https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94033
--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think it doesn't show up for gcc-9 because there are also std::tuple changes
on master that make the bug show up.
And I think what's happening is another instance of PR 41437. When
std::optional<mutation> is instantiated it causes
std::is_constructible<mutation> to be instantiated, but gives the wrong answer
because of PR 41437.
Later when the value of std::is_nothrow_default_constructible<mutation> is
needed, the bad instantiation of is_constructible<mutation> is reused, which
means that the first condition in the __and_ here is true (but shouldn't be)
and so we try the second condition, which is ill-formed:
/// is_nothrow_default_constructible
template<typename _Tp>
struct is_nothrow_default_constructible
: public __and_<is_default_constructible<_Tp>,
__is_nt_default_constructible_impl<_Tp>>
{ };
/home/jwakely/gcc/10/include/c++/10.0.1/type_traits:966:47: error:
'mutation::mutation()' is private within this context
966 | : public integral_constant<bool, noexcept(_Tp())>
| ^~~~~
We should never try to construct _Tp() when it isn't default constructible. The
cached bad value of is_constructible<mutation> makes us try to do something
impossible.
Lending weight to this theory is the fact that explicitly checking
is_constructible_v<mutation> *before* instantiating optional<mutation> makes
the bug go away:
#include <optional>
#include <tuple>
template <bool B> struct abc {};
template <typename T>
struct future : public
abc<std::is_trivially_constructible<std::tuple<T>>::value> {};
class mutation {
mutation();
friend class std::optional<mutation>;
};
#ifdef FIX
static_assert( !std::is_nothrow_default_constructible<mutation>::value );
#endif
using mutation_opt = std::optional<mutation>;
future<mutation_opt> foo();
template <typename Consumer> future<mutation_opt> consume_partitions() {
return foo();
}
future<mutation_opt> bar() { return consume_partitions<int>(); }
future<mutation> zed();
future<mutation> apply_counter_update() { return zed(); }
Define FIX to make the bug vanish.
