[Bug c++/95772] New: warning desired when default operator= cannot be constructued

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95772

            Bug ID: 95772
           Summary: warning desired when default operator= cannot be
                    constructued
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marcpawl at gmail dot com
  Target Milestone: ---

https://godbolt.org/z/eXckPe

#include <type_traits>

class Const {
 public:
  int const i_;

  Const(int i) : i_(i) {}

  Const& operator=(Const& rhs) = default;
};

int main(int argc, char**) {
  Const c{argc};
  static_assert(!std::is_assignable<Const, Const>::value,
                "should not be able to assign i_");
#ifdef ASSIGN
  Const d{argc + 1};
  d = c;
#endif
  return c.i_;
}

I expected a diagnostic saying that operator= cannot be defaulted which is seen
if the ASSIGN code is enabled.  The code compiles cleanly.

When I wrote this bug in a large code base  with -03 and -flto the code will
fail with illegal instructions and other memory corruption errors.  No problem
without optimization.