[Bug c++/58709] [c++11] Bogus? int vs. scoped enum printf warning when -fabi-version >= 6

["ppluzhnikov at google dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58709

Paul Pluzhnikov <ppluzhnikov at google dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |jdennett at google dot com
         Resolution|---                         |INVALID

--- Comment #4 from Paul Pluzhnikov <ppluzhnikov at google dot com> ---
James Dennett says:

I believe that the code is incorrect, and the warning is valid.

For calls via varargs:

       .       If the argument has integral or enumeration type that is subject
to the integral promotions (4.5), or a floating point type that is subject to
the floating point promotion (4.6), the value of the argument is converted to
the promoted type before the call. These promotions are referred to as the
default argument promotions.

And scoped enumerations are not "subject to the integral promotions"; only
unscoped enums are:

       .       The value of an enumerator or an object of an unscoped
enumeration type is converted to an integer by integral promotion (4.5).

So it (the scoped enumeration value) is not passed as an int, and using va_arg
to pull it out as an int is undefined behavior.  Well, the full story is even
worse; va_arg is defined by the C standard (not the C++ standard) and in
particular is defined in terms of "compatible" types.  C says that its enums
are compatible with some underlying (but unspecified) integral type, but
naturally C has nothing to say about scoped enumerations.  It would be
reasonable for C++ to specify that varargs calls apply the default argument
promotions to scoped enumerations also, but that's not how C++ is written
today.

The intent of scoped enums is that they never implicitly act like integers,
from which perspective it's good to require people to convert to the underlying
type if they must use varargs.