[Bug libstdc++/85813] New: make_exception_ptr should support __cxa_init_primary_exception path even with -fno-exceptions

["redbeard0531 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85813

            Bug ID: 85813
           Summary: make_exception_ptr should support
                    __cxa_init_primary_exception path even with
                    -fno-exceptions
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redbeard0531 at gmail dot com
  Target Milestone: ---

It looks like nothing in that path actually needs exception support. This would
allow make_exception_ptr to produce a valid exception_ptr even in TUs that
don't support exceptions. While that may not sound valuable, the exception_ptr
could be handed to TUs that do support exceptions where it can be rethrown (a
misnomer in this case...) and caught.

Also, if my paper http://wg21.link/p1066 is accepted, that would provide full
support for handling the exception without ever throwing it. And if this ticket
is impossible to implement for some reason, I'd love to know that before
Rapperswil ;)

The nullptr return was added in response to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64241, but it looks like that was
done before the __cxa_init_primary_exception path existed. When that was added
in
https://github.com/gcc-mirror/gcc/commit/cce8e59224e18858749a2324bce583bcfd160d6c#diff-e1b5856b8cc940de58c12ef252d63c34R188,
I can't tell if it was put in the #if __cpp_exceptions block for a good reason
or just by default.