diff options
Diffstat (limited to 'absl/meta/type_traits.h')
-rw-r--r-- | absl/meta/type_traits.h | 44 |
1 files changed, 44 insertions, 0 deletions
diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h index 8d3264f1..457b8908 100644 --- a/absl/meta/type_traits.h +++ b/absl/meta/type_traits.h @@ -44,6 +44,7 @@ namespace absl { namespace type_traits_internal { + template <typename... Ts> struct VoidTImpl { using type = void; @@ -61,6 +62,49 @@ struct default_alignment_of_aligned_storage<Len, static constexpr size_t value = Align; }; +//////////////////////////////// +// Library Fundamentals V2 TS // +//////////////////////////////// + +// NOTE: The `is_detected` family of templates here differ from the library +// fundamentals specification in that for library fundamentals, `Op<Args...>` is +// evaluated as soon as the type `is_detected<Op, Args...>` undergoes +// substitution, regardless of whether or not the `::value` is accessed. That +// is inconsistent with all other standard traits and prevents lazy evaluation +// in larger contexts (such as if the `is_detected` check is a trailing argument +// of a `conjunction`. This implementation opts to instead be lazy in the same +// way that the standard traits are (this "defect" of the detection idiom +// specifications has been reported). + +template <class Enabler, template <class...> class Op, class... Args> +struct is_detected_impl { + using type = std::false_type; +}; + +template <template <class...> class Op, class... Args> +struct is_detected_impl<typename VoidTImpl<Op<Args...>>::type, Op, Args...> { + using type = std::true_type; +}; + +template <template <class...> class Op, class... Args> +struct is_detected : is_detected_impl<void, Op, Args...>::type {}; + +template <class Enabler, class To, template <class...> class Op, class... Args> +struct is_detected_convertible_impl { + using type = std::false_type; +}; + +template <class To, template <class...> class Op, class... Args> +struct is_detected_convertible_impl< + typename std::enable_if<std::is_convertible<Op<Args...>, To>::value>::type, + To, Op, Args...> { + using type = std::true_type; +}; + +template <class To, template <class...> class Op, class... Args> +struct is_detected_convertible + : is_detected_convertible_impl<void, To, Op, Args...>::type {}; + } // namespace type_traits_internal // void_t() |