Using consteval certainly makes this simpler, but you can force constant evaluation in C++11 and with C++14 / C++17 features you can clean it up a bit. The key point is that template arguments must be evaluated at compile time and so if you pass a constexpr value through a template parameter then it will either be evaluated at compile time or fail to compile. You can then wrap this behaviour like this (this is C++17, which has been the default for everything that I’ve worked on for so long that I can’t remember which bits don’t work with C++14 / C++11, but you can do the same thing with slightly more clunky syntax in C++11):

#include <type_traits>
template<auto Val>
struct force_const
{
        using type = std::decay_t<decltype(Val)>;
        static constexpr type value = Val;
};

template<auto Val>
inline constexpr auto force_const_v = force_const<Val>::value;

int definitely_works = force_const_v<12>;

constexpr auto fn()
{
        ...
}

auto works_if_fn_evaluates_at_compile_time = force_const_v<fn()>;

If fn fails to evaluate at compile time then you’ll get a compile error.