`to_chars` is wrong for negative values of `int128_t`

[pps83]

sample code:

#define BOOST_INT128_ALLOW_SIGN_CONVERSION
#include <boost/int128.hpp>
#include <boost/int128/charconv.hpp>


void toChars(char* ptr, char* ptrEnd, boost::int128::int128_t i128)
{
    auto r = boost::charconv::to_chars(ptr, ptrEnd, i128);
    *r.ptr = '\0';
}

void toChars(char* ptr, char* ptrEnd, boost::int128::uint128_t u128)
{
    auto r = boost::charconv::to_chars(ptr, ptrEnd, u128);
    *r.ptr = '\0';
}

int main()
{
    boost::int128::int128_t i128 = -123;
    boost::int128::uint128_t u128 = 123;

    for (int i=1; i < 5; ++i)
    {
        i128 *= i;
        u128 *= i;

        char buf[64] = {};
        toChars(buf, buf + 64, i128); printf("i128 val: %s\n", buf);
        toChars(buf, buf + 64, u128); printf("u128 val: %s\n", buf);
    }
}

this is the output I get with VS2022:

i128 val: -18446744073709551493
u128 val: 123
i128 val: -18446744073709551370
u128 val: 246
i128 val: -18446744073709550878
u128 val: 738
i128 val: -18446744073709548664
u128 val: 2952

[mborland]

Interesting since the example examples/charconv.cpp runs fine:

=== to_chars ===
uint128_t max (decimal): 340282366920938463463374607431768211455
int128_t min (decimal):  -170141183460469231731687303715884105728
uint128_t (hex): 0xdeadbeefcafebabe12345678
int128_t 511 (octal): 0777

=== from_chars ===
Parsed "340282366920938463463374607431768211455"
  Result: 340282366920938463463374607431768211455
  Equals max? true
Parsed "-170141183460469231731687303715884105728"
  Result: -170141183460469231731687303715884105728
  Equals min? true
Parsed hex "DEADBEEFCAFEBABE12345678"
  Result: 68915718020162848918556923512

I'll take a look.

[mborland]

This is actually an upstream bug in boost.charconv. Probably has the same breaking behavior for __int128.

[mborland]

Will be fixed by: boostorg/charconv#289