[Bug c++/86439] CTAD with deleted copy constructor fails due to deduction-guide taking by value

[Bug c++/86439] CTAD with deleted copy constructor fails due to deduction-guide taking by value

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86439

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |ppalka at gcc dot gnu.org

[Bug c++/86439] CTAD with deleted copy constructor fails due to deduction-guide taking by value

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86439

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Another side effect of this is that GCC incorrectly rejects the following use
of CTAD + braced-init-lists:

struct B { };
struct C { };

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

template<class T>
A(T, C) -> A<T>;

A a(0, {});

The problem is that building the call to the deduction guide has the side
effect of changing the type of the {} to C, which causes later overload
resolution to fail because C isn't convertible to B.

[Bug c++/86439] CTAD with deleted copy constructor fails due to deduction-guide taking by value

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86439

--- 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:3eecc1db4c691a87ef4a229d059aa863066d9a1c

commit r12-1744-g3eecc1db4c691a87ef4a229d059aa863066d9a1c
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jun 23 08:24:34 2021 -0400

    c++: CTAD and deduction guide selection [PR86439]

    During CTAD, we select the best viable deduction guide using
    build_new_function_call, which performs overload resolution on the set
    of candidate guides and then forms a call to the guide.  As the PR
    points out, this latter step is unnecessary and occasionally incorrect
    since a call to the selected guide may be ill-formed, or forming the
    call may have side effects such as prematurely deducing the type of a {}.

    So this patch introduces a specialized subroutine based on
    build_new_function_call that stops short of building a call to the
    selected function, and makes do_class_deduction use this subroutine
    instead.  And since a call is no longer built, do_class_deduction
    doesn't need to set tf_decltype or cp_unevaluated_operand anymore.

    This change causes us to reject some container CTAD examples in the
    libstdc++ testsuite due to deduction failure for {}, which AFAICT is the
    correct behavior.  Previously in e.g. the first removed example

      std::map{{std::pair{1, 2.0}, {2, 3.0}, {3, 4.0}}, {}},

    the type of the {} would get deduced to less<int> as a side effect of
    forming a call to the chosen guide

      template<typename _Key, typename _Tp, typename _Compare = less<_Key>,
               typename _Allocator = allocator<pair<const _Key, _Tp>>>
          map(initializer_list<pair<_Key, _Tp>>,
              _Compare = _Compare(), _Allocator = _Allocator())
          -> map<_Key, _Tp, _Compare, _Allocator>;

    which made later overload resolution for the constructor call
    unambiguous.  Now, the type of the {} remains undeduced until
    constructor overload resolution, and we complain about ambiguity
    for the two equally good constructor candidates

      map(initializer_list<value_type>,
          const _Compare& = _Compare(),
          const allocator_type& = allocator_type())

      map(initializer_list<value_type>, const allocator_type&).

    This patch fixes these problematic container CTAD examples by giving
    the {} an appropriate concrete type.  Two of these adjusted CTAD
    examples (one for std::set and one for std::multiset) end up triggering
    an unrelated CTAD bug on trunk, PR101174, so these two adjusted examples
    are commented out for now.

            PR c++/86439

    gcc/cp/ChangeLog:

            * call.c (print_error_for_call_failure): Constify 'args' parameter.
            (perform_dguide_overload_resolution): Define.
            * cp-tree.h: (perform_dguide_overload_resolution): Declare.
            * pt.c (do_class_deduction): Use perform_dguide_overload_resolution
            instead of build_new_function_call.  Don't use tf_decltype or
            set cp_unevaluated_operand.  Remove unnecessary NULL_TREE tests.

    libstdc++-v3/ChangeLog:

            * testsuite/23_containers/map/cons/deduction.cc: Replace ambiguous
            CTAD examples.
            * testsuite/23_containers/multimap/cons/deduction.cc: Likewise.
            * testsuite/23_containers/multiset/cons/deduction.cc: Likewise.
            Mention one of the replaced examples is broken due to PR101174.
            * testsuite/23_containers/set/cons/deduction.cc: Likewise.
            * testsuite/23_containers/unordered_map/cons/deduction.cc: Replace
            ambiguous CTAD examples.
            * testsuite/23_containers/unordered_multimap/cons/deduction.cc:
            Likewise.
            * testsuite/23_containers/unordered_multiset/cons/deduction.cc:
            Likewise.
            * testsuite/23_containers/unordered_set/cons/deduction.cc:
Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/class-deduction88.C: New test.
            * g++.dg/cpp1z/class-deduction89.C: New test.
            * g++.dg/cpp1z/class-deduction90.C: New test.

[Bug c++/86439] CTAD with deleted copy constructor fails due to deduction-guide taking by value

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86439

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |toe-ger at web dot de

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
*** Bug 88026 has been marked as a duplicate of this bug. ***

[Bug c++/86439] CTAD with deleted copy constructor fails due to deduction-guide taking by value

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86439

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 12