[Bug c++/95606] New: [10/11 regression] <any> conflicts with std::is_constructible

[kuzniar95 at o2 dot pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95606

            Bug ID: 95606
           Summary: [10/11 regression] <any> conflicts with
                    std::is_constructible
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kuzniar95 at o2 dot pl
  Target Milestone: ---

Created attachment 48713
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48713&action=edit
main.ii + verbose gcc invoke command

Hi! I'm coming from nlohmann/json project on GitHub where we detected a
possible regression in GCC 10 and later.

Please follow this link for details:
https://github.com/nlohmann/json/issues/2129 You will find minimal code example
there with some detailed analysis of "compilation path" but I need your help
whether it actually is a regression or not. For example, I'm not sure if the
compiler should jump from decltype(swap(std::declval<_Tp&>(),
std::declval<_Tp&>())) with some implicit conversions to std::any constructor
(maybe it should be inaccessible) as clang seems to dismiss std::any which I
discovered by poisoning std::any constructor. I'm also attaching some files as
requested by "bug reporting instructions".

Even if you think GCC works fine, maybe it could be improved so it gives better
error messages in the future in such convoluted scenarios. Please let me know
what you think.

P.S. I've taken a while to bisect and build GCC compilers and I discovered this
was the first broken commit:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=608a080c3f6e5a8338ff99658b27e226cbba7a67