[Bug c++/100470] std::is_nothrow_move_constructible incorrect behavior for explicitly defaulted members

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100470

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |wrong-code
   Last reconfirmed|                            |2022-09-20

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1286r2.html says that
S" has a potentially-throwing move constructor.

It looks like the __is_nothrow_constructible built-in gets this wrong:

using size_t = decltype(sizeof(0));
void* operator new(size_t, void* p) noexcept { return p; }

namespace std {
template<typename T> T&& declval() noexcept;

template<typename T>
constexpr bool is_nothrow_move_constructible_v
  = noexcept(new (declval<void*>()) T(declval<T>()));
}

struct S1{
    S1(S1&&) noexcept(false);
};
struct S2{
    S2(S2&&) noexcept(false) = default;
};
struct S3{
    S3(S3&&) noexcept(false){}
};
struct S4{
    S4(S4&&) = default;
};

static_assert(!std::is_nothrow_move_constructible_v<S1>);  // OK
static_assert(!std::is_nothrow_move_constructible_v<S2>);  // OK
static_assert(!std::is_nothrow_move_constructible_v<S3>);  // OK
static_assert( std::is_nothrow_move_constructible_v<S4>);  // OK

static_assert(!__is_nothrow_constructible(S1, S1));  // OK
static_assert(!__is_nothrow_constructible(S2, S2));  // failed
static_assert(!__is_nothrow_constructible(S3, S3));  // OK
static_assert( __is_nothrow_constructible(S4, S4));  // OK

This makes it a libstdc++ regression in GCC 11.1 and later, because we use
__is_nothrow_construcitble now, instead of a pure library implementation for
the traits.