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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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Refactor aggregate_initialize_overload using `is_convertible_withou… 
…t_narrowing_v`
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yaito3014 committed Mar 12, 2026
commit 9ba01d357dc6152332d38a69e544e87057b65f29
46 changes: 20 additions & 26 deletions include/iris/type_traits.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -222,6 +222,25 @@ template<class T>
constexpr bool is_trivially_swappable_v = is_trivially_swappable<T>::value;


// P0870R7: is_convertible_without_narrowing
// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p0870r7.html
template<class From, class To>
struct is_convertible_without_narrowing
: std::false_type
{};

template<class From, class To>
requires
std::is_convertible_v<From, To> &&
requires { std::type_identity_t<To[]>{std::declval<From>()}; }
struct is_convertible_without_narrowing<From, To>
: std::true_type
{};

template<class From, class To>
inline constexpr bool is_convertible_without_narrowing_v = is_convertible_without_narrowing<From, To>::value;


namespace detail {

template<std::size_t I, class Ti>
Expand All @@ -231,19 +250,12 @@ struct aggregate_initialize_tag
using type = Ti;
};

// This version works better than MSVC's, does not break IntelliSense or ReSharper
template<std::size_t I, class Ti>
struct aggregate_initialize_overload
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{
using TiA = Ti[];

// https://eel.is/c++draft/dcl.init.general#14
// https://eel.is/c++draft/dcl.init.list#3.4
// https://eel.is/c++draft/dcl.init.aggr#3

template<class T>
auto operator()(Ti, T&&) -> aggregate_initialize_tag<I, Ti>
requires requires(T&& t) { { TiA{std::forward<T>(t)} }; } // emulate `Ti x[] = {std::forward<T>(t)};`
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 •
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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
}
Expand Down Expand Up @@ -283,24 +295,6 @@ struct aggregate_initialize_resolution<
template<class T, class... Ts>
struct aggregate_initialize_resolution : detail::aggregate_initialize_resolution<void, T, Ts...> {};

// P0870R7: is_convertible_without_narrowing
// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p0870r7.html
template<class From, class To>
struct is_convertible_without_narrowing
: std::false_type
{};

template<class From, class To>
requires
std::is_convertible_v<From, To> &&
requires { std::type_identity_t<To[]>{std::declval<From>()}; }
struct is_convertible_without_narrowing<From, To>
: std::true_type
{};

template<class From, class To>
inline constexpr bool is_convertible_without_narrowing_v = is_convertible_without_narrowing<From, To>::value;

} // iris

#endif