[Bug c++/110000] GCC should implement exclude_from_explicit_instantiation

["ldionne.2 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110000

--- Comment #9 from Louis Dionne <ldionne.2 at gmail dot com> ---
(In reply to Andrew Pinski from comment #3)
> I am getting a feeling this attribute is well defined enough.
> 
> Is it really just supposed to block explicit instantiation of templates?
> Is there a decent set of testcases that can be used to match up the
> implementations here? Because I suspect without those it will be implemented
> slightly different.

Is there anything specific you're thinking about that would be insufficiently
defined? It's possible that that's the case, and if so then we can define it
properly and make sure Clang/GCC are aligned on the semantics.

This is quite a painful issue for libc++ on GCC since the current solution is
to use `always_inline`, which has too many downsides. It used to be just an
annoyance, but with the addition of libraries like `std::format`, using
`always_inline` leads to a library that is literally unusable in some cases
(compile times and code size just skyrockets). So yeah, we're much willing to
collaborate in order to make this work.