[Bug libstdc++/69388] New: Allow functexcept.cc definitions to be replaced

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69388

            Bug ID: 69388
           Summary: Allow functexcept.cc definitions to be replaced
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Choosing the non-throwing versions of the __throw_* definitions in
src/c++11/functexcept.cc requires rebuilding libstdc++ with -fno-exceptions,
which is not usually practical.

It would be helpful to allow them to be replaced by the application, so that a
custom libstdc++ isn't needed. Maybe they could be declared weak, and we could
provide an extension header for users to include (or an object file to link to)
that has alternative definitions.

This would mean that even exceptions thrown from inside the compiled library
code (rather than just inline/template code in headers) could be transformed
into abort calls per-application without using a rebuilt libstdc++.so

The Taller Technologies guys are looking into using different multlib builds
for -fexceptions / -fno-exceptions, this might make that unnecesary (at least
for targets that support symbol interposition and/or weak symbols).