On Wed, 2025-09-10 at 08:06 +0100, Jonathan Wakely wrote:
> On Wed, 10 Sept 2025, 04:49 Xi Ruoyao wrote:
> >
> > On Tue, 2025-09-09 at 23:45 +0200, Arsen Arsenović via Gcc-help wrote:
> > > We claim to be able to build using an ISO C++14 compiler, though (and
> > > presumably a C compiler of similar age), so perhaps we shouldn't use
> > > alloca.
> >
> > The claim is not fully true anyway... for example we perform narrowing
> > conversions from constant, but C++14 does not allow to do so.
>
> Could you give an example in the GCC source where that's the case?
>
> C++14 allows narrowing from constants if the value fits in the
> destination type, and requires a diagnostic if the value doesn't fit.
> GCC conforms, allowing the former case and giving an error for the
> latter:
>
> const int i = 42;
> char c{i}; // not narrowing
> char x{i * i};
>
> narrow.cc:3:10: error: narrowing conversion of ‘1764’ from ‘int’ to
> ‘char’ [-Wnarrowing]
> 3 | char x{i * i};
> | ~~^~~
>
> So I don't think the problem you described can happen.
>
> C++14 does not allow it for a **non-constant**, and GCC warns in that case:
>
> int i = 42; // not const
> char c{i};
>
> arrow.cc:2:8: warning: narrowing conversion of ‘i’ from ‘int’ to
> ‘char’ [-Wnarrowing]
> 2 | char c{i};
> | ^
>
> Maybe you're thinking of cases like this, where a non-constant is narrowed?
>
> That is not allowed by C++14 and we should insert the necessary casts
> (and build state1 with -Werror=narrowing).
Oh sorry, my memory is outdated. Those has been fixed since
https://gcc.gnu.org/r15-3826.
--
Xi Ruoyao <xry111@xxxxxxxxxxx>
