How to assert that a constexpr if else clause never happen?

by

You have to make the discarded statement dependent of the template parameters

template <class...> constexpr std::false_type always_false{};

if constexpr(condition1){
    ...
} else if constexpr (condition2) {
   ....
} else if constexpr (condition3) {
  ....
} else {       
    static_assert(always_false<T>);
}

This is so because

[temp.res]/8 – The program is ill-formed, no diagnostic required, if

no valid specialization can be generated for a template or a substatement of a constexpr if statement within a template and the template is not instantiated, or …

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