[Linux] std::codecvt_byname("*.UTF-8").in() to_next greater than expected

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

int main ()
{
    const std::locale utf8 ("en_US.UTF-8");
    typedef std::codecvt<wchar_t, char, std::mbstate_t> UTF8_Cvt;

    const UTF8_Cvt &cvt = std::use_facet<UTF8_Cvt>(utf8);

    const char src[] = "abc";
    wchar_t dst [2] = { L'\0' };

    const char* from_next;

    wchar_t* to_next;

    std::mbstate_t state = std::mbstate_t ();

    const std::codecvt_base::result res =
        cvt.in (state,
                src, src + 1, from_next,
                dst, dst + 2, to_next);

    assert (1 == from_next - src);
    assert (1 == to_next - dst);
    assert ('a' == dst [0]);
}

gcc -c -I/home/mbrown/stdcxx/include/ansi -D_RWSTDDEBUG    -I/home/mbrown/stdcxx/include -I/build/mbrown/stdcxx-gcc-4.1.1-11S/include -I/home/mbrown/stdcxx/examples/include  -pedantic -nostdinc++ -g   -W -Wall -Wcast-qual -Winline -Wshadow -Wwrite-strings -Wno-long-long -Wcast-align   t.cpp
t.cpp: In function 'int main()':
t.cpp:21: warning: unused variable 'res'
gcc t.o -o t  -L/build/mbrown/stdcxx-gcc-4.1.1-11S/lib  -lstd11S -lsupc++ -lm 
t: t.cpp:26: int main(): Assertion `1 == from_next - src' failed.
Aborted