https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103755
--- Comment #16 from Jonathan Wakely <redi at gcc dot gnu.org> --- Unfortunately this change causes a regression for libs that were statically linked to libstdc++.a before the PR 91057 fix. Any object which has the buggy std::locale::id::_M_id() code linked into it can get corrupted locale::_Impl::_M_facet arrays, where the facets are at the wrong indices. Before the introduction of __try_use_facet those corrupted _M_facet arrays would result in a failed dynamic_cast and so has_facet would be false and use_facet would throw. With the new code in GCC 13 the static_cast succeeds, but with undefined behaviour. So to avoid a regression from detecting the bug and throwing an exception to crashing with a segfault, I think we need to change __try_use_facet to use dynamic_cast, unfortunately. We will still retain the use of __try_use_facet in std::basic_ios::_M_cache_locale, so we'll still only do three dynamic_casts not six, so that's still a bit better than it was before.
