sgetn() fails to extract characters from an unbuffered stream

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

int main ()
{
    static int x = '0';

    struct: std::streambuf {
        // unbuffered, always successfully reads one character
        int_type underflow () { return x++; }
    } sb;

    // "endless" stream that never reaches EOF
    std::istream endless (&sb);

    char s [4] = "";

    endless.read (s, sizeof s);
 
    // expect to extract as many characters as requested
    assert (endless.good ());
    assert (sizeof s == endless.gcount ());
    assert ('0' == s [0] && '1' == s [1] && '2' == s [2] && '3' == s [3]);
    
}
gcc -c -I/home/sebor/stdcxx-4.2.1/include/ansi \
       -I/home/sebor/stdcxx-4.2.1/include \
       -I/build/sebor/stdcxx-4.2.1-gcc-4.3.0-8D/include \
       -I/home/sebor/stdcxx-4.2.1/examples/include \
       -pedantic -nostdinc++ -O2   -W -Wall \
       -Wcast-qual -Winline -Wshadow -Wwrite-strings -Wno-long-long -Wcast-align   t.cpp
t.cpp: In function 'int main()':
t.cpp:9: warning: inlining failed in call to 'virtual main()::<anonymous struct>::~._26()': call is unlikely and code size would grow
t.cpp:24: warning: called from here
t.cpp:9: warning: inlining failed in call to 'virtual main()::<anonymous struct>::~._26()': call is unlikely and code size would grow
t.cpp:24: warning: called from here
gcc t.o -o t  -L/build/sebor/stdcxx-4.2.1-gcc-4.3.0-8D/lib  -Wl,-R/build/sebor/stdcxx-4.2.1-gcc-4.3.0-8D/lib -lstd8D -lsupc++ -lm 
t: t.cpp:22: int main(): Assertion `endless.good ()' failed.
Aborted