https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113099
--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #8)
> (In reply to Peter Dimov from comment #7)
> > You don't necessarily need dynamic_cast because facets are always installed
> > and obtained by their exact type, not via a reference to base.
>
> Is that true? std::use_facet<X> will return a reference to a facet with
> X::id but it could actually be something derived from X. e.g. a user can
> install their own facet derived from std::ctype<char>, which overrides some
> members. Code that does std::use_facet<std::ctype<char>>(loc) will get the
> user's facet, but via reference to base.
And conversely, for std::use_facet<Y> the locale might contain a base class of
Y, with the same id as Y::id, but there is no Y present. For example, given the
OP's my_codecvt, the following code must throw std::bad_cast:
std::locale loc;
(void) std::use_facet<my_codecvt>(loc);
There is no my_codecvt present. There is a facet with the same id, but its
dynamic type is std::codecvt<wchar_t, char, mbstate_t> not my_codecvt.
I don't see a way to avoid using dynamic_cast without an ABI change to the
std::locale in libstdc++.
