From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id C0CEB3854833; Sun,  8 Nov 2020 16:26:38 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org C0CEB3854833
From: "egor_suvorov at mail dot ru" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/97755] New: Explicit default constructor is called during
 copy-list-initialization with a warning only
Date: Sun, 08 Nov 2020 16:26:38 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 10.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: egor_suvorov at mail dot ru
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-97755-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Sun, 08 Nov 2020 16:26:38 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97755

            Bug ID: 97755
           Summary: Explicit default constructor is called during
                    copy-list-initialization with a warning only
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: egor_suvorov at mail dot ru
  Target Milestone: ---

Consider the following test case:
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dblob;f=3Dgcc/testsuite/g%2B%2B.dg/=
cpp0x/initlist40.C

// PR c++/54835, DR 1518
// { dg-do compile { target c++11 } }
struct A
{
  explicit A(int =3D 42);
};
int main()
{
  A a1 =3D { };                   // { dg-error "explicit" }
  A a2 =3D { 24 };                // { dg-error "explicit" }
}

GCC fails to compile it, but the line with 'a1' emits only a warning:
"converting to 'A' from initializer list would use explicit constructor
'A::A(int)'". Hence, if I comment out the line with 'a2', compilation succe=
eds.

However, if I modify the test case slightly:

struct A
{
  explicit A();
  explicit A(int);
};
int main()
{
  A a1 =3D { };                   // { dg-error "explicit" }
  A a2 =3D { 24 };                // { dg-error "explicit" }
}

Both messages become errors.

I believe it's a regression between GCC 5 (correctly fails both test cases)=
 and
GCC 6 (emits warning instead of error): https://godbolt.org/z/1o81h1

Looks like the change was brought by this commit:
https://gcc.gnu.org/git/?p=3Dgcc.git&a=3Dcommit;h=3De7838ec9d2ea06e844ef236=
60862781b81a26329
from this bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D54835

I'm suspicious that the code says "When converting from an init list we
consider explicit constructors, but actually trying to call one is an error=
.",
but then proceeds to call `pedwarn` instead of `error` in some cases.=