We had a discussion with colleagues on why in the following code we cannot simply use
static_assert(false)
but need to do a trick with ‘always_false’:
#include <type_traits>
template<typename>
struct always_false : std::false_type {};
template<typename Type>
constexpr int Get()
{
if constexpr (std::is_same_v<Type, int>)
{
return 1;
}
else if constexpr (std::is_same_v<Type, bool>)
{
return 2;
}
else {
static_assert(always_false<Type>::value);
}
}
int main()
{
constexpr auto result = Get<bool>(); //ok
//compile error: static assertion failed
//constexpr auto result = Get<float>();
return result;
}
In GCC and CLang ‘static_assert(false)’ always fails independently of Type, but in MSVC it does not.
Finally we found the answer on cppreference.com, see ‘dependent_false’.
