Number conversions do not round to even

Converting a floating-point number using QString::number, arg() etc. to a string rounds away from zero (JavaScript semantics), instead of rounding half to even (IEEE754's default, i.e. printf / std::format / std::to_chars / Python / Rust / ... semantics).

Example:

qDebug() << QString::number(0.25, 'f', 1);
qDebug() << QString("%1").arg(0.25, 0, 'f', 1);

prints "0.3" (twice) instead of "0.2". 0.25 is precise, and should be rounded to even to 0.2.

Indeed:

printf("%.1f\n", 0.25);
std::println("{:.1f}", 0.25);

prints "0.2".

This is due to libdouble-conversion, which has JS semantics, and documents the choice:

  //   'v'. If there are two at the same distance, than the one farther away
  //   from 0 is chosen (halfway cases - ending with 5 - are rounded up).

However this also means that we get inconsistent outputs between builds with and without libdouble-conversion (bootstrap; different build configurations; and complicates porting Qt < 5.6 code towards Qt 5.15 / Qt 6). Also, moving away from libdouble-conversion and towards standardized solutions (std::to_chars / std::from_chars) will also change output back to IEEE754's.