static-if.hh

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#pragma once

#include <utility>

#include <vcsn/misc/type_traits.hh>

namespace vcsn
{
  namespace detail
  {
    template <typename Then, typename Else>
    auto static_if_impl(std::true_type, Then&& then, Else&&)
    {
      return std::forward<Then>(then);
    }

    template <typename Then, typename Else>
    auto static_if_impl(std::false_type, Then&&, Else&& else_)
    {
      return std::forward<Else>(else_);
    }

    template <bool cond, typename Then, typename Else>
    auto static_if(Then&& then, Else&& else_)
    {
      return static_if_impl(bool_constant<cond>{},
                            std::forward<Then>(then),
                            std::forward<Else>(else_));
    }

    template <bool cond, typename Then>
    auto static_if(Then&& then)
    {
      // Applies to both GCC and Clang.  Cannot be done inside a
      // statement.
#if defined __GNUC__
# pragma GCC diagnostic push
# pragma GCC diagnostic ignored "-Wunused-parameter"
#endif
      return static_if<cond>(std::forward<Then>(then),
#if (defined __clang__ && __clang_major__ == 3 && __clang_minor__ < 6   \
     || defined __GNUC__ && !defined __clang__ && __GNUC__ < 5)
                     // Clang 3.5 and GCC 4.9 require that we name the argument.
                             [](auto&&... args){}
#else
                             [](auto&&...){}
#endif
                             );
#if defined __GNUC__
# pragma GCC diagnostic pop
#endif
    }
  }
}