How to Guarantee Exclusive Execution of Constexpr if/else Branches in C ?

Ensuring Exclusive Execution of Constexpr if/else Branches

In C , when constructing a series of constexpr if/else statements, it's desirable to ensure that the else branch is never executed. This is typically done by repeating the evaluation of all conditions in the else clause, as seen in:

static_assert(condition1 || condition2 || condition3);

However, such repetition can become tedious, especially with complex conditions.

Employing Dependent Statements

A more elegant solution involves making the discarded statement dependent on the template parameters. This is achieved by:

1. Defining a constexpr std::false_type template that always returns false:

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

2. Using this template within the else clause of the if/else statement:

else {       
    static_assert(always_false<T>);
}

This approach is valid because:

• According to [temp.res]/8 of the C standard, the program is ill-formed if no valid specialization can be generated for the constexpr if/else statement within a template.
• Since the always_false template always returns false, no specialization can be generated for the else clause under any circumstances.
• Therefore, the else clause will never be executed at compile time.

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