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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/113099] locale without RTTI uses dynamic_cast before
 gcc 13.2 or has ODR violation since gcc 13.2
Date: Sun, 24 Dec 2023 15:30:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113099

--- Comment #9 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 insta=
lled
> > and obtained by their exact type, not via a reference to base.
>=20
> 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 so=
me
> members. Code that does std::use_facet<std::ctype<char>>(loc) will get the
> user's facet, but via reference to base.

And I guess if the user's derived facet uses virtual inheritance from
std::locale::facet, then this could break with the
static_cast<std::ctype<char>*> in libstdc++ today. Hmm.=