ceph/src/boost/libs/hana/test/integral_constant/constexpr_init.cpp

   1 // Copyright Louis Dionne 2013-2017
   2 // Distributed under the Boost Software License, Version 1.0.
   3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   4
   5 #include <boost/hana/bool.hpp>
   6 #include <boost/hana/integral_constant.hpp>
   7 namespace hana = boost::hana;
   8
   9
  10 /*
  11 When we use `int_<...>` in a template, Clang 3.5 says:
  12
  13 --------------------------------
  14 include/boost/hana/integral_constant.hpp:80:20: error: constexpr variable 'int_<1>' must be initialized by a constant expression
  15     constexpr auto int_ = integral<int, i>;
  16                    ^      ~~~~~~~~~~~~~~~~
  17 test/integral/constexpr_bug.cpp:41:37: note: in instantiation of variable template specialization 'boost::hana::int_' requested here
  18 constexpr auto check_int() { return int_<1>; }
  19                                     ^
  20 include/boost/hana/integral_constant.hpp:80:27: note: subexpression not valid in a constant expression
  21     constexpr auto int_ = integral<int, i>;
  22                           ^
  23 include/boost/hana/integral_constant.hpp:80:27: note: in call to 'integral_type(integral)'
  24 --------------------------------
  25
  26 if we define int_ & friends like
  27
  28     template <int i>
  29     constexpr auto int_ = integral<int, i>;
  30
  31 Instead, we do
  32
  33     template <int i>
  34     constexpr decltype(integral<int, i>) int_{};
  35
  36 which is equivalent but uglier. Note that everything works just fine when
  37 we're not in a template.
  38 */
  39
  40 template <typename T>
  41 constexpr auto check_int() { return hana::int_c<1>; }
  42
  43 template <typename T>
  44 constexpr auto check_true() { return hana::true_c; }
  45
  46 template <typename T>
  47 constexpr auto check_size_t() { return hana::size_c<0>; }
  48
  49
  50 int main() { }