cpp/experimental/special functions/expint

@1@ Computes the exponential integral of. @4@ A set of overloads or a function template accepting an argument of any integral type. Equivalent to after casting the argument to.

Return value
If no errors occur, value of the exponential integral of, that is $-$, is returned.

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 the argument is ±0, -∞ is returned