[Bug libstdc++/89464] shared_ptr_base.h: error: '__tag' was not declared in this scope (gcc-8.3.0 regression?)

[gcc at nmacleod dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89464

--- Comment #6 from Milhouse <gcc at nmacleod dot com> ---
Andrew, can you tell me what the correct algnas() definition should be, as
everywhere it is used seems to be passing a single argument. This is the usage
in shared_ptr_base.h:

alignas(type_info) static constexpr char __tag[sizeof(type_info)] = { };


Also, if possible can you explain what change in gcc-8.3.0 is triggering this
new behaviour (considering gcc-8.2.0 doesn't complain).

Many thanks in advance!