plain_type.hpp

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#ifndef STAN_MATH_PRIM_META_PLAIN_TYPE_HPP
#define STAN_MATH_PRIM_META_PLAIN_TYPE_HPP
 
#include <stan/math/prim/meta/is_eigen.hpp>
#include <stan/math/prim/meta/is_detected.hpp>
#include <stan/math/prim/meta/is_var_matrix.hpp>
#include <type_traits>
 
namespace stan {
 
template <typename T, typename Enable = void>
struct plain_type {
  using type = std::decay_t<T>;
};
 
template <typename T>
using plain_type_t = typename plain_type<T>::type;
 
template <typename T>
struct eval_return_type {
  using T1 = plain_type_t<T>;
  using type = std::conditional_t<std::is_same<std::decay_t<T>, T1>::value,
                                  const T1&, T1>;
};
 
template <typename T>
using eval_return_type_t = typename eval_return_type<T>::type;
 
namespace internal {
// primary template handles types that have no nested ::type member:
template <class, class = void>
struct has_plain_object : std::false_type {};
 
// specialization recognizes types that do have a nested ::type member:
template <class T>
struct has_plain_object<T, void_t<typename std::decay_t<T>::PlainObject>>
    : std::true_type {};
 
// primary template handles types that have no nested ::type member:
template <class, class = void>
struct has_eval : std::false_type {};
 
// specialization recognizes types that do have a nested ::type member:
template <class T>
struct has_eval<T, void_t<decltype(std::declval<std::decay_t<T>&>().eval())>>
    : std::true_type {};
 
}  // namespace internal
 
template <typename T>
struct plain_type<T, require_t<bool_constant<internal::has_eval<T>::value
                                             && is_eigen<T>::value>>> {
  using type = std::decay_t<decltype(std::declval<T&>().eval())>;
};
 
template <typename T>
struct plain_type<
    T, require_t<bool_constant<!internal::has_eval<T>::value
                               && internal::has_plain_object<T>::value
                               && is_eigen<T>::value>>> {
  using type = typename std::decay_t<T>::PlainObject;
};
 
}  // namespace stan
 
#endif  // STAN_MATH_PRIM_META_PLAIN_TYPE_HPP