[Bug c++/95609] New: span<T> could have better layout

["s_gccbugzilla at nedprod dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95609

            Bug ID: 95609
           Summary: span<T> could have better layout
           Product: gcc
           Version: 10.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: s_gccbugzilla at nedprod dot com
  Target Milestone: ---

I would assume that the ABI ship has sailed, as usual, but if libstdc++'s
span<T> could instead have the layout:

{
  T *p;
  size_t l;
}

... then a span<byte> would be layout-compatible with struct iovec, which would
save libstdc++ having to reimplement span<> with a struct iovec compatible
layout for any future std::file_handle::buffer_type, if that gets standardised.

I put notice of this out onto lib-ext last year requesting this of span<T>
implementations. libc++ heeded my request, MSVC have very kindly undone their
initial implementation to meet the pointer + size layout plus add standard
layout, so they'll be on that too going forth.

In the end, this request is about as unimportant for right now as you can get.
It's purely for saving you, Jonathan, avoidable work later on.

Niall