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From: Tim Song <t.canens.cpp@gmail.com>
To: Tim Shen <timshen@google.com>
Cc: Jonathan Wakely <jwakely@redhat.com>,
	"libstdc++" <libstdc++@gcc.gnu.org>,
		gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] PR libstdc++/80939 Remove unmeetable constexpr specifiers
Date: Fri, 02 Jun 2017 19:40:00 -0000	[thread overview]
Message-ID: <CAPQZVxv2JEUXDsPag7tw5Tx1iVi7ZTHp2PMjiKymS2fRX22xGg@mail.gmail.com> (raw)
In-Reply-To: <CAG4ZjN=0A_7LDVudhxin6VP0b5mCO1QuXt9No2Kcu+vvkLS4tg@mail.gmail.com>

On Fri, Jun 2, 2017 at 3:19 PM, Tim Shen <timshen@google.com> wrote:
> On Fri, Jun 2, 2017 at 6:07 AM, Jonathan Wakely wrote:
>> As the PR points out, we aren't qualifying calls to __ref_cast, and
>> have 'constexpr' on function templates that can never be usable in
>> constant expressions.
>
> Apology for the constexpr trolling, but that was not intentional. :)
>
> I'm curious why no tests break. Is it because constexpr in a template
> function is a no-op instead of a hard error, when the function
> definition is not constexpr?
>
> The patch looks good.
>

A non-template, non-default constexpr function that can never be used
in a constant expression is ill-formed NDR. ([dcl.constexpr]/5)

A constexpr function template for which no specialization that
satisfies the requirements for a constexpr function (when considered
as a non-template) can be generated is ill-formed NDR.
([dcl.constexpr]/6)

It's not really clear to me whether the second rule incorporates the
first or if it's just talking about the requirements in
[dcl.constexpr]/3, but regardless it's not required to break.

  reply	other threads:[~2017-06-02 19:40 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-06-02 13:07 Jonathan Wakely
2017-06-02 19:19 ` Tim Shen via gcc-patches
2017-06-02 19:40   ` Tim Song [this message]
2017-06-05 16:49   ` Jonathan Wakely
2017-06-11  0:15 ` Tim Song

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