[Development] Houston, qint128 has a problem

[Marc Mutz]

Hi,

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 following is /my/ professional opinion, Ivan may chime in himself:

I think we need to mandate that if you want qint128 support, then you 
must compile with gnu++NN, which is actually the default on both GCC and 
Clang. We seem to switch that off (-ansi on).

We cannot fight a broken stdlib. If the compiler says that these things 
don't exist as integral types, then who is Qt to disagree? It slows down 
development, and all those work-arounds slow down compilations, too:

  - std::is_integral_v<T>
  + std::disjunction_v<std::is_integral<T>,
  +                    std::is_same<T, qint128>,
  +                    std::is_same<T, quint128>>

Unless someone can present a solution where <type_traits> and <limits> 
work for q(u)int128 under -ansi — even for users — I would undef 
QT_SUPPORTS_INT128 when __STRICT_ANSI__ is in effect, then drop the 
-ansi from Qt builds to make sure the symbols that contain qint128 are 
present in the library. User projects can then use whichever one they want.

We have a similar issue with qfloat16::NativeFloat. On GCC-13, the 
traits only work (tested: <limits>) when compiling in C++23 mode, yet we 
insist on using the types before, eagerly grabbing _some_ definition 
from wherever we can (C20, platform-specifics, ...).

And yes, that means we'll have BC issues from that, too.

References:
- https://bugreports.qt.io/browse/QTBUG-119901
- 
https://codereview.qt-project.org/c/qt/qtbase/+/478199/comment/bcbe8aca_d5d65018/
- https://codereview.qt-project.org/c/qt/qtbase/+/507383

Opinions? Comments?

Thanks,
Marc

-- 
Marc Mutz <marc.mutz at qt.io>
Principal Software Engineer

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