https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93923
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
You can make the program valid by ensuring you do not try to do the
is_constructible when performing overload resolution for copy initialization of
B:
#include <type_traits>
class A;
template<typename T>
struct B {
template<typename... Args>
static constexpr bool not_copy_constructor_candidate
= sizeof...(Args) != 1 || (std::is_same_v<B, Args> && ...);
template<
class... Args,
std::enable_if_t<not_copy_constructor_candidate<Args...>, int> = 0,
std::enable_if_t<std::is_constructible<T, Args...>::value, int> = 0 >
B(Args && ... args) {}
};
struct A {
A(const B<A>&) {}
};
int main()
{
static_assert(std::is_copy_constructible<A>::value, "");
}
I think GCC is correct to reject the original code.
