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#ifndef _CUDA_FUNCTIONAL_
#define _CUDA_FUNCTIONAL_

#include <cuda/std/detail/__config>

#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
#  pragma GCC system_header
#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
#  pragma clang system_header
#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
#  pragma system_header
#endif // no system header

#include <cuda/std/__algorithm/max.h>
#include <cuda/std/__algorithm/min.h>
#include <cuda/std/functional>
#include <cuda/std/type_traits>
#include <cuda/std/utility>

_LIBCUDACXX_BEGIN_NAMESPACE_CUDA
namespace __detail
{

template <class _Ret, class _DecayFn>
class __return_type_wrapper
{
private:
  _DecayFn __fn_;

public:
  __return_type_wrapper() = delete;

  template <class _Fn,
            class = _CUDA_VSTD::__enable_if_t<_CUDA_VSTD::is_same<_CUDA_VSTD::__decay_t<_Fn>, _DecayFn>::value>>
  _LIBCUDACXX_INLINE_VISIBILITY _CCCL_CONSTEXPR_CXX14 explicit __return_type_wrapper(_Fn&& __fn) noexcept
      : __fn_(_CUDA_VSTD::forward<_Fn>(__fn))
  {}

  template <class... _As>
  _LIBCUDACXX_INLINE_VISIBILITY _CCCL_CONSTEXPR_CXX14 _Ret operator()(_As&&... __as) & noexcept
  {
#if !defined(__NVCC__) || defined(__CUDA_ARCH__)
    static_assert(_CUDA_VSTD::is_same<_Ret, typename _CUDA_VSTD::__invoke_of<_DecayFn&, _As...>::type>::value,
                  "Return type shall match the proclaimed one exactly");
#endif

    return _CUDA_VSTD::__invoke(__fn_, _CUDA_VSTD::forward<_As>(__as)...);
  }

  template <class... _As>
  _LIBCUDACXX_INLINE_VISIBILITY _CCCL_CONSTEXPR_CXX14 _Ret operator()(_As&&... __as) && noexcept
  {
#if !defined(__NVCC__) || defined(__CUDA_ARCH__)
    static_assert(_CUDA_VSTD::is_same<_Ret, typename _CUDA_VSTD::__invoke_of<_DecayFn, _As...>::type>::value,
                  "Return type shall match the proclaimed one exactly");
#endif

    return _CUDA_VSTD::__invoke(_CUDA_VSTD::move(__fn_), _CUDA_VSTD::forward<_As>(__as)...);
  }

  template <class... _As>
  _LIBCUDACXX_INLINE_VISIBILITY _CCCL_CONSTEXPR_CXX14 _Ret operator()(_As&&... __as) const& noexcept
  {
#if !defined(__NVCC__) || defined(__CUDA_ARCH__)
    static_assert(_CUDA_VSTD::is_same<_Ret, typename _CUDA_VSTD::__invoke_of<const _DecayFn&, _As...>::type>::value,
                  "Return type shall match the proclaimed one exactly");
#endif

    return _CUDA_VSTD::__invoke(__fn_, _CUDA_VSTD::forward<_As>(__as)...);
  }

  template <class... _As>
  _LIBCUDACXX_INLINE_VISIBILITY _CCCL_CONSTEXPR_CXX14 _Ret operator()(_As&&... __as) const&& noexcept
  {
#if !defined(__NVCC__) || defined(__CUDA_ARCH__)
    static_assert(_CUDA_VSTD::is_same<_Ret, typename _CUDA_VSTD::__invoke_of<const _DecayFn, _As...>::type>::value,
                  "Return type shall match the proclaimed one exactly");
#endif

    return _CUDA_VSTD::__invoke(_CUDA_VSTD::move(__fn_), _CUDA_VSTD::forward<_As>(__as)...);
  }
};

} // namespace __detail

template <class _Ret, class _Fn>
inline _LIBCUDACXX_INLINE_VISIBILITY __detail::__return_type_wrapper<_Ret, _CUDA_VSTD::__decay_t<_Fn>>
proclaim_return_type(_Fn&& __fn) noexcept
{
  return __detail::__return_type_wrapper<_Ret, _CUDA_VSTD::__decay_t<_Fn>>(_CUDA_VSTD::forward<_Fn>(__fn));
}
_LIBCUDACXX_END_NAMESPACE_CUDA

#endif // _CUDA_FUNCTIONAL_
