Re: C++ constexpr woes while updating firefox

On 5/11/26 11:36, Christoph Moench-Tegeder wrote:
> Hi,
> 
> I'm stumbling on a problem (propably) related to C++ constexpr while
> updating firefox (current beta).
> 
> The frame of reference here is FreeBSD 14.4-STABLE amd64, with llvm19
> from ports (default) (but switching to llvm21 makes no difference).
> 
> The relevant build failure seems start with this:
> 
> In file included from /wrkdirs/usr/ports/www/firefox/work/firefox-151.0/js/src/builtin/intl/SharedIntlData.h:21:
> /wrkdirs/usr/ports/www/firefox/work/firefox-151.0/js/src/util/LanguageId.h:234:66: error: constexpr function's return type 'mozilla::Maybe<std::pair<LanguageId, size_t>>' (aka 'Maybe<pair<js::LanguageId, unsigned long>>') is not a literal type
>   234 |   static constexpr mozilla::Maybe<std::pair<LanguageId, size_t>> from(
>       |                                                                  ^
> /wrkdirs/usr/ports/www/firefox/work/.build/dist/include/mozilla/Maybe.h:367:7: note: 'Maybe<std::pair<js::LanguageId, unsigned long>>' is not literal because it has base class 'detail::MaybeStorage<std::pair<js::LanguageId, unsigned long>>' of non-literal type
>   367 |     : private detail::MaybeStorage<T>,
>       |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I have vague  memories of FreeBSD having an ABI issue for backwards
compatibility that affects/limits some types of usage of pair and such
in modern C++: what is allowed vs. how the language standards progressed
--related to not updating the ABI.

But it could take notable time to track down whatever the actual issue
was vs. my vague association. It all could be irrelevant here.

> 
> The code in question is here (the build systems does some shuffling):
> - Maybe.h: https://github.com/mozilla-firefox/firefox/blob/beta/mfbt/Maybe.h
>   Especially detail::MaybeStorage is defined starting here:
>   https://github.com/mozilla-firefox/firefox/blob/beta/mfbt/Maybe.h#L238
> - LanguageId.h: https://github.com/mozilla-firefox/firefox/blob/beta/js/src/util/LanguageId.h
> 
> As far as I can see, the whole class setup is independent of OS and
> compiler - that is, the other (major) platforms have no problem compiling
> this.
> I feel like I'm missing some major thing here - I've exhausted my sparse
> C++ knowledge and I'm now officially out of my depths.
> 
> Has anyone a hint how to proceed here?
> I can provide full logs, but the output is about 15MB and rather
> uneventful (I believe). I'll ask the Mozilla people in parallel; but this
> seems to be rather specific...
> 
> Regards,
> Christoph
> 


-- 
===
Mark Millard
marklmi at yahoo.com