Re: [PATCH 01/14] c++: Implement __is_integral built-in trait

[Jonathan Wakely <jwakely@redhat.com>]

On Wed, 17 Jan 2024 at 00:31, Jason Merrill wrote:
>
> On 1/10/24 04:22, Ken Matsui wrote:
> > +/* Return true if T is an integral type.  With __STRICT_ANSI__, __int128 and
> > +   unsigned __int128 are not integral types.  */
>
> This really needs a rationale, since they are actually integer types.  I
> know __int128 is considered an extension rather than an extended integer
> type under the standard, but is there a writeup we can point to for why?
>
> And even if we don't want to subject it to all the standard requirements
> of an extended integer type, why not still say it's an integral type?
> flag_iso is only supposed to disable features that could conflict with
> obscure but standard-conforming code, and since __int128 is in the
> reserved namespace, I'd think it should be safe to support (to the
> degree that we do) regardless of flag_iso.

The reason for __int128 not being an integral type is because the
standard says that intmax_t must be the largest standard or extended
integer type, and intmax_t is fixed by ABI to be a 64-bit type. As a
result, GCC has historically said that 128-bit integer types are not
part of the "standard or extended integer type"classification, in
Joseph's words they're sui generis types. But C2x and C++23 changed
this, and now we can just do the obvious, simple thing and say that
128-bit integer types are integer types.
This changed with https://cplusplus.github.io/LWG/issue3828 for C++.

So we can remove the dependency on __STRICT_ISO__ for 128-bit integer
types, and implementing std::is_integral with a built-in seems like
the perfect time to do that. But that seems like stage 1 material, as
we need to go through the library and see what needs to change.