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libstdc++: Simplify std::variant construction using variable templates

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libstdc++-v3/ChangeLog:

	* include/std/variant (_Build_FUN::_S_fun): Define fallback case
	as deleted.
	(__accepted_index, _Extra_visit_slot_needed): Replace class
	templates with variable templates.
This commit is contained in:
Jonathan Wakely 2022-06-23 18:41:17 +01:00
parent 6177f60d91
commit da55353e95
1 changed files with 21 additions and 22 deletions
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43
libstdc++-v3/include/std/variant
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@ -764,7 +764,7 @@ namespace __variant
{
// This function means 'using _Build_FUN<I, T, Ti>::_S_fun;' is valid,
// but only static functions will be considered in the call below.
void _S_fun();
void _S_fun() = delete;
};
// "... for which Ti x[] = {std::forward<T>(t)}; is well-formed."
@ -795,29 +795,28 @@ namespace __variant
// The index selected for FUN(std::forward<T>(t)), or variant_npos if none.
template<typename _Tp, typename _Variant, typename = void>
struct __accepted_index
: integral_constant<size_t, variant_npos>
{ };
inline constexpr size_t
__accepted_index = variant_npos;
template<typename _Tp, typename _Variant>
struct __accepted_index<_Tp, _Variant, void_t<_FUN_type<_Tp, _Variant>>>
: _FUN_type<_Tp, _Variant>
{ };
inline constexpr size_t
__accepted_index<_Tp, _Variant, void_t<_FUN_type<_Tp, _Variant>>>
= _FUN_type<_Tp, _Variant>::value;
template <typename _Maybe_variant_cookie, typename _Variant>
struct _Extra_visit_slot_needed
{
template <typename> struct _Variant_never_valueless;
template<typename _Maybe_variant_cookie, typename _Variant,
typename = __remove_cvref_t<_Variant>>
inline constexpr bool
__extra_visit_slot_needed = false;
template <typename... _Types>
struct _Variant_never_valueless<variant<_Types...>>
: bool_constant<__variant::__never_valueless<_Types...>()> {};
template<typename _Var, typename... _Types>
inline constexpr bool
__extra_visit_slot_needed<__variant_cookie, _Var, variant<_Types...>>
= !__variant::__never_valueless<_Types...>();
static constexpr bool value =
(is_same_v<_Maybe_variant_cookie, __variant_cookie>
|| is_same_v<_Maybe_variant_cookie, __variant_idx_cookie>)
&& !_Variant_never_valueless<__remove_cvref_t<_Variant>>::value;
};
template<typename _Var, typename... _Types>
inline constexpr bool
__extra_visit_slot_needed<__variant_idx_cookie, _Var, variant<_Types...>>
= !__variant::__never_valueless<_Types...>();
// Used for storing a multi-dimensional vtable.
template<typename _Tp, size_t... _Dimensions>
@ -874,7 +873,7 @@ namespace __variant
using _Variant = typename _Nth_type<__index, _Variants...>::type;
static constexpr int __do_cookie =
_Extra_visit_slot_needed<_Ret, _Variant>::value ? 1 : 0;
__extra_visit_slot_needed<_Ret, _Variant> ? 1 : 0;
using _Tp = _Ret(*)(_Visitor, _Variants...);
@ -953,7 +952,7 @@ namespace __variant
_S_apply_all_alts(_Array_type& __vtable,
std::index_sequence<__var_indices...>)
{
if constexpr (_Extra_visit_slot_needed<_Result_type, _Next>::value)
if constexpr (__extra_visit_slot_needed<_Result_type, _Next>)
(_S_apply_single_alt<true, __var_indices>(
__vtable._M_arr[__var_indices + 1],
&(__vtable._M_arr[0])), ...);
@ -1372,7 +1371,7 @@ namespace __variant
template<typename _Tp>
static constexpr size_t __accepted_index
= __detail::__variant::__accepted_index<_Tp, variant>::value;
= __detail::__variant::__accepted_index<_Tp, variant>;
template<size_t _Np, typename = enable_if_t<(_Np < sizeof...(_Types))>>
using __to_type = typename _Nth_type<_Np, _Types...>::type;