Section: 18.4.9 [concept.swappable] Status: New Submitter: Jiang An Opened: 2024-01-20 Last modified: 2024-01-21 07:32:54 UTC
Priority: Not Prioritized
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Discussion:
18.4.9 [concept.swappable] bullet (2.3.1) currently requires T to be a literal type in order to make the swapping expression a constant expression in that case. The requirement was likely automatically enforced by the core language rules in C++20 and thus essentially redundant.
However, as P2448R2 relaxed the restrictions on constexpr functions, it seems that the swapping expression can be a constant expression even if T is not a literal type. E.g. the following program is accepted by GCC/libstdc++ in C++23 mode (demo).
#include <concepts>
struct NonLiteral {
NonLiteral() {} // non-constexpr
constexpr NonLiteral(const NonLiteral&) noexcept {};
constexpr NonLiteral& operator=(const NonLiteral&) noexcept { return *this; };
};
int main()
{
NonLiteral x;
static_assert((std::ranges::swap(x, x), true));
}
IMO there's no good reason to additionally require literal types since C++23, which would complicate implementations.
Proposed resolution:
This wording is relative to N4971.
Modify 18.4.9 [concept.swappable] as indicated:
-2- The name ranges::swap denotes a customization point object (16.3.3.3.5 [customization.point.object]). The expression ranges::swap(E1, E2) for subexpressions E1 and E2 is expression-equivalent to an expression S determined as follows:
(2.1) — […]
(2.2) — […]
(2.3) — Otherwise, if E1 and E2 are lvalues of the same type T that models move_constructible<T> and assignable_from<T&, T>, S is an expression that exchanges the denoted values. S is a constant expression if
(2.3.1) — T is a literal type (6.8.1 [basic.types.general]),(2.3.2) — both E1 = std::move(E2) and E2 = std::move(E1) are constant subexpressions (3.14 [defns.const.subexpr]), and
(2.3.3) — the full-expressions of the initializers in the declarations
T t1(std::move(E1)); T t2(std::move(E2));are constant subexpressions.
(2.4) — Otherwise, ranges::swap(E1, E2) is ill-formed.
noexcept(S) is equal to is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>.
(2.4) — Otherwise, ranges::swap(E1, E2) is ill-formed.
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