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Exclude in-place tags from variant's converting constructor

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Like std::variant, etl::variant should exclude in_place_type_t and
in_place_index_t specializations from its converting constructor. This
prevents implicit conversions from treating an in-place tag as an
ordinary value while preserving dedicated in-place construction.
This commit is contained in:
Thomas Altenbach 2026-10-19 01:38:44 +02:00
parent cba156b44f
commit cff9da11bd
2 changed files with 30 additions and 2 deletions
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26
include/etl/private/variant_variadic.h
View File

@ -312,6 +312,24 @@ namespace etl
template <typename T, typename... TTypes>
using select_alternative_index = decltype(conversion_overloads<T, TTypes...>::select(etl::declval<T>()));
//*************************************************************************
/// Helpers for identifying in-place tags.
//*************************************************************************
template <typename T>
struct is_in_place_tag : etl::false_type
{
};
template <typename T>
struct is_in_place_tag<etl::in_place_type_t<T> > : etl::true_type
{
};
template <size_t Index>
struct is_in_place_tag<etl::in_place_index_t<Index> > : etl::true_type
{
};
//*******************************************
/// Constexpr get by index from variadic_union.
//*******************************************
@ -1075,7 +1093,9 @@ namespace etl
//***************************************************************************
#include "diagnostic_uninitialized_push.h"
template < typename T, bool Trivial_ = Is_Trivially_Destructible_Suite,
etl::enable_if_t<!etl::is_same<etl::remove_cvref_t<T>, variant>::value && !Trivial_, int> = 0,
etl::enable_if_t< !etl::is_same<etl::remove_cvref_t<T>, variant>::value && !Trivial_
&& !private_variant::is_in_place_tag<etl::remove_cvref_t<T> >::value,
int> = 0,
size_t AlternativeIndex = private_variant::select_alternative_index<T, TTypes...>::value,
typename TAlternative = type_from_index<AlternativeIndex>, etl::enable_if_t<etl::is_constructible<TAlternative, T>::value, int> = 0>
ETL_CONSTEXPR14 variant(T&& value)
@ -1085,7 +1105,9 @@ namespace etl
}
template < typename T, bool Trivial_ = Is_Trivially_Destructible_Suite,
etl::enable_if_t<!etl::is_same<etl::remove_cvref_t<T>, variant>::value && Trivial_, int> = 0,
etl::enable_if_t<!etl::is_same<etl::remove_cvref_t<T>, variant>::value && Trivial_
&& !private_variant::is_in_place_tag<etl::remove_cvref_t<T> >::value,
int> = 0,
size_t AlternativeIndex = private_variant::select_alternative_index<T, TTypes...>::value,
typename TAlternative = type_from_index<AlternativeIndex>, etl::enable_if_t<etl::is_constructible<TAlternative, T>::value, int> = 0 >
constexpr variant(T&& value)

6
test/test_variant_variadic.cpp
View File

@ -918,6 +918,12 @@ namespace
CHECK(!(std::is_constructible<etl::variant<int, long>, AmbiguousIntLong>::value));
CHECK(!(std::is_constructible<etl::variant<std::string, int, long>, AmbiguousIntLong>::value));
// The converting constructor should not accept in-place type or index tags.
CHECK((!std::is_constructible<etl::variant<etl::in_place_type_t<int>>, etl::in_place_type_t<int>>::value));
CHECK((!std::is_constructible<etl::variant<std::string, etl::in_place_type_t<int>>, etl::in_place_type_t<int>>::value));
CHECK((!std::is_constructible<etl::variant<etl::in_place_index_t<1U>>, etl::in_place_index_t<1U>>::value));
CHECK((!std::is_constructible<etl::variant<std::string, etl::in_place_index_t<2U>>, etl::in_place_index_t<2U>>::value));
}
//*************************************************************************