从 istream 读取 bitset<0> 总是失败？

Question

I was recently working with std::bitset<0> and noticed something interesting with operator>> :

The "specification" states that:

If no characters are extracted, is.setstate(ios_base::failbit) is called.

Does this still apply when "all possible N characters" (with N == 0) have been read? If so, does this imply that trying to read from an istream into a bitset<0> must always set failbit ?

The following unit test exhibits no consistent behavior across the compilers I had at hand:

BOOST_AUTO_TEST_CASE(io0) {
    try {
        std::stringstream ss{"1"};
        ss.exceptions(std::ios::badbit | std::ios::failbit);
        std::bitset<0> b;
        ss >> b;
        BOOST_TEST(false);
    } catch(...) {
        BOOST_TEST(true);
    }
}

MSVC 2019 sets the failbit, whilst GCC 7.3.0 does not!

Answer 1

The specification (no scare quotes) said the same thing ( when this question was asked ), so yes. (Of course, skipping whitespace first may provoke some other kind of failure instead.) I thought this a clear error in the standard, so I filed an issue which has since been resolved for C++20.