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From: "davveston at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110958] New: [CWG 2137][accepts-invalid]
 Copy-list-initialization with single element of same class only considers
 converting constructors as viable
Date: Wed, 09 Aug 2023 10:44:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110958

            Bug ID: 110958
           Summary: [CWG 2137][accepts-invalid] Copy-list-initialization
                    with single element of same class only considers
                    converting constructors as viable
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: davveston at gmail dot com
  Target Milestone: ---

There are several (closed) bug reports relating to CWG 2137, particularly
highlighting the still present implementation divergence between e.g. GCC a=
nd
Clang. These mostly relate to whether or not a non-aggregate class initiali=
zed
via single element init-list of same-class resolves to the rules of
[over.match.list] or not. See e.g.
https://gcc.gnu.org/bugzilla//show_bug.cgi?id=3D85577 and its duplicates

I interpret the wording the same way as GCC, such that after CWG 2137
initializer list constructors take precedence over e.g. copy constructors
([over.match.list]/1.1).

However, when falling through to [over.match.list]/1.2, GCC seems to consid=
er
only converting constructors as if following [over.match.ctor]/1, whereas by
[over.match.list]/1.2 explicit constructors are still viable, although lead=
ing
to an ill-formed program if picked as most viable.=20

The following example is accepted by GCC:

struct S {
  S()            =3D default;
  S(S const&)    =3D default; // #1
  explicit S(S&) =3D delete;  // #2
};

S a;=20
S b =3D {a}; // #3

As #3 picks #1 for the copy-list-initialization. As of CWG 2137 I believe it
should be rejected as ill-formed as #2 should be selected instead: ill-form=
ed
not due to deleted definition but due to the last paragraph of
[over.match.list]/1.

(Holds for various GCC versions and C++ language versions).=