[LWG #596] std::filebuf::open(..., ios::app) unexpectedly succeeds

According to filebuf.members, p2, filebuf::open(const char*, openmode mode) is not some combination of the flags listed in Table 113 the call is supposed to fail. Since ios::app only appears in conjunction with ios::out (and ios::binary|ios::out) the call in the program below should fail.

$ cat t.cpp && make t && ./t
#include <cassert>
#include <fstream>

int main ()
{
std::filebuf fb;

const std::filebuf* const pfb = fb.open ("dummy", std::ios::app);

assert (0 == pfb);
}
gcc -c -I/build/sebor/dev/stdlib/include/ansi -D_RWSTDDEBUG -D_RWSTD_USE_CONFIG -I/build/sebor/dev/stdlib/include -I/build/sebor/gcc-4.1.0-11s/include -I/build/sebor/dev/stdlib/../rwtest -I/build/sebor/dev/stdlib/../rwtest/include -I/build/sebor/dev/stdlib/tests/include -pedantic -nostdinc++ -g -W -Wall -Wcast-qual -Winline -Wshadow -Wwrite-strings -Wno-long-long t.cpp
gcc t.o -o t -L/build/sebor/gcc-4.1.0-11s/rwtest -lrwtest11s -L/build/sebor/gcc-4.1.0-11s/lib -lstd11s -lsupc++ -lm
Assertion failed: 0 == pfb, file t.cpp, line 10
Abort (core dumped)