[Bug c++/106366] New: CTAD fails to prioritize initializer_list when done via Deduction guide and inherited CTORS

[Bug c++/106366] New: CTAD fails to prioritize initializer_list when done via Deduction guide and inherited CTORS

[peter.cpp at sommerlad dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106366

            Bug ID: 106366
           Summary: CTAD fails to prioritize initializer_list when done
                    via Deduction guide and inherited CTORS
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: peter.cpp at sommerlad dot ch
  Target Milestone: ---

The following code fails to compile on trunk, 12.1 and all 11 versions of g++ I
tried and compiles fine with clang and msvc:

#include <vector>
template<typename T>
struct Adapter: std::vector<T>{
    using std::vector<T>::vector;
    // Adapter(std::initializer_list<T> l)
    // :std::vector<T>::vector(l){}
};
template<typename T>
Adapter(std::initializer_list<T>) -> Adapter<T>;

int main(){
    Adapter x{1,2,3};
}

see https://compiler-explorer.com/z/hj9zPPa8d

defining the constructor instead of the (not yet inherited) deduction guide
makes it work.

[Bug c++/106366] CTAD fails to prioritize initializer_list when done via Deduction guide and inherited CTORS

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106366

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
                 CC|                            |ppalka at gcc dot gnu.org
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2022-07-20

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed, reduced:

#include <initializer_list>

template<class T>
struct A { A(...); };

template<class T>
A(std::initializer_list<T>) -> A<T>;

A a{1,2,3};

This seems valid according to [over.match.class.deduct]/4.  In order for the
initializer-list phase of overload resolution to kick in the class template
doesn't necessarily need an initializer-list constructor, it suffices to just
have a guide that looks like one.

[Bug c++/106366] CTAD fails to prioritize initializer_list when done via Deduction guide and inherited CTORS

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106366

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:f77bbc8f86900b21abdec457b4153b30512e192d

commit r13-1805-gf77bbc8f86900b21abdec457b4153b30512e192d
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Jul 22 18:42:02 2022 -0400

    c++: CTAD from initializer list [PR106366]

    During CTAD, we currently perform the first phase of overload resolution
    from [over.match.list] only if the class template has a list constructor.
    But according to [over.match.class.deduct]/4 it should be enough to just
    have a guide that looks like a list constructor (which is a more general
    criterion in light of user-defined guides).

            PR c++/106366

    gcc/cp/ChangeLog:

            * pt.cc (do_class_deduction): Don't consider TYPE_HAS_LIST_CTOR
            when setting try_list_ctor.  Reset args even when try_list_ctor
            is true and there are no list candidates.  Call resolve_args on
            the reset args.  Rename try_list_ctor to try_list_cand.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/class-deduction112.C: New test.

[Bug c++/106366] CTAD fails to prioritize initializer_list when done via Deduction guide and inherited CTORS

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106366

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |13.0

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 13.