base/template_util.h

// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_TEMPLATE_UTIL_H_
#define BASE_TEMPLATE_UTIL_H_

#include <stddef.h>
#include <iosfwd>
#include <iterator>
#include <type_traits>
#include <utility>
#include <vector>

#include "build/build_config.h"

// Some versions of libstdc++ have partial support for type_traits, but misses
// a smaller subset while removing some of the older non-standard stuff. Assume
// that all versions below 5.0 fall in this category, along with one 5.0
// experimental release. Test for this by consulting compiler major version,
// the only reliable option available, so theoretically this could fail should
// you attempt to mix an earlier version of libstdc++ with >= GCC5. But
// that's unlikely to work out, especially as GCC5 changed ABI.
#define CR_GLIBCXX_5_0_0 20150123
#if (defined(__GNUC__) && __GNUC__ < 5) || \
    (defined(__GLIBCXX__) && __GLIBCXX__ == CR_GLIBCXX_5_0_0)
#define CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX
#endif

// This hacks around using gcc with libc++ which has some incompatibilies.
// - is_trivially_* doesn't work: https://llvm.org/bugs/show_bug.cgi?id=27538
// TODO(danakj): Remove this when android builders are all using a newer version
// of gcc, or the android ndk is updated to a newer libc++ that works with older
// gcc versions.
#if !defined(__clang__) && defined(_LIBCPP_VERSION)
#define CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX
#endif

namespace base {

template <class T> struct is_non_const_reference : std::false_type {};
template <class T> struct is_non_const_reference<T&> : std::true_type {};
template <class T> struct is_non_const_reference<const T&> : std::false_type {};

namespace internal {

// Implementation detail of base::void_t below.
template <typename...>
struct make_void {
  using type = void;
};

}  // namespace internal

// base::void_t is an implementation of std::void_t from C++17.
//
// We use |base::internal::make_void| as a helper struct to avoid a C++14
// defect:
//   http://en.cppreference.com/w/cpp/types/void_t
//   http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1558
template <typename... Ts>
using void_t = typename ::base::internal::make_void<Ts...>::type;

namespace internal {

// Uses expression SFINAE to detect whether using operator<< would work.
template <typename T, typename = void>
struct SupportsOstreamOperator : std::false_type {};
template <typename T>
struct SupportsOstreamOperator<T,
                               decltype(void(std::declval<std::ostream&>()
                                             << std::declval<T>()))>
    : std::true_type {};

// Used to detech whether the given type is an iterator.  This is normally used
// with std::enable_if to provide disambiguation for functions that take
// templatzed iterators as input.
template <typename T, typename = void>
struct is_iterator : std::false_type {};

template <typename T>
struct is_iterator<T,
                   void_t<typename std::iterator_traits<T>::iterator_category>>
    : std::true_type {};

}  // namespace internal

// is_trivially_copyable is especially hard to get right.
// - Older versions of libstdc++ will fail to have it like they do for other
//   type traits. This has become a subset of the second point, but used to be
//   handled independently.
// - An experimental release of gcc includes most of type_traits but misses
//   is_trivially_copyable, so we still have to avoid using libstdc++ in this
//   case, which is covered by CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX.
// - When compiling libc++ from before r239653, with a gcc compiler, the
//   std::is_trivially_copyable can fail. So we need to work around that by not
//   using the one in libc++ in this case. This is covered by the
//   CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX define, and is discussed in
//   https://llvm.org/bugs/show_bug.cgi?id=27538#c1 where they point out that
//   in libc++'s commit r239653 this is fixed by libc++ checking for gcc 5.1.
// - In both of the above cases we are using the gcc compiler. When defining
//   this ourselves on compiler intrinsics, the __is_trivially_copyable()
//   intrinsic is not available on gcc before version 5.1 (see the discussion in
//   https://llvm.org/bugs/show_bug.cgi?id=27538#c1 again), so we must check for
//   that version.
// - When __is_trivially_copyable() is not available because we are on gcc older
//   than 5.1, we need to fall back to something, so we use __has_trivial_copy()
//   instead based on what was done one-off in bit_cast() previously.

// TODO(crbug.com/554293): Remove this when all platforms have this in the std
// namespace and it works with gcc as needed.
#if defined(CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX) || \
    defined(CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX)
template <typename T>
struct is_trivially_copyable {
// TODO(danakj): Remove this when android builders are all using a newer version
// of gcc, or the android ndk is updated to a newer libc++ that does this for
// us.
#if _GNUC_VER >= 501
  static constexpr bool value = __is_trivially_copyable(T);
#else
  static constexpr bool value =
      __has_trivial_copy(T) && __has_trivial_destructor(T);
#endif
};
#else
template <class T>
using is_trivially_copyable = std::is_trivially_copyable<T>;
#endif

#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ <= 7
// Workaround for g++7 and earlier family.
// Due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80654, without this
// Optional<std::vector<T>> where T is non-copyable causes a compile error.
// As we know it is not trivially copy constructible, explicitly declare so.
template <typename T>
struct is_trivially_copy_constructible
    : std::is_trivially_copy_constructible<T> {};

template <typename... T>
struct is_trivially_copy_constructible<std::vector<T...>> : std::false_type {};
#else
// Otherwise use std::is_trivially_copy_constructible as is.
template <typename T>
using is_trivially_copy_constructible = std::is_trivially_copy_constructible<T>;
#endif

}  // namespace base

#undef CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX
#undef CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX

#endif  // BASE_TEMPLATE_UTIL_H_