[Bug c++/103049] New: [C++23] P0849R8 - auto(x): decay-copy in the language

[Bug c++/103049] New: [C++23] P0849R8 - auto(x): decay-copy in the language

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103049

            Bug ID: 103049
           Summary: [C++23] P0849R8 - auto(x): decay-copy in the language
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

See https://wg21.link/p849r8

[Bug c++/103049] [C++23] P0849R8 - auto(x): decay-copy in the language

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103049

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-11-02

[Bug c++/103049] [C++23] P0849R8 - auto(x): decay-copy in the language

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103049

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

[Bug c++/103049] [C++23] P0849R8 - auto(x): decay-copy in the language

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103049

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Patch on review.

[Bug c++/103049] [C++23] P0849R8 - auto(x): decay-copy in the language

[webrown.cpp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103049

--- Comment #2 from W E Brown <webrown.cpp at gmail dot com> ---
(In reply to Marek Polacek from comment #1)
> Patch on review.

In the proposed patch, I respectfully recommend a slight rewording of the new
pedwarn messages in gcc/cp/semantics.c and gcc/cp/typeck2.c:

  either s/only available with/available only with/
  or     s/only available with/unavailable without/
  or     s/only available with/requires/

would emphasize the required flag rather than focus on the feature's
availability.  (FWIW, of the above suggestions, the last ["requires"] seems to
me the most direct and simplest to understand.)

Thank you.

[Bug c++/103049] [C++23] P0849R8 - auto(x): decay-copy in the language

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103049

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I'm afraid "only available with" is the customary form we use throughout the
front end, so in the patch I'm just sticking to that form.

[Bug c++/103049] [C++23] P0849R8 - auto(x): decay-copy in the language

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103049

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Yeah.  If we change the wording, we should change it consistently, and perhaps
it would be useful to do that because the current wording suggests only a
single pair of -std= options, while we mean that level and anything later (I
don't think we have something supported std >= 11 && std <= 17 or similar ATM,
just >= version or <= version).

[Bug c++/103049] [C++23] P0849R8 - auto(x): decay-copy in the language

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103049

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=93769

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
PR 93769 covers pretty much the same topic.

[Bug c++/103049] [C++23] P0849R8 - auto(x): decay-copy in the language

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103049

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:93810fd673654db9ff16170624a6d36449eab241

commit r12-5386-g93810fd673654db9ff16170624a6d36449eab241
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Nov 3 11:04:22 2021 -0400

    c++: Implement C++23 P0849R8 - auto(x) [PR103049]

    This patch implements P0849R8 which allows auto in a functional cast,
    the result of which is a prvalue.

    [expr.type.conv]/1 says that the type is determined by placeholder type
    deduction.  We only accept 'auto', not 'decltype(auto)' -- that the
    type shall be auto comes from [dcl.type.auto.deduct].  Therefore the
    rules are like for [temp.deduct.call], deducing template arguments from
    a function call, so the result type will never be a reference, and we
    decay arrays/functions.

            PR c++/103049

    gcc/cp/ChangeLog:

            * semantics.c (finish_compound_literal): Accept C++23 auto{x}.
            * typeck2.c (build_functional_cast_1): Accept C++23 auto(x).

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/auto25.C: Adjust dg-error.
            * g++.dg/cpp0x/auto9.C: Likewise.
            * g++.dg/cpp2a/concepts-pr84979-2.C: Likewise.
            * g++.dg/cpp2a/concepts-pr84979-3.C: Likewise.
            * g++.dg/cpp23/auto-fncast1.C: New test.
            * g++.dg/cpp23/auto-fncast2.C: New test.
            * g++.dg/cpp23/auto-fncast3.C: New test.
            * g++.dg/cpp23/auto-fncast4.C: New test.
            * g++.dg/cpp23/auto-fncast5.C: New test.
            * g++.dg/cpp23/auto-fncast6.C: New test.

[Bug c++/103049] [C++23] P0849R8 - auto(x): decay-copy in the language

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103049

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Implemented in GCC 12.