[Bug c++/113300] New: GCC rejects valid program involving explicit and non explicit default constructors

[Bug c++/113300] New: GCC rejects valid program involving explicit and non explicit default constructors

[jlame646 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113300

            Bug ID: 113300
           Summary: GCC rejects valid program involving explicit and non
                    explicit default constructors
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlame646 at gmail dot com
  Target Milestone: ---

The following valid program is rejected by gcc and clang but accepted by msvc.
https://godbolt.org/z/Y3x8nP11M

```
struct A
{
  explicit A(int = 10);
  A()= default;
};

A a = {}; //msvc ok but gcc and clang fails here

```
GCC says:

```
<source>:8:8: error: conversion from '<brace-enclosed initializer list>' to 'A'
is ambiguous
    8 | A a = {}; //msvc ok but gcc and clang fails here
      |        ^
<source>:5:3: note: candidate: 'constexpr A::A()'
    5 |   A()= default;
      |   ^
<source>:4:12: note: candidate: 'A::A(int)'
    4 |   explicit A(int = 10);
```
The reason why this is valid is discussed here:
https://stackoverflow.com/questions/77788861/ambiguous-constructor-error-in-gcc-but-not-in-msvc

[Bug c++/113300] GCC rejects valid program involving explicit and non explicit default constructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113300

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There is a C++ defect report in this area even ...

[Bug c++/113300] GCC rejects valid program involving explicit and non explicit default constructors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113300

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|GCC rejects valid program   |GCC rejects valid program
                   |involving copy list         |involving explicit and non
                   |initialization A a = {} of  |explicit default
                   |a class with explicit and   |constructors
                   |non explicit default        |
                   |constructors                |
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=84849,
                   |                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=60027

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://wg21.link/cwg1228

[Bug c++/113300] GCC rejects valid program involving copy list initialization A a = {} of a class with explicit and non explicit default constructors

[jlame646 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113300

Jason Liam <jlame646 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|GCC rejects valid program   |GCC rejects valid program
                   |involving explicit and non  |involving copy list
                   |explicit default            |initialization A a = {} of
                   |constructors                |a class with explicit and
                   |                            |non explicit default
                   |                            |constructors

--- Comment #3 from Jason Liam <jlame646 at gmail dot com> ---
(In reply to Andrew Pinski from comment #1)
> There is a C++ defect report in this area even ...

Can you post a link to the DR that you're referencing? Or are you referring to
this https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0398r0.html

[Bug c++/113300] GCC rejects valid program involving copy list initialization A a = {} of a class with explicit and non explicit default constructors

[jlame646 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113300

--- Comment #4 from Jason Liam <jlame646 at gmail dot com> ---
(In reply to Andrew Pinski from comment #1)
> There is a C++ defect report in this area even ...

Looks like that gcc and clang are non-compliant here as per this discussion in
CWG issue list: https://github.com/cplusplus/CWG/issues/486 

Also, the cwg 1228 is categorised as NAD. https://wg21.link/cwg1228 so
this(that the program should compile) looks intentional.

[Bug c++/113300] GCC rejects valid program involving copy list initialization A a = {} of a class with explicit and non explicit default constructors

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113300

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-01-10
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

[Bug c++/113300] GCC rejects valid program involving copy list initialization A a = {} of a class with explicit and non explicit default constructors

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113300

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Not a regression so I think it has to wait until GCC 15.  I'd like to take a
look then.

I've updated https://gcc.gnu.org/projects/cxx-dr-status.html

[Bug c++/113300] GCC rejects valid program involving copy list initialization A a = {} of a class with explicit and non explicit default constructors

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113300

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
This is CWG 2856 https://cplusplus.github.io/CWG/issues/2856.html