How Can I Guarantee Exclusive Constexpr if-else Paths in C ?

Ensuring Exclusive Constexpr if-else Paths

In constexpr if-else clauses, it's critical to ensure that the else case is never taken. Traditionally, one might use static_assert(false); to raise an error in such cases. However, this approach raises a compilation error that unexpectedly asserts the statement as true.

Instead, to enforce that the else case is unreachable, the discarded statement must be made dependent on template parameters. This can be achieved using the always_false template, which returns std::false_type regardless of the template arguments.

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

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

This solution is based on the fact that in C , a program is considered ill-formed if no valid specialization can be generated for a template or constexpr if-else substatement. Therefore, when the else clause becomes unreachable, the compiler will issue a compilation error, ensuring that the code remains valid.

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