From: Jason Merrill <jason@redhat.com>
To: Marek Polacek <polacek@redhat.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: C++ PATCH to fix rejects-valid with constexpr ctor in C++17 (PR c++/83692)
Date: Fri, 02 Feb 2018 19:11:00 -0000 [thread overview]
Message-ID: <CADzB+2mRAk9djHzTyErtmFKf4QhhDwTf8_HdH4AWpj7BeRC-mA@mail.gmail.com> (raw)
In-Reply-To: <20180125211639.GA2620@redhat.com>
On Thu, Jan 25, 2018 at 4:16 PM, Marek Polacek <polacek@redhat.com> wrote:
> This is a similar problem to 83116: we'd cached a constexpr call, but after a
> store the result had become invalid, yet we used the wrong result again when
> encountering the same call later. This resulted in evaluating a THROW_EXPR
> which doesn't work. Details in
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83692#c5
>
> The fix for 83116 didn't work here, because when evaluating the body of the
> ctor via store_init_value -> cxx_constant_value we are in STRICT, so we do
> cache.
> It seems that we may no longer rely on the constexpr call table when we
> do cxx_eval_store_expression, because that just rewrites *valp, i.e. the
> value of an object. Might be too big a hammer again, but I couldn't think
> of how I could guard the caching of a constexpr call.
> This doesn't manifest in C++14 because build_special_member_call in C++17 is
> more aggressive with copy elisions (as required by P0135 which changed how we
> view prvalues). In C++14 build_special_member_call produces a CALL_EXPR, so
> expand_default_init calls maybe_constant_init, for which STRICT is false, so
> we avoid caching as per 83116.
So it sounds like the problem is using cxx_constant_value for the
diagnostic when it has different semantics from the
maybe_constant_init that follows right after. I guess we want a
cxx_constant_init function that is a hybrid of the two.
Jason
next prev parent reply other threads:[~2018-02-02 19:11 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-25 21:58 Marek Polacek
2018-01-25 22:37 ` Marek Polacek
2018-02-02 19:11 ` Jason Merrill [this message]
2018-02-05 13:38 ` Marek Polacek
2018-02-05 18:45 ` Jason Merrill
2018-02-16 21:10 ` Jason Merrill
2018-02-23 14:30 ` Marek Polacek
2018-02-24 1:55 ` Jason Merrill
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