[Bug c++/111512] New: GCC's __builtin_memcpy can trigger ADL

[Bug c++/111512] New: GCC's __builtin_memcpy can trigger ADL

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111512

            Bug ID: 111512
           Summary: GCC's __builtin_memcpy can trigger ADL
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

When using libstdc++ in GCC 14, the following program compiles with Clang but
is rejected by GCC (https://godbolt.org/z/WKbe1c3v7).

```
#include <array>
#include <utility>

struct incomplete;

template<class T>
struct holder {
    T t;
};

using validator = holder<incomplete>*;

int main()
{
    validator a[1]{};
    (void) std::to_array(a);
    (void) std::to_array(std::move(a));
}
```

The implementation can be made to work for GCC if `::` is added before the call
to __builtin_memcpy (https://godbolt.org/z/6Mx11T3f9).

So the reason for failure seems to be that GCC's __builtin_memcpy can trigger
ADL like a real function.

[Bug c++/111512] GCC's __builtin_memcpy can trigger ADL

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111512

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-09-21
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, that seems undesirable.

Reduced:

template<typename T>
void to_array(T& from)
{
  T to;
  __builtin_memcpy(&to, &from, sizeof(T));
}

struct incomplete;

template<class T>
struct holder {
    T t;
};

using validator = holder<incomplete>*;

int main()
{
    validator a[1]{};
    ::to_array(a);
}

Casting the arguments to void avoids the problem:

  __builtin_memcpy((void*)&to, (void*)&from, sizeof(T));

[Bug c++/111512] GCC's __builtin_memcpy can trigger ADL

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111512

--- Comment #2 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:77cf3773021b0a20d89623e09d620747a05588ec

commit r14-4252-g77cf3773021b0a20d89623e09d620747a05588ec
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 21 09:14:57 2023 +0100

    libstdc++: Prevent unwanted ADL in std::to_array [PR111512]

    As noted in PR c++/111512, GCC does ADL for __builtin_memcpy if it is
    unqualified, which can cause errors for template argument types which
    cannot be completed.

    Casting the memcpy arguments to void* prevents ADL from considering the
    problem type.

    libstdc++-v3/ChangeLog:

            PR libstdc++/111511
            PR c++/111512
            * include/std/array (to_array): Cast memcpy arguments to void*.
            * testsuite/23_containers/array/creation/111512.cc: New test.

[Bug c++/111512] GCC's __builtin_memcpy can trigger ADL

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111512

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The library has a workaround, but the front end still does unwanted ADL for
__builtin_memcpy (and probably other built-ins).

[Bug c++/111512] GCC's __builtin_memcpy can trigger ADL

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111512

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:161a477f0ce93de1d083d379247d0770d4ef96af

commit r13-7844-g161a477f0ce93de1d083d379247d0770d4ef96af
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 21 09:14:57 2023 +0100

    libstdc++: Prevent unwanted ADL in std::to_array [PR111512]

    Qualify the calls to the __to_array helper to prevent ADL, so we don't
    try to complete associated classes.

    libstdc++-v3/ChangeLog:

            PR libstdc++/111511
            PR c++/111512
            * include/std/array (to_array): Qualify calls to __to_array.
            * testsuite/23_containers/array/creation/111512.cc: New test.

    (cherry picked from commit 77cf3773021b0a20d89623e09d620747a05588ec)

[Bug c++/111512] GCC's __builtin_memcpy can trigger ADL

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111512

--- Comment #5 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:685a8c45d8e2c8c10171e672888484ea2c50662e

commit r12-9894-g685a8c45d8e2c8c10171e672888484ea2c50662e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 21 09:14:57 2023 +0100

    libstdc++: Prevent unwanted ADL in std::to_array [PR111512]

    Qualify the calls to the __to_array helper to prevent ADL, so we don't
    try to complete associated classes.

    libstdc++-v3/ChangeLog:

            PR libstdc++/111511
            PR c++/111512
            * include/std/array (to_array): Qualify calls to __to_array.
            * testsuite/23_containers/array/creation/111512.cc: New test.

    (cherry picked from commit 77cf3773021b0a20d89623e09d620747a05588ec)

[Bug c++/111512] GCC's __builtin_memcpy can trigger ADL

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111512

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:97a33ab114187e7c6cd6c6c0f06cd8225e8aeef5

commit r11-11021-g97a33ab114187e7c6cd6c6c0f06cd8225e8aeef5
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 21 09:14:57 2023 +0100

    libstdc++: Prevent unwanted ADL in std::to_array [PR111512]

    Qualify the calls to the __to_array helper to prevent ADL, so we don't
    try to complete associated classes.

    libstdc++-v3/ChangeLog:

            PR libstdc++/111511
            PR c++/111512
            * include/std/array (to_array): Qualify calls to __to_array.
            * testsuite/23_containers/array/creation/111512.cc: New test.

    (cherry picked from commit 77cf3773021b0a20d89623e09d620747a05588ec)