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).
