<xlocnum>: Incorrect rounding when parsing long floating point numbers just below midpoints

[statementreply]

Describe the bug
The decimal code path of num_get::do_get records the initial 768 significant digits. If there are any remaining non-zero digits, it adds 1 to the last recorded digit (with carry). This is incorrect when the result is just below the rounding boundary.

Command-line test case 1

D:\Temp>type test.cpp

#include <iostream>
#include <sstream>
#include <string>
using namespace std;

int main() {
    // just below 2^-1022 + 2^-1074 + 2^-1075
    istringstream stream(
        "2.225073858507202124188701479202220329072405282794390378143031338374351073192441946867544064325638818513821882"
        "18502438069999947733013005649884107791928741341929297200970481951993067993290969042784064731682041565926728632"
        "93363047467012331685298342215274451726083585965456631928283524478778779989431077978383369915928859455521371418"
        "11284582511455843192230798975043950868594124572308917389461693683723211913736589779777232866988403563902510444"
        "43035457396733706583981055420456693824658413747607155981176573877626747665912387199931904006317334709003012790"
        "18817520344719025002806127777791679839109057858400646471594381051148915428277504117468219413395246668250343130"
        "61815878293790042053923750720833666932415800027583911188541886415131684784363130802375962957739830017089843749"
        "e-308");
    cout.precision(17);
    double x;
    if (stream >> x) {
        cout << "actual:   " << x << "\n";
    } else {
        cout << "actual:   (failure)\n";
    }
    cout << "expected: 2.2250738585072019e-308\n";
    return 0;
}

D:\Temp>cl /EHsc /W4 test.cpp
用于 x64 的 Microsoft (R) C/C++ 优化编译器 19.35.32213 版
版权所有(C) Microsoft Corporation。保留所有权利。

test.cpp
Microsoft (R) Incremental Linker Version 14.35.32213.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test.exe
test.obj

D:\Temp>.\test
actual:   2.2250738585072024e-308
expected: 2.2250738585072019e-308

Command-line test case 2

D:\Temp>type test.cpp

#include <cfenv>
#include <iostream>
#include <sstream>
#include <string>
using namespace std;

#pragma fenv_access(on)

int main() {
    istringstream stream("1.4" + string(1000, '9'));

    fesetround(FE_DOWNWARD);
    double x;
    stream >> x;
    fesetround(FE_TONEAREST);

    cout.precision(17);
    if (stream) {
        cout << "actual:   " << x << "\n";
    } else {
        cout << "actual:   (failure)\n";
    }
    cout << "expected: 1.4999999999999998\n";
    return 0;
}

D:\Temp>cl /EHsc /W4 test.cpp
用于 x64 的 Microsoft (R) C/C++ 优化编译器 19.35.32213 版
版权所有(C) Microsoft Corporation。保留所有权利。

test.cpp
Microsoft (R) Incremental Linker Version 14.35.32213.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test.exe
test.obj

D:\Temp>.\test
actual:   1.5
expected: 1.4999999999999998

STL version

Microsoft Visual Studio Community 2022 (64 位) - Preview
版本 17.5.0 Preview 5.0