std::filebuf::overflow(EOF) writes garbage in unbuffered mode

The program below dumps core:

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

struct MyBuf: std::filebuf {
int_type oflow ()

{ return overflow (traits_type::eof ()); }

};

int main ()
{
MyBuf f;

assert (0 != f.open ("test.file", std::ios::out | std::ios::trunc));
assert (0 != f.pubsetbuf (0, 0));

assert (MyBuf::traits_type::eof () != f.oflow ());

assert (0 == f.pubseekoff (0, std::ios::end, std::ios::out));
}
aCC -c -I/amd/devco/sebor/dev/stdlib/include/ansi -I/usr/include -D_RWSTDDEBUG -mt -D_RWSTD_USE_CONFIG -I/amd/devco/sebor/dev/stdlib/include -I/build/sebor/aCC-3.63-15S/include -I/amd/devco/sebor/dev/stdlib/../rwtest -I/amd/devco/sebor/dev/stdlib/../rwtest/include -I/amd/devco/sebor/dev/stdlib/tests/include -Aa +nostl -g +d +DD64 +w +W392 +W655 +W684 +W818 +W819 +W849 t.cpp
aCC t.o -o t -L/build/sebor/aCC-3.63-15S/rwtest -lrwtest15S -Aa +nostl -Wl,+s -Wl,+vnocompatwarnings -L/build/sebor/aCC-3.63-15S/lib -mt +DD64 -L/build/sebor/aCC-3.63-15S/lib -lstd15S -lm
Assertion failed: 0 == f.pubseekoff (0, std::ios::end, std::ios::out), file t.cpp, line 19
ABORT instruction (core dumped)