https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128657
Bug ID: 128657
Summary: std::is_swappable no longer reflects actual
swappability
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: ykakeyama3014 at gmail dot com
Target Milestone: ---
Since r16-2273-g178dc3367643 "Constrain std::swap using concepts in C++20", the
std::swap templates in bits/move.h use requires-clauses in C++20, but
<type_traits> still declares them with enable_if return types. This makes
std::is_swappable disagree with whether swap can actually be called.
Repro code is as follows:
#include <concepts>
#include <type_traits>
#include <utility>
namespace ns {
struct S {};
template<std::same_as<S> T>
void swap(T&, T&)
{
}
} // namespace ns
static_assert(std::is_swappable_v<ns::S>); // pass
void func(ns::S& a, ns::S& b)
{
using std::swap;
swap(a, b); // error
}
With -std=c++20, the static_assert passes but the swap call is ambiguous. GCC
15 accepts both.
