[Bug libstdc++/113099] locale without RTTI uses dynamic_cast before gcc 13.2 or has ODR violation since gcc 13.2

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113099

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to andysem from comment #11)
> > I'm not sure what you mean by "the compiler is free to generate code that takes it into account." Takes what into account? What problem does that freedom cause?
> 
> I mean the compiler could move (some part of) dynamic_cast to precede the
> check for the standard facet. I.e. of something like this:
> 
>   template< typename _Facet >
>   const _Facet* __try_use_facet(locale const& loc)
>   {
>     const size_t __i = _Facet::id._M_id();
>     const locale::facet** __facets = __loc._M_impl->_M_facets;
>     const _Facet* __dyn_facet = __dynamic_cast< const _Facet*
> >(__facets[__i]);
> 
>     // checks for every standard facet type
>     if (__is_same(_Facet, ...))
>       return static_cast<const _Facet*>(__facets[__i]);
> 
>     return __dyn_facet;
>   }

But why? Maybe I'm being slow but I still don't understand what problem is
being solved here.

Also this would defeat the optimization that avoids the unnecessary overhead of
dynamic_cast for standard facets.

> AFAIK, the standard or libstdc++ docs do not require RTTI for std::locale to
> function.

The standard requires RTTI, period. Using -fno-rtti is completely non-standard
and so the standard has nothing to say about it.