[Bug libstdc++/94033] [10 Regression] is_trivially_copy_constructible<> fails with compiler error on complicated object with private default constructor

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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.