[Bug libstdc++/64128] Let vector take advantage of malloc_usable_size for malloc-using allocators

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64128

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Actually, I don't think we can reasonably do anything here without extensions
to the allocator interface. We can't rely on malloc_usable_size if we don't
know if we're using glibc's malloc at run-time.