[Bug libstdc++/109242] New: C++2b std::optional::transform omits required std::remove_cv_t from return optional type

[Bug libstdc++/109242] New: C++2b std::optional::transform omits required std::remove_cv_t from return optional type

[pkasting at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109242

            Bug ID: 109242
           Summary: C++2b std::optional::transform omits required
                    std::remove_cv_t from return optional type
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pkasting at google dot com
  Target Milestone: ---

The implementation of C++2b's std::optional::transform() on HEAD omits a call
to std::remove_cv_t when determining the value type of the returned optional.
As a result valid code such as the following example is rejected:
https://godbolt.org/z/hG5hnz59e

#include <optional>

struct S {
  int i;
};

void foo() {
  std::optional<S>().transform(&S::i);
}

Per https://en.cppreference.com/w/cpp/utility/optional/transform, the returned
type should always be computed via std::remove_cv_t<> on the invoke_result, but
per
https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libstdc%2B%2B-v3/include/std/optional;hb=HEAD,
_Up consistently omits this.

[Bug libstdc++/109242] C++2b std::optional::transform omits required std::remove_cv_t from return optional type

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109242

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-03-21
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

[Bug libstdc++/109242] C++2b std::optional::transform omits required std::remove_cv_t from return optional type

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109242

TC <rs2740 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rs2740 at gmail dot com

--- Comment #1 from TC <rs2740 at gmail dot com> ---
The missing remove_cv_t is real, but this example is invalid. As the linked
cppreference page notes, you cannot pass a PMD to transform.

[Bug libstdc++/109242] C++2b std::optional::transform omits required std::remove_cv_t from return optional type

[pkasting at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109242

--- Comment #2 from Peter Kasting <pkasting at google dot com> ---
(In reply to TC from comment #1)
> The missing remove_cv_t is real, but this example is invalid. As the linked
> cppreference page notes, you cannot pass a PMD to transform.

Ah, true! How about this then: https://godbolt.org/z/TaccPEsY1

#include <optional>

struct S {
  int& i() const;
};

void foo() {
  std::optional<S>().transform(&S::i);
}

[Bug libstdc++/109242] C++2b std::optional::transform omits required std::remove_cv_t from return optional type

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109242

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Peter Kasting from comment #2)
> struct S {
>   int& i() const;
> };
> 
> void foo() {
>   std::optional<S>().transform(&S::i);

The U type is a reference here, which isn't allowed.


This shows the bug:

struct A { };
const A f(int);
std::optional<int> o;
A&& p = *o.transform(f);

[Bug libstdc++/109242] C++2b std::optional::transform omits required std::remove_cv_t from return optional type

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109242

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:ee122a2eeaea2ffec0e32577c7372bd4e2289e11

commit r13-6937-gee122a2eeaea2ffec0e32577c7372bd4e2289e11
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 30 00:42:11 2023 +0100

    libstdc++: Fix filename of new test [PR109242]

    libstdc++-v3/ChangeLog:

            PR libstdc++/109242
            * testsuite/20_util/optional/monadic/pr109340.cc: Moved to...
            * testsuite/20_util/optional/monadic/pr109242.cc: ...here.

[Bug libstdc++/109242] C++2b std::optional::transform omits required std::remove_cv_t from return optional type

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109242

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.3

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk by commit r13-6934-g31a909712014b75fc6ae2ca5eaa425f218bb5f32

I'll backport to gcc-12 too.

[Bug libstdc++/109242] C++2b std::optional::transform omits required std::remove_cv_t from return optional type

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109242

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:82e8d1685a1e5fac4880e987ed9684248378bce2

commit r12-9369-g82e8d1685a1e5fac4880e987ed9684248378bce2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 29 22:16:55 2023 +0100

    libstdc++: Use std::remove_cv_t in std::optional::transform [PR109242]

    We need to strip cv-qualifiers from the result of the callable passed to
    std::optional::transform.

    libstdc++-v3/ChangeLog:

            PR libstdc++/109242
            * include/std/optional (transform): Use std::remove_cv_t.
            * testsuite/20_util/optional/monadic/pr109242.cc: New test.

    (cherry picked from commit 31a909712014b75fc6ae2ca5eaa425f218bb5f32)

[Bug libstdc++/109242] C++2b std::optional::transform omits required std::remove_cv_t from return optional type

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109242

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

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

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for 12.3, thanks for the report.