[Bug c++/110958] New: [CWG 2137][accepts-invalid] Copy-list-initialization with single element of same class only considers converting constructors as viable

["davveston at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110958

            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 and
Clang. These mostly relate to whether or not a non-aggregate class initialized
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=85577 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 consider
only converting constructors as if following [over.match.ctor]/1, whereas by
[over.match.list]/1.2 explicit constructors are still viable, although leading
to an ill-formed program if picked as most viable. 

The following example is accepted by GCC:

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

S a; 
S b = {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-formed
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).