Function datum_is_nan 

fn datum_is_nan(datum: Datum<'_>) -> bool

Whether a datum is a floating-point or numeric NaN.

NaN sorts as the maximum of the numeric/float Datum ordering, but it is a fixed point of most functions we treat as monotone (e.g. negation maps NaN to NaN while flipping the sign of every other value). A range whose bounds include NaN therefore breaks the monotonicity assumption that ResultSpec::flat_map relies on, so the range-narrowing shortcut must be skipped in that case.