[Bug libstdc++/102090] New: Placement-new is not constexpr

[Bug libstdc++/102090] New: Placement-new is not constexpr

[friedkeenan at protonmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102090

            Bug ID: 102090
           Summary: Placement-new is not constexpr
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: friedkeenan at protonmail dot com
  Target Milestone: ---

When calling placement-new in a constant expression, a compiler error occurs
saying that `void* operator new(std::size_t, void*)` is not constexpr; see
https://godbolt.org/z/8enoG9G57 for a minimal example.

As far as I can tell, the fix should just be adding constexpr to the signature.

[Bug libstdc++/102090] Placement-new is not constexpr

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102090

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I don't think inplacement new is valid for constexpr.  See PR 61105 also.

[Bug libstdc++/102090] Placement-new is not constexpr

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102090

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
clang produces:

<source>:3:16: error: consteval function never produces a constant expression
[-Winvalid-constexpr]
consteval auto fuck() {
               ^
<source>:5:23: note: call to placement 'operator new'
    const auto data = new(buf) int(1);
                      ^

[Bug libstdc++/102090] Placement-new is not constexpr

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102090

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
DR 1312 also mentions something similar with respect to casting to void*.

[Bug libstdc++/102090] Placement-new is not constexpr

[friedkeenan at protonmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102090

--- Comment #4 from friedkeenan at protonmail dot com ---
I think you're correct. That's really strange, but I guess that's what
std::construct_at is for? But that also confuses me, how is GCC's
implementation of std::construct_at working if placement-new isn't constexpr:
https://github.com/gcc-mirror/gcc/blob/16e2427f50c208dfe07d07f18009969502c25dc8/libstdc%2B%2B-v3/include/bits/stl_construct.h#L97

[Bug libstdc++/102090] Placement-new is not constexpr

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102090

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I know cppreference is not the standard but from:
https://en.cppreference.com/w/cpp/memory/construct_at

except that construct_at may be used in evaluation of constant expressions.

(In reply to friedkeenan from comment #4)
> I think you're correct. That's really strange, but I guess that's what
> std::construct_at is for? But that also confuses me, how is GCC's
> implementation of std::construct_at working if placement-new isn't
> constexpr:
> https://github.com/gcc-mirror/gcc/blob/
> 16e2427f50c208dfe07d07f18009969502c25dc8/libstdc%2B%2B-v3/include/bits/
> stl_construct.h#L97

Because it is still broken ...

[Bug libstdc++/102090] Placement-new is not constexpr

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102090

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It is not broken.  The C++ FE knows std::construct_at is special and allows
placement new in there as implementation detail in constant expressions.
See uses of is_std_construct_at in gcc/cp/constexpr.c.

[Bug libstdc++/102090] Placement-new is not constexpr

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102090

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
http://eel.is/c++draft/new.syn doesn't say placement new should be constexpr.

[Bug libstdc++/102090] Placement-new is not constexpr

[friedkeenan at protonmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102090

--- Comment #8 from friedkeenan at protonmail dot com ---
Sorry for wasting your time with this bug report. It just didn't make sense to
me why placement-new couldn't be constexpr

[Bug libstdc++/102090] Placement-new is not constexpr

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102090

--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
One of the reasons might be that placement new returns void *, and cast of that
to pointer to some other type is not valid in constant expressions.
std::construct_at bypasses all of that just by being a magic function, it is up
to the implementation how it achieves what it is supposed to do.