From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <jwakely.gcc@gmail.com>
Received: from mail-wr1-x42d.google.com (mail-wr1-x42d.google.com
 [IPv6:2a00:1450:4864:20::42d])
 by sourceware.org (Postfix) with ESMTPS id 77C1B3858D34
 for <libstdc++@gcc.gnu.org>; Sun, 12 Sep 2021 18:19:24 +0000 (GMT)
DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 77C1B3858D34
Received: by mail-wr1-x42d.google.com with SMTP id u15so4996043wru.6
 for <libstdc++@gcc.gnu.org>; Sun, 12 Sep 2021 11:19:24 -0700 (PDT)
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
 d=1e100.net; s=20210112;
 h=x-gm-message-state:mime-version:references:in-reply-to:from:date
 :message-id:subject:to:cc;
 bh=giIm6bQm43DxCyESQ4cY9h4Vyiw6JvDctmdaTEZZAbs=;
 b=IIoS6PPTqLn6RG6v3HFUC40ZxmEXxAdohKLYVATN3Ai+s01VTCYf99M4vWbSyUGARu
 NI80NJy84EfF7Xz5AIakNh6u4MukT+Klb6S8ougcYgC6neC4tHNvyva/nW8B67VNj+Hj
 KbpSDtbauqNz9VSu18IS3ET77nrD8y8xBZ6Iv2VLTSyAMyHas/HySZH6/xt8VRcyNvCN
 9K9vMdmrcI7p9D5f/bl+23572px1fe22wEpqCKok5CI0PnEh2n/0vufi9z9/ure1y0Tn
 gYpTcYnEpIB+jg4C3Yqr0ddKhl+gShn+EeQ0mi5o8+b8A9BVVDZ+DqHeVO5azUz81VOa
 HTsg==
X-Gm-Message-State: AOAM530jaTz4bJVSX0GqXWpl5voayk2CJIJXPioILa6yH9+7b9ZgX88x
 KtTnV0drkaIdSbkVsIReYcmNx+wNecjuyBavYhY=
X-Google-Smtp-Source: ABdhPJxsCodKYNuZ8rrqUq+jrZNVxm9+YwnJzPH7srOEceQpNR13goyfV6atz6CJfjuQ8ZDMXQNqk6OWWfOhJhdeasg=
X-Received: by 2002:adf:d1b2:: with SMTP id w18mr8863691wrc.6.1631470763492;
 Sun, 12 Sep 2021 11:19:23 -0700 (PDT)
MIME-Version: 1.0
References: <63CE1E9B-77D5-4E17-95FA-554B45C4028D@globus.software>
In-Reply-To: <63CE1E9B-77D5-4E17-95FA-554B45C4028D@globus.software>
From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Sun, 12 Sep 2021 19:19:11 +0100
Message-ID: <CAH6eHdTQFfS8KaE6tCftbJSx1mp7UtPs683bex2kj5ATDeRBfA@mail.gmail.com>
Subject: Re: Forward declaration and std::future
To: Evgen Bodunov <evgen@globus.software>
Cc: "libstdc++" <libstdc++@gcc.gnu.org>
X-Spam-Status: No, score=-1.4 required=5.0 tests=BAYES_00, DKIM_SIGNED,
 DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, HTML_MESSAGE,
 RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS,
 TXREP autolearn=ham autolearn_force=no version=3.4.4
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on
 server2.sourceware.org
Content-Type: text/plain; charset="UTF-8"
X-Content-Filtered-By: Mailman/MimeDel 2.1.29
X-BeenThere: libstdc++@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Libstdc++ mailing list <libstdc++.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/libstdc++>,
 <mailto:libstdc++-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/libstdc++/>
List-Post: <mailto:libstdc++@gcc.gnu.org>
List-Help: <mailto:libstdc++-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/libstdc++>,
 <mailto:libstdc++-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Sun, 12 Sep 2021 18:19:26 -0000

On Sun, 12 Sep 2021, 10:36 Evgen Bodunov, <evgen@globus.software> wrote:

> Hello everyone,
>
> During development of valhalla routing project I stumbled upon compilation
> error on gcc + libstdc++ 11 and newer when std::future constructed from
> forward declared class. Live example is there:
> https://godbolt.org/z/rEx3Eonsz It builds on gcc prior to v11. and on any
> clang with libc++.
>
> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/type_traits: In
> instantiation of 'struct std::is_destructible<test_data>':
> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future:1065:21:
>  required from 'class std::promise<test_data>'
> <source>:8:29:   required from here
> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/type_traits:849:52:
> error: static assertion failed: template argument must be a complete class
> or an unbounded array
>   849 |
>  static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
>       |
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/type_traits:849:52:
> note: 'std::__is_complete_or_unbounded<std::__type_identity<test_data>
> >((std::__type_identity<test_data>{}, std::__type_identity<test_data>()))'
> evaluates to false
> In file included from <source>:1:
> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future: In
> instantiation of 'class std::promise<test_data>':
> <source>:8:29:   required from here
> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future:1065:21:
> error: static assertion failed: result type must be destructible
>  1065 |       static_assert(is_destructible<_Res>{},
>       |                     ^~~~~~~~~~~~~~~~~~~~~~~
> /opt/compiler-explorer/gcc-11.1.0/include/c++/11.1.0/future:1065:21: note:
> 'std::is_destructible<test_data>{}' evaluates to false
> Compiler returned: 1
>
> Is that static assert is really required? May be then it should correctly
> process forward declared types?
>

The standard says your code has undefined behaviour. The assertion is
pointing out your mistake.