How to cause static error in constexpr if-else chain?

Question

In the following C++20 function template:

template<int i>
void f() {
    if constexpr (i == 1)
       g();
    else if constexpr (i == 2)
       h();
    else
       ??? // <--error
}

Is there something we can write in ??? such that a call of f<3>() will fail at compile-time?

Answer 1

The problem is that the discarded statement of constexpr if can't be ill-formed for every possible specialization. [temp.res.general]/6

(emphasis mine)

The validity of a template may be checked prior to any instantiation.

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

You can use a type-dependent expression that is always false. E.g.

template<int i> struct dependent_false : std::false_type {};

template<int i>
void f() {
    if constexpr (i == 1)
       g();
    else if constexpr (i == 2)
       h();
    else
       static_assert(dependent_false<i>::value, "Must be 1 or 2");
}