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

[Jonathan Wakely]

On 04/03/19 01:26 +0200, Ville Voutilainen wrote:
>On Wed, 6 Feb 2019 at 15:12, Ville Voutilainen
><ville.voutilainen@gmail.com> wrote:
>
>> And, to emphasize, the most important reason for this was to be able
>> to write straightforward
>> code for the special member functions, with the hope that it wouldn't
>> have a negative codegen
>> effect. Our Microsoft friends described the general technique as "has
>> crazy-good codegen",
>> but I have no idea what their starting point was; our starting point
>> probably wasn't bad
>> to begin with.
>
>However, the codegen should be somewhat improved; this patch removes a
>bag of run-time ifs from the implementation.
>
>An amended patch attached. This gets rid of all __erased* stuff,
>including hash, swap, constructors, relops.
>I consider variant to no longer be in the state of sin after this.
>Since this is touching just a C++17 facility with no
>impact elsewhere, we could consider landing it in GCC 9 as a late
>change. Failing that, it certainly seems safe enough
>to put into GCC 9.2.

As it doesn't touch anything pre-C++17 it's OK for trunk now.

Thanks.