cpp/numeric/special functions/assoc legendre

@1-3@ Computes the of the degree, order , and argument. @A@ Additional overloads are provided for all integer types, which are treated as.

Return value
If no errors occur, value of the associated Legendre polynomial of, that is , is returned (where  is the unassociated Legendre polynomial, ).

Note that the Condon-Shortley phase term is omitted from this definition.

Error handling
Errors may be reported as specified in math_errhandling
 * If the argument is NaN, NaN is returned and domain error is not reported
 * If $|x| > 1$, a domain error may occur
 * If is greater or equal to 128, the behavior is implementation-defined