[v3 PATCH, RFC] Rewrite variant. Also PR libstdc++/85517

[Ville Voutilainen]

On Mon, 4 Mar 2019 at 01:43, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:
>
> On Mon, 4 Mar 2019 at 01:26, Ville Voutilainen
> <ville.voutilainen@gmail.com> wrote:
> > I consider variant to no longer be in the state of sin after this.
>
> Sigh, except for the places where it self-destructs or placement-news
> over things that it shouldn't. That's hopefully
> the next bit that I'll rectify, Real Soon Now.

Here we go. Variant, go forth and sin no more. Tested locally on
linux-x64, will run the full suite
on linux-ppc64. OK for trunk if tests pass?

2019-03-05  Ville Voutilainen  <ville.voutilainen@gmail.com>

    Rewrite variant.
    Also PR libstdc++/85517
    * include/std/variant (__do_visit): New.
    (__variant_cast): Likewise.
    (__variant_cookie): Likewise.
    (__erased_*): Remove.
    (_Variant_storage::_S_vtable): Likewise.
    (_Variant_storage::__M_reset_impl): Adjust to use __do_visit.
    (_Variant_storage::__M_reset): Adjust.
    (__variant_construct): New.
    (_Copy_ctor_base(const _Copy_ctor_base&)): Adjust to use
    __variant_construct.
    (_Move_ctor_base(_Move_ctor_base&&)): Likewise.
    (_Move_ctor_base::__M_destructive_copy): New.
    (_Move_ctor_base::__M_destructive_move): Adjust to use
    __variant_construct.
    (_Copy_assign_base::operator=): Adjust to use __do_visit.
    (_Copy_assign_alias): Adjust to check both copy assignment
    and copy construction for triviality.
    (_Move_assign_base::operator=): Adjust to use __do_visit.
    (_Multi_array): Add support for visitors that accept and return
    a __variant_cookie.
    (__gen_vtable_impl::_S_apply_all_alts): Likewise.
    (__gen_vtable_impl::_S_apply_single_alt): Likewise.
    (__gen_vtable_impl::__element_by_index_or_cookie): New. Generate
    a __variant_cookie temporary for a variant that is valueless and..
    (__gen_vtable_impl::__visit_invoke): ..adjust here.
    (__gen_vtable::_Array_type): Conditionally make space for
    the __variant_cookie visitor case.
    (__variant_construct_by_index): New.
    (get_if): Adjust to use std::addressof.
    (relops): Adjust to use __do_visit.
    (variant): Add __variant_cast and __variant_construct_by_index
    as friends.
    (variant::emplace): Use _M_reset() and __variant_construct_by_index
    instead of self-destruction.
    (variant::swap): Adjust to use __do_visit.
    (visit): Reimplement in terms of __do_visit.
    (__variant_hash_call_base_impl::operator()): Adjust to use __do_visit.
    * testsuite/20_util/variant/compile.cc: Adjust.
    * testsuite/20_util/variant/run.cc: Likewise.
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