[libcxx] `operator>>(integer)` consumes too much input characters when `basefield == 0`

[morinmorin]

When basefield == 0, __stage2_int_loop in num.h does not distinguish 'x'/'X' from digit characters (0–9, a–f, A–F). As a result, reading from "10xFFx01" (with basefield == 0) consumes the whole input.

Testcase:

#include <iostream>
#include <iomanip>
#include <sstream>

int main()
{
    std::istringstream is("10xFFx0.1=FF");
    int x;

    is >> std::setbase(0);
    is >> x;
    bool failbit = is.fail();
    if (failbit) is.clear();
    auto next_pos = is.tellg();

    std::cout << "value: " << x << '\n'; // expected: 10, actual: 0
    std::cout << "fail: " << failbit << '\n'; // expected: 0, actual: 1
    std::cout << "next: " << next_pos << '\n'; // expected: 2 (1st 'x'), actual: 7 ('.')
}

It looks like PR #121795 fixes this issue.

[morinmorin]

A similar bug affects the floating-point operator>>: with the same input ("10xFFx0.1=FF") as the above testcase, double x; is >> x; consumes "10xFFx0.1".

It's been tracked in Issue #18156 for roughly 10 years and is close to going stale.