[Bug c++/64867] warning for passing non-POD to varargs function

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64867

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Tom Tromey from comment #9)
> However my belief is that because this class has a user-provided
> default constructor, it is not trivial.

True, but ...

> I tested this by adding "#include <type_traits>" and then
> 
> static_assert(!std::is_trivial<ConstUTF8CharsZ>::value, "whoops");

That's the wrong thing to assert:

  Passing a potentially-evaluated argument of class type (Clause 9)
  having a non-trivial copy constructor, a non-trivial move constructor,
  or a non-trivial destructor, with no corresponding parameter, is
  conditionally-supported with implementation-defined semantics.

Your type has a trivial copy constructor and trivial move constructor and
trivial destructor, so passing it to a varargs function is well-defined.

Give it a non-trivial destructor and you get a warning:

cs.cc: In function âvoid m(const char*)â:
cs.cc:33:12: warning: passing objects of non-trivially-copyable type âclass
ConstUTF8CharsZâ through â...â is conditionally supported
[-Wconditionally-supported]
   zzz(m, cu);
            ^