gdbsupport/traits.h

/* Copyright (C) 2017-2021 Free Software Foundation, Inc.

   This file is part of GDB.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#ifndef COMMON_TRAITS_H
#define COMMON_TRAITS_H

#include <type_traits>

/* GCC does not understand __has_feature.  */
#if !defined(__has_feature)
# define __has_feature(x) 0
#endif

/* HAVE_IS_TRIVIALLY_COPYABLE is defined as 1 iff
   std::is_trivially_copyable is available.  GCC only implemented it
   in GCC 5.  */
#if (__has_feature(is_trivially_copyable) \
     || (defined __GNUC__ && __GNUC__ >= 5))
# define HAVE_IS_TRIVIALLY_COPYABLE 1
#endif

/* HAVE_IS_TRIVIALLY_CONSTRUCTIBLE is defined as 1 iff
   std::is_trivially_constructible is available.  GCC only implemented it
   in GCC 5.  */
#if (__has_feature(is_trivially_constructible) \
     || (defined __GNUC__ && __GNUC__ >= 5))
# define HAVE_IS_TRIVIALLY_CONSTRUCTIBLE 1
#endif

namespace gdb {

/* Pre C++14-safe (CWG 1558) version of C++17's std::void_t.  See
   <http://en.cppreference.com/w/cpp/types/void_t>.  */

template<typename... Ts>
struct make_void { typedef void type; };

template<typename... Ts>
using void_t = typename make_void<Ts...>::type;

/* Implementation of the detection idiom:

   - http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4502.pdf
   - http://en.cppreference.com/w/cpp/experimental/is_detected

*/

struct nonesuch
{
  nonesuch () = delete;
  ~nonesuch () = delete;
  nonesuch (const nonesuch &) = delete;
  void operator= (const nonesuch &) = delete;
};

namespace detection_detail {
/* Implementation of the detection idiom (negative case).  */
template<typename Default, typename AlwaysVoid,
	 template<typename...> class Op, typename... Args>
struct detector
{
  using value_t = std::false_type;
  using type = Default;
};

/* Implementation of the detection idiom (positive case).  */
template<typename Default, template<typename...> class Op, typename... Args>
struct detector<Default, void_t<Op<Args...>>, Op, Args...>
{
  using value_t = std::true_type;
  using type = Op<Args...>;
};

/* Detect whether Op<Args...> is a valid type, use Default if not.  */
template<typename Default, template<typename...> class Op,
	 typename... Args>
using detected_or = detector<Default, void, Op, Args...>;

/* Op<Args...> if that is a valid type, otherwise Default.  */
template<typename Default, template<typename...> class Op,
	 typename... Args>
using detected_or_t
  = typename detected_or<Default, Op, Args...>::type;

} /* detection_detail */

template<template<typename...> class Op, typename... Args>
using is_detected
  = typename detection_detail::detector<nonesuch, void, Op, Args...>::value_t;

template<template<typename...> class Op, typename... Args>
using detected_t
  = typename detection_detail::detector<nonesuch, void, Op, Args...>::type;

template<typename Default, template<typename...> class Op, typename... Args>
using detected_or = detection_detail::detected_or<Default, Op, Args...>;

template<typename Default, template<typename...> class Op, typename... Args>
using detected_or_t = typename detected_or<Default, Op, Args...>::type;

template<typename Expected, template<typename...> class Op, typename... Args>
using is_detected_exact = std::is_same<Expected, detected_t<Op, Args...>>;

template<typename To, template<typename...> class Op, typename... Args>
using is_detected_convertible
  = std::is_convertible<detected_t<Op, Args...>, To>;

/* A few trait helpers, mainly stolen from libstdc++.  Uppercase
   because "and/or", etc. are reserved keywords.  */

template<typename Predicate>
struct Not : public std::integral_constant<bool, !Predicate::value>
{};

template<typename...>
struct Or;

template<>
struct Or<> : public std::false_type
{};

template<typename B1>
struct Or<B1> : public B1
{};

template<typename B1, typename B2>
struct Or<B1, B2>
  : public std::conditional<B1::value, B1, B2>::type
{};

template<typename B1,typename B2,typename B3, typename... Bn>
struct Or<B1, B2, B3, Bn...>
  : public std::conditional<B1::value, B1, Or<B2, B3, Bn...>>::type
{};

template<typename...>
struct And;

template<>
struct And<> : public std::true_type
{};

template<typename B1>
struct And<B1> : public B1
{};

template<typename B1, typename B2>
struct And<B1, B2>
  : public std::conditional<B1::value, B2, B1>::type
{};

template<typename B1, typename B2, typename B3, typename... Bn>
struct And<B1, B2, B3, Bn...>
  : public std::conditional<B1::value, And<B2, B3, Bn...>, B1>::type
{};

/* Concepts-light-like helper to make SFINAE logic easier to read.  */
template<typename Condition>
using Requires = typename std::enable_if<Condition::value, void>::type;
}

#endif /* COMMON_TRAITS_H */