[Bug c++/103480] New: [10,11,12]Werror=useless-cast with constexpr destructor

["peeceseven at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103480

            Bug ID: 103480
           Summary: [10,11,12]Werror=useless-cast with constexpr
                    destructor
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: peeceseven at gmail dot com
  Target Milestone: ---

Created attachment 51901
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51901&action=edit
sample that causes the bug.

https://godbolt.org/z/vsdd6M6cc

Issue: useless cast error triggers somehow when we have a constexpr destructor.

GCC 9 doesn't support c++20's constexpr destructor. But this problem exists in
10,11,12, and trunk.

Code compiles on clang and msvc.



//error output
<source>: In instantiation of 'static constexpr auto scope_guard::array(void
(*)()) [with long unsigned int count = 2]':
<source>:82:45:   required from here
<source>:69:12:   in 'constexpr' expansion of 'std::array<scope_guard, 2>(r)'
<source>:69:12: error: useless cast to type 'struct scope_guard&&'
[-Werror=useless-cast]
   69 |     return r;
      |            ^
<source>: In function 'int main()':
<source>:82:51:   in 'constexpr' expansion of 'scope_guard::array(void (*)())
[with long unsigned int count = 2]()'
<source>:69:12:   in 'constexpr' expansion of 'std::array<scope_guard, 2>(r)'
<source>:82:51: error: useless cast to type 'struct scope_guard&&'
[-Werror=useless-cast]
   82 |     constexpr auto t = scope_guard::array<2>(&test);
      |                                                   ^
<source>:82:51:   in 'constexpr' expansion of 'scope_guard::array(void (*)())
[with long unsigned int count = 2]()'
<source>:69:12:   in 'constexpr' expansion of 'std::array<scope_guard, 2>(r)'
<source>:82:51: error: useless cast to type 'struct scope_guard&&'
[-Werror=useless-cast]
cc1plus: some warnings being treated as errors
Compiler returned: 1