A lot of `if`s can be replaced by `if constexpr`

[jachymb]

This is based on review by @SteveBronder in my PR #3157 (comment) but it apparently pertains to many other already implemented distributions as well as functions

When I run the following command

grep -rE if \(!?is_constant stan/math

The output has 1146 lines.

If I understand correctly the value is known at compile time and these could be replaced by if constexpr

similarly:

grep -rE if \(!?include_summand stan/math

has 285 lines.

This is just for illustration, there are others that can be found that can be found by filtering for <.*> or ::value etc. The grep may not find all in case of formatting line breaks. Perhaps this can be done by a more sophisticated regex search and replace through the codebase?

[SteveBronder]

Yes! I have not had time, but what I would really like to do is replace !is_constant and !is_all_constant with a helper variable template like is_autodiff_type_v. Then use constexpr everywhere we have is_constant. So lines like

if (!is_constant_all<T_beta, T_x, T_alpha>::value) {

would be

if constexpr (is_all_autodiff_type_v<T_beta, T_x, T_alpha>)

The helper types would look like

/**
 * Checks whether the `scalar_type` of a type is an autodiff type
 * @tparam T Any type
 */
template <typename T>
struct is_autodiff_type : is_autodiff<scalar_type_t<T>> {};

template <typename T>
inline constexpr bool is_autodiff_type_v = is_autodiff_type<T>::value;

template <typename... Types>
inline constexpr bool is_all_autodiff_type_v = std::conjunction<is_autodiff_type<Types>...>::value;

[spinkney]

Yes! I have not had time, but what I would really like to do is replace !is_constant and !is_all_constant with a helper variable template like is_autodiff_type_v. Then use constexpr everywhere we have is_constant. So lines like

if (!is_constant_all<T_beta, T_x, T_alpha>::value) {

would be

if constexpr (is_all_autodiff_type_v<T_beta, T_x, T_alpha>)

The helper types would look like

/**
 * Checks whether the `scalar_type` of a type is an autodiff type
 * @tparam T Any type
 */
template <typename T>
struct is_autodiff_type : is_autodiff<scalar_type_t<T>> {};

template <typename T>
inline constexpr bool is_autodiff_type_v = is_autodiff_type<T>::value;

template <typename... Types>
inline constexpr bool is_all_autodiff_type_v = std::conjunction<is_autodiff_type<Types>...>::value;

Does having this template make it easier to get user declared derivatives and marking parameters as constants, ie don't differentiate? or par variable syntax which possible

[SteveBronder]

It's mostly to prettify the code a bit. It would allow you to do different things for scalars and vectors which can be nice, but otherwise everything should be the same