[Development] Houston, qint128 has a problem

[Ville Voutilainen]

On Sat, 9 Dec 2023 at 18:16, Thiago Macieira <thiago.macieira at intel.com> wrote:
>
> On Friday, 8 December 2023 18:51:19 PST Marc Mutz via Development wrote:
> > After spending countless hours between Ivan and myself fighting GCC's
> > mysteriously-vanishing <type_traits> support for __int128_t (= qint128),
> > it turns out that with -ansi/-std=c++NN, not even the most basic of
> > <type_traits> work: std::is_signed_v<qint128> is _false_! I repeat: _FALSE_.
>
> The issue here is that GCC devs' reading of the standard, as worded, was that
> they weren't allowed to make an extended integer type return true for those
> traits because they weren't in the list of types that the standard said it
> should. Specifically, the question was whether std::is_integral is true.
>
> I thought C++23 had clarified that they can, indeed, be integral. But I can't
> find the change and I can't reproduce it. In fact, it the code seems to still
> be there, so I must be mis-remembering the conclusion. libstdc++ reacts to the
> compiler defining __GLIBCXX_TYPE_INT_N_0 to say this is a supported type That
> macro is defined in c-cppbuiltin.cc[1], which has a !flag_iso check.
>
> It's a little difficult to do more investigation right now because Qt Creator
> keeps crashing as soon as I open type_traits[2]. There's something wrong with
> the build I made yesterday.

It all boils down to intmax_t. Extended int types couldn't be larger
than that and be integers, but that restriction was lifted
in C, via https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2889.htm,
and C++ inherits that fix.