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Add is_convertible_without_narrowing #54

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384edff
Add `is_convertible_without_narrowing`
yaito3014 Mar 11, 2026
37bf953
Add test
yaito3014 Mar 12, 2026
6443dfa
Styling fix
yaito3014 Mar 12, 2026
65e4638
Fix for MSVC
yaito3014 Mar 12, 2026
9ba01d3
Refactor `aggregate_initialize_overload` using `is_convertible_withou…
yaito3014 Mar 12, 2026
0b16dda
Add edge cases
yaito3014 Mar 12, 2026
1a0b140
Add `is_assignable_without_narrowing`
yaito3014 Mar 12, 2026
1a35a62
Add clarification
yaito3014 Mar 12, 2026
8a4f052
Revert `is_assignable_without_narrowing` related change
yaito3014 Mar 12, 2026
b9c6216
Cleanup
yaito3014 Mar 12, 2026
cdec1b1
Refine `is_convertible_without_narrowing`
yaito3014 Mar 12, 2026
daab5d1
MSVC workaround
yaito3014 Mar 12, 2026
1380b7e
Rename "aggregate_initialize" to "no_narrowing"
yaito3014 Mar 13, 2026
66ca64b
Rewrite const-west to east-const
yaito3014 Mar 13, 2026
de99433
Add element-to-array test for `is_convertible_without_narrowing`
yaito3014 Mar 13, 2026
c378972
Revert "Rename "aggregate_initialize" to "no_narrowing""
yaito3014 Mar 13, 2026
80ad201
Rename "aggregate_initialize" to "no_narrowing" v2
yaito3014 Mar 13, 2026
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Revert "Rename "aggregate_initialize" to "no_narrowing"" 
This reverts commit 1380b7e.
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@yaito3014
yaito3014 committed Mar 13, 2026
commit c378972919699dc3ea4ca3bcad2d954902ed2dda
14 changes: 7 additions & 7 deletions include/iris/rvariant/rvariant.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -620,10 +620,10 @@ class rvariant : private detail::rvariant_base_t<Ts...>
(!std::is_same_v<std::remove_cvref_t<T>, rvariant>) &&
(!is_ttp_specialization_of_v<std::remove_cvref_t<T>, std::in_place_type_t>) &&
(!is_ctp_specialization_of_v<std::remove_cvref_t<T>, std::in_place_index_t>) &&
std::is_constructible_v<typename no_narrowing_resolution<T, Ts...>::type, T>
std::is_constructible_v<typename aggregate_initialize_resolution<T, Ts...>::type, T>
constexpr /* not explicit */ rvariant(T&& t)
noexcept(std::is_nothrow_constructible_v<typename no_narrowing_resolution<T, Ts...>::type, T>)
: base_type(std::in_place_index<no_narrowing_resolution<T, Ts...>::index>, std::forward<T>(t))
noexcept(std::is_nothrow_constructible_v<typename aggregate_initialize_resolution<T, Ts...>::type, T>)
: base_type(std::in_place_index<aggregate_initialize_resolution<T, Ts...>::index>, std::forward<T>(t))
{}

IRIS_RVARIANT_ALWAYS_THROWING_UNREACHABLE_BEGIN
Expand All @@ -632,12 +632,12 @@ IRIS_RVARIANT_ALWAYS_THROWING_UNREACHABLE_BEGIN
template<class T>
requires
(!std::is_same_v<std::remove_cvref_t<T>, rvariant>) &&
detail::variant_assignable<typename no_narrowing_resolution<T, Ts...>::type, T>::value
detail::variant_assignable<typename aggregate_initialize_resolution<T, Ts...>::type, T>::value
constexpr rvariant& operator=(T&& t)
noexcept(detail::variant_nothrow_assignable<typename no_narrowing_resolution<T, Ts...>::type, T>::value)
noexcept(detail::variant_nothrow_assignable<typename aggregate_initialize_resolution<T, Ts...>::type, T>::value)
{
using Tj = no_narrowing_resolution<T, Ts...>::type; // either plain type or wrapped with recursive_wrapper
constexpr std::size_t j = no_narrowing_resolution<T, Ts...>::index;
using Tj = aggregate_initialize_resolution<T, Ts...>::type; // either plain type or wrapped with recursive_wrapper
constexpr std::size_t j = aggregate_initialize_resolution<T, Ts...>::index;
static_assert(j != std::variant_npos);

this->raw_visit([this, &t]<std::size_t i, class Ti>(std::in_place_index_t<i>, [[maybe_unused]] Ti& ti)
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23 changes: 11 additions & 12 deletions include/iris/type_traits.hpp
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Expand Up @@ -272,38 +272,37 @@ struct aggregate_initialize_tag
};

template<std::size_t I, class Ti>
struct no_narrowing_overload
struct aggregate_initialize_overload
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{
template<class T>
auto operator()(T&&) -> aggregate_initialize_tag<I, Ti>
auto operator()(Ti, T&&) -> aggregate_initialize_tag<I, Ti>
requires is_convertible_without_narrowing_v<T, Ti>
Comment on lines 277 to +279

@saki7 saki7 Mar 13, 2026

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I think the first parameter Ti can be omitted, as it existed for checking is_convertible by passing-by-value

@yaito3014 yaito3014 Mar 13, 2026 •
edited
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No, the Ti parameter cannot be omitted, since we should consider overload resolution for Ti including implicit conversion sequence's priority.

For example, constructing rvariant<int, long> from int should be resolved to index() == 0 though both int and long are equally convertible from int without narrowing. This is because identity conversion should occur prior to integral promotion in standard conversion.

{
return {}; // silence MSVC warning
}
};

template<class Is, class... Ts>
struct no_narrowing_fun;
struct aggregate_initialize_fun;

// Imaginary function FUN of https://eel.is/c++draft/variant#ctor-14
template<std::size_t... Is, class... Ts>
struct no_narrowing_fun<std::index_sequence<Is...>, Ts...>
: no_narrowing_overload<Is, Ts>...
struct aggregate_initialize_fun<std::index_sequence<Is...>, Ts...>
: aggregate_initialize_overload<Is, Ts>...
{
using no_narrowing_overload<Is, Ts>::operator()...;
using aggregate_initialize_overload<Is, Ts>::operator()...;
};

template<class... Ts>
using aggregate_initialize_fun_for = no_narrowing_fun<std::index_sequence_for<Ts...>, Ts...>;
using aggregate_initialize_fun_for = aggregate_initialize_fun<std::index_sequence_for<Ts...>, Ts...>;

template<class Enabled, class T, class... Ts>
struct no_narrowing_resolution {};
struct aggregate_initialize_resolution {};

template<class T, class... Ts>
struct no_narrowing_resolution<
struct aggregate_initialize_resolution<
std::void_t<decltype(aggregate_initialize_fun_for<Ts...>{}(std::declval<T>(), std::declval<T>()))>, T, Ts...
>
{
> {
using tag = decltype(aggregate_initialize_fun_for<Ts...>{}(std::declval<T>(), std::declval<T>()));
using type = tag::type;
static constexpr std::size_t index = tag::index;
Expand All @@ -315,7 +314,7 @@ struct no_narrowing_resolution<
// because they would lead to unnecessarily nested instantiation for
// legitimate infinite recursion errors on recursive types.
template<class T, class... Ts>
struct no_narrowing_resolution : detail::no_narrowing_resolution<void, T, Ts...> {};
struct aggregate_initialize_resolution : detail::aggregate_initialize_resolution<void, T, Ts...> {};

} // iris

Expand Down