Function ferrers_real_degree 

pub(crate) fn ferrers_real_degree<const DERIVATIVE: bool>(
    v: f64,
    m: i32,
    x: f64,
) -> Result<(f64, f64)>

Real-degree Ferrers function P_v^m(x) (degree v not an integer) and, with DERIVATIVE, its analytic derivative in x; without it the second element is 0.0.

Upstream: treams.special.lpmv(m, v, x) at a non-integer degree. Differences: degrees above 128, orders above 130 and x outside (-1, 1] give Error::InvalidInput, and results beyond the double range give Error::SpecialFunction, as does the infinite derivative at x = 1 for |m| = 1.