[Bug c++/102609] [C++23] P0847R7 - Deducing this

[waffl3x at protonmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102609

--- Comment #15 from waffl3x <waffl3x at protonmail dot com> ---
Created attachment 55793
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55793&action=edit
inital support for P0847 explicit-object-parameter

Alright, I finalized something that I hope is worthy of criticism. I haven't
ran tests on it yet but I think it should be relatively stable. My first time
around I made the mistake of having hard failing TREE_CHECKs in if conditions,
and I'm pretty sure that was causing problems with the tests (I saw a lot of
segfaults), but I'm fairly sure it should be good this time (not that I shared
the first one).

I will probably start tests first thing tomorrow, hopefully I can figure out
how to make it take less than 4-8 hours.

I'm pretty happy with where I put most things, I didn't add anything to the
core tree nodes, I instead used tree_decl_common::decl_flag_3 for PARM_DECL,
but I added a member to lang_decl_base, hopefully this is satisfactory.

As for what I know works right now, the below program outputs 15, 25, 35, 45 as
you would expect. I haven't tried lambda's but I am sure they don't work. I
have not tried anything with inheritance, I wouldn't bet on it working but I
wouldn't bet against it. I have not tried implicit conversions, but I have a
feeling they probably work. I was planning on implementing rejection of
qualifiers on xobj member functions but I forgot, so that will come tomorrow. I
also have to implement errors when trying to declare a xobj parameter in a
function type. I haven't tried taking the address of an xobj member function,
but I have a hunch it will work, they are almost entirely treated as static
member functions at the moment. Speaking of that, the function declaration gets
pretty printed as a static function at the moment too.

So as you can see, there's still lots to do, but it shouldn't be as hard now
that I am more familiar with the code base.

Something I'm not especially happy with is how the error checking is strewn
around. I tried to put it where it is most relevant, but prioritized putting it
where I could get the best diagnostics, but I'm probably missing some things I
could be doing to better group it together. I am also not happy with the
quality of all of the diagnostics, I want to improve on that as I learn the
quirks of the utilities.

Please criticize, I am certain I am still doing some stuff wrong, so I would
appreciate any input so I can correct those mistakes.

Here is the aforementioned program that I know to work on my system. Surely
nothing can go wrong, but who knows.

#include <cstdio>
struct S {
    int _a;
    int my_func(this S& s) {
        return s._a + 5;
    }
    int my_func(this S const& s) {
        return s._a + 15;
    }
    int my_func(this S&& s) {
        return s._a + 25;
    }
    int my_func(this S const&& s) {
        return s._a + 35;
    }
    template<typename Self>
    int my_func_dispatch(this Self&& self) {
        return static_cast<Self&&>(self).my_func();
    }
};

int main()
{
    S s{10};

    printf("%d\n", s.my_func_dispatch());
    printf("%d\n", static_cast<S const&>(s).my_func_dispatch());
    printf("%d\n", static_cast<S&&>(s).my_func_dispatch());
    printf("%d\n", static_cast<S const&&>(s).my_func_dispatch());
}