Real types

Detailed Description

require type traits for types that are either arithmetic, var, or fvar.

Modules
	autodiff <br>
	Require return type from parameter pack satisfies Check
	double_or_int <br>
	fvar <br>
	stan_scalar <br>
	stan_scalar_or_eigen <br>
	var <br>
	var_or_arithmetic <br>
	arithmetic <br>
	floating_point <br>
	integral <br>