https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95606
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The testcase is:
#include <any>
#include <string>
template<typename>
class basic_json;
using json = basic_json<std::string>;
class json_ref
{
public:
template <typename T,
bool = std::is_constructible<json, T>::value>
json_ref(T &&){}
};
template<typename>
class basic_json
{
public:
basic_json(json_ref) {}
};
namespace std {
template<>
void swap<json>(json&, json&) noexcept {}
}
int main() {}
This fails when compiled with -std=gnu++17 or later.
Specializing std::swap like this is wrong (and in C++20 explicitly results in
undefined behaviour). Replacing it with a normal overload in the same namespace
as the type avoids the error.
Do you have an example that doesn't involve a bogus specialization in namespace
std?
