[Bug c++/48780] New: [C++0x] scoped enumerations and va_arg (default argument promotions)

[hstong at ca dot ibm.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48780

           Summary: [C++0x] scoped enumerations and va_arg (default
                    argument promotions)
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: hstong@ca.ibm.com
                CC: michaelw@ca.ibm.com
              Host: powerpc64-unknown-linux-gnu
            Target: powerpc64-unknown-linux-gnu


In the 2011 FDIS, default argument promotions are defined to leave arguments of
scoped enumeration types as-is (without conversion).

This appears to be an interaction with old wording and I am not aware of any
rationale behind this.

For cases where the underlying type has rank lower than that of int, GCC
performs the default argument promotions as-if the enumeration is unscoped.
This manifests itself as a warning message and a run-time abend.

In 5.2.2 [expr.call],
If the argument has integral or enumeration type that is subject to the
integral promotions (4.5), ..., the value of the argument is converted to the
promoted type before the call. These promotions are referred to as the default
argument promotions.

In 4.5 [conv.prom], integral promotions are not specified to apply to scoped
enumerations.


### Self-contained source (a.cpp):
typedef __builtin_va_list __gnuc_va_list;
typedef __gnuc_va_list va_list;

enum struct A : short { X };

void foo(int x, ...) {
   va_list vl;
   __builtin_va_start(vl, x);
   enum A t = __builtin_va_arg(vl, enum A);
   __builtin_va_end(vl);
}

int main() {
   foo(0, A::X);
}


### Compiler Invocation:
g++ -std=c++0x -Wall -W -pedantic -c a.cpp


### Compiler Output (return code 0):
/data/a.cpp: In function 'void foo(int, ...)':
/data/a.cpp:9:11: warning: unused variable 't' [-Wunused-variable]
/data/a.cpp:9:42: warning: 'A' is promoted to 'int' when passed through '...'
[enabled by default]
/data/a.cpp:9:42: note: (so you should pass 'int' not 'A' to 'va_arg')
/data/a.cpp:9:42: note: if this code is reached, the program will abort


### Run output:
Program received signal SIGTRAP, Trace/breakpoint trap.


### g++ -v output:
Using built-in specs.
Target: powerpc64-unknown-linux-gnu
Configured with: ./configure --prefix=/data/gcc --program-suffix=-4.6.0
--disable-libssp --disable-libgcj --enable-version-specific-runtime-libs
--with-cpu=default32 --enable-secureplt --with-long-double-128 --enable-shared
--enable-__cxa_atexit --enable-threads=posix --enable-languages=c,c++,fortran
--with-gmp=/usr/local
Thread model: posix
gcc version 4.6.0 (GCC)