return out;
  }
} // namespace fn_traits
 
template <typename T> struct fn_traits;
 
template <typename Ret, typename... Args> struct fn_traits<std::function<Ret(Args...)>> {
    using fn_ptr_t = Ret(Args...);
    using function_t = std::function<fn_ptr_t>;
    using ret = Ret;
 
    static Ret c_run_callback(const void *cb, replace_fn_t<Args>... args) {
        return (*reinterpret_cast<const function_t *>(cb))(fn_trait_helpers::replace<Args>(args)...);
    }
 
    template<typename T, typename E, typename TOut>
    static TOut c_run_callback_result(const void *cb, replace_fn_t<Args>... args) {
      result<T, E> res = c_run_callback(cb, args...);
 
      return fn_trait_helpers::replace_result<T, E, TOut>(res);
    }
 
    // For DiplomatOption<>
    template<typename T, typename TOut>
    static TOut c_run_callback_diplomat_option(const void *cb, replace_fn_t<Args>... args) {
      std::optional<T> ret = c_run_callback(cb, args...);
 
      return fn_trait_helpers::replace_optional_ret<TOut>(ret);
    }
 
    // All we need to do is just convert one pointer to another, while keeping the arguments the same:
    template<typename T>
    static T c_run_callback_diplomat_opaque(const void* cb, replace_fn_t<Args>... args) {
      Ret out = c_run_callback(cb, args...);
 
      if constexpr(std::is_pointer_v<Ret>) {
        return out->AsFFI();
      } else {
        return out.AsFFI();
      }
    }
 
    static void c_delete(const void *cb) {
        delete reinterpret_cast<const function_t *>(cb);
    }
 
    fn_traits(function_t) {} // Allows less clunky construction (avoids decltype)
};
 
// additional deduction guide required
template<class T>
fn_traits(T) -> fn_traits<T>;
 
// Trait for extracting inner types from either T*, std::optional, or std::unique_ptr.
// These are the three potential types returned by next() functions
template<typename T> struct inner { /* only T*, std::optional, and std::unique_ptr are supported */ };
template<typename T> struct inner<T*> { using type = T; };
template<typename T> struct inner<std::unique_ptr<T>> { using type = T; };
template<typename T> struct inner<std::optional<T>>{ using type = T; };
 
template<typename T, typename U = typename inner<T>::type>
inline const U get_inner_if_present(T v) {
  if constexpr(std::is_same_v<T,U>) {
    return std::move(v);
  } else {
    return *std::move(v);
  }
}
 
// Adapter for iterator types
template<typename T, typename U = void> struct has_next : std::false_type {};
template<typename T> struct has_next < T, std::void_t<decltype(std::declval<T>().next())>> : std::true_type {};
template<typename T> constexpr bool has_next_v = has_next<T>::value;
 
template<typename T>
struct next_to_iter_helper {
  static_assert(has_next_v<T>, "next_to_iter_helper may only be used with types implementing next()");
  using next_type = decltype(std::declval<T>().next());
 
  // STL Iterator trait definitions
  using value_type = typename inner<next_type>::type;
  using difference_type = void;
  using reference = std::add_lvalue_reference_t<value_type>;
  using iterator_category = std::input_iterator_tag;
 
  next_to_iter_helper(std::unique_ptr<T>&& ptr) : _ptr(std::move(ptr)), _curr(_ptr->next()) {}
 
  // https://en.cppreference.com/w/cpp/named_req/InputIterator requires that the type be copyable
  next_to_iter_helper(const next_to_iter_helper& o) : _ptr(o._ptr), _curr(o._curr) {}
 
  void operator++() { _curr = _ptr->next(); }
  void operator++(int) { ++(*this); }
  const value_type& operator*() const { return *_curr; }
 
  bool operator!=(std::nullopt_t) {
    return (bool)_curr;
  }
 
  std::shared_ptr<T> _ptr; // shared to satisfy the copyable requirement
  next_type _curr;
};
 
template<typename T>
using maybe_null = T;
 
} // namespace diplomat
} // namespace icu4x
#endif