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[libc++] Implement P0608R3 - A sane variant converting constructor

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Summary:
Prefer user-defined conversions over narrowing conversions and conversions to bool.

References:
 http://wg21.link/p0608

Reviewers: EricWF, mpark, mclow.lists

Reviewed By: mclow.lists

Subscribers: zoecarver, ldionne, libcxx-commits, cfe-commits, christof

Differential Revision: https://reviews.llvm.org/D44865

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@363692 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Zhihao Yuan
2019-06-18 15:26:50 +00:00
parent 8ea6252cec
commit f8f62ac93c
6 changed files with 238 additions and 6 deletions
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32
include/variant
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@@ -1098,11 +1098,39 @@ struct __overload<> { void operator()() const; };
template <class _Tp, class... _Types>
struct __overload<_Tp, _Types...> : __overload<_Types...> {
using __overload<_Types...>::operator();
__identity<_Tp> operator()(_Tp) const;
static auto __test(_Tp (&&)[1]) -> __identity<_Tp>;
template <class _Up>
auto operator()(_Tp, _Up&& __t) const
-> decltype(__test({ _VSTD::forward<_Up>(__t) }));
};
template <class _Base, class _Tp>
struct __overload_bool : _Base {
using _Base::operator();
template <class _Up, class _Ap = __uncvref_t<_Up>>
auto operator()(bool, _Up&&) const
-> enable_if_t<is_same_v<_Ap, bool>, __identity<_Tp>>;
};
template <class... _Types>
struct __overload<bool, _Types...>
: __overload_bool<__overload<_Types...>, bool> {};
template <class... _Types>
struct __overload<bool const, _Types...>
: __overload_bool<__overload<_Types...>, bool const> {};
template <class... _Types>
struct __overload<bool volatile, _Types...>
: __overload_bool<__overload<_Types...>, bool volatile> {};
template <class... _Types>
struct __overload<bool const volatile, _Types...>
: __overload_bool<__overload<_Types...>, bool const volatile> {};
template <class _Tp, class... _Types>
using __best_match_t = typename result_of_t<__overload<_Types...>(_Tp&&)>::type;
using __best_match_t =
typename invoke_result_t<__overload<_Types...>, _Tp, _Tp>::type;
} // __variant_detail