[Bug libstdc++/109049] New: std::declval gives wrong result for cv void

[Bug libstdc++/109049] New: std::declval gives wrong result for cv void

[fsb4000 at yandex dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109049

            Bug ID: 109049
           Summary: std::declval gives wrong result for cv void
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fsb4000 at yandex dot ru
  Target Milestone: ---

A. Jiang <de34@live.cn> found that:

Quote:

"The return type is... cursed for cv void.

It seems that decltype(std::declval<const void>) should be const void() (until
C++17) / const void() noexcept (since C++17) (same for volatile void and const
volatile void), but the cv-qualifiers are dropped"

Code example:

#include <utility>
#include <type_traits>

int main() {
    static_assert(std::is_same_v<decltype(std::declval<const void>), const void
() noexcept>);
    static_assert(std::is_same_v<decltype(std::declval<volatile void>),
volatile void () noexcept>);
}


https://gcc.godbolt.org/z/TMe3v9sxh

I've created the same issue for libc++:
https://github.com/llvm/llvm-project/issues/61232

[Bug libstdc++/109049] std::declval gives wrong result for cv void

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109049

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Who cares? Taking the address of std::declval is forbidden, and you can't call
that specialization anyway.

[Bug libstdc++/109049] std::declval gives wrong result for cv void

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109049

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The reason it behaves this way is that we define declval as:

  template<typename _Tp>
    auto declval() noexcept -> decltype(__declval<_Tp>(0));

and decltype(__declval<_Tp>(0)) drops the cv-qualifiers. This implementation is
much faster to compile, and so we're not going to stop using it to "fix" the
useless testcase in this bug. Nobody cares about decltype(std::declval<const
void>) but they do care about compile times.

[Bug libstdc++/109049] std::declval gives wrong result for cv void

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109049

--- Comment #3 from Jiang An <de34 at live dot cn> ---
I've mailed to LWG Chair to request legitimation of libc++ and libstdc++'s
current strategy.

[Bug libstdc++/109049] std::declval gives wrong result for cv void

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109049

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I thought it was already allowed, because std::declval is not an addressable
function ([namespace.std]) but that only covers forming pointers and references
to functions. We should extend that to cover this case.