bug: Literal comparison orders -NaN as greater than +NaN, contradicting the documented total ordering

[LuciferYang]

Summary

Literal::operator<=> orders a negative NaN as greater than a positive NaN, the opposite of the total ordering the code documents and tests elsewhere. The helper CompareFloat (src/iceberg/expression/literal.cc) returns lhs_is_negative <=> rhs_is_negative for the both-NaN case, so -NaN <=> +NaN evaluates to true <=> false = greater. The adjacent comment states "-NAN < NAN", and FloatSpecialValuesComparison / DoubleSpecialValuesComparison assert the full ordering -NaN < -Infinity < ... < +Infinity < +NaN. The both-NaN sign branch contradicts both.

Root Cause

bool lhs_is_negative = std::signbit(lhs);
bool rhs_is_negative = std::signbit(rhs);
return lhs_is_negative <=> rhs_is_negative;

For lhs = -NaN (signbit = true) and rhs = +NaN (signbit = false), this is true <=> false = std::strong_ordering::greater, i.e. it reports -NaN > +NaN. A negative sign bit should sort below a positive one, so the operands must be swapped: rhs_is_negative <=> lhs_is_negative.

The existing FloatNaNComparison / DoubleNaNComparison tests only cover same-sign NaN pairs (qNaN vs sNaN, which are equivalent), so the mixed-sign case was never exercised.

Reproduction

auto neg_nan = Literal::Float(-std::numeric_limits<float>::quiet_NaN());
auto pos_nan = Literal::Float(std::numeric_limits<float>::quiet_NaN());

// Expected per the documented ordering: less
// Actual: greater
(neg_nan <=> pos_nan);

Proposed Fix

Swap the operands so a negative sign bit compares as less, consistent with the -NaN < +NaN comment and the special-values ordering tests:

return rhs_is_negative <=> lhs_is_negative;

I have the fix and a mixed-sign NaN regression test (float and double) ready and will open a PR.