[FIXED] How to suppress inline function used but never defined warning in g++

Issue

Context: I'm doing some compile time programming which involves manipulating types using constexpr functions that are only evaluated in a decltype context.

For example a pop function that removes the first type from a list of types:

template <typename T0, typename... T1toN>
constexpr auto pop(List<T0, T1toN...>) -> List<T1toN...>;

These functions are used like this

decltype(pop(my_type_list))

In other words, they are never 'executed'. Yet, the compiler (g++) gives me warnings such as warning: inline function ‘constexpr List<T1toN ...> pop(List<T0, T1toN ...>) [with T0 = Type1; T1toN = {Type2, Type3}]’ used but never defined

EDIT: It turns out the the warning only appears when the functions are evaluated in a decltype indirectly. i.e. A top level function which is only evaluated in a decltype uses a subfunction. results in a warning for the subfunction. Jarod42 created a nice reproduction scenario here

Question: Is there any way I can suppress this used but never defined warning? And is it possible to only do it for functions evaluated (indirectly) in a decltype context?

I am using g++ version 8.3

Solution

In

template <typename ...> struct List{};

template <typename T0, typename... T1toN>
constexpr auto pop(List<T0, T1toN...>) -> List<T1toN...>;

template <typename T0, typename... T1toN>
constexpr auto pop2(List<T0, T1toN...> l) { return pop(l); } // You use pop

constexpr List<int, int> my_type_list;
using type = decltype(pop2(my_type_list));

Warning demo

pop2 should not have definition, and be:

template <typename T0, typename... T1toN>
constexpr auto pop2(List<T0, T1toN...> l) -> decltype(pop(l));

Answered By - Jarod42

Answer Checked By - Katrina (PHPFixing Volunteer)