Error case in a series of "if constexpr" statements

I am refactoring a code that previously used SFINAE in C++14 to handle different categories of types (float, ints, strings, etc.). Something like the following:

template<typename T, typename Enable = void>
struct S;

template<typename Float>
struct S<Float, std::enable_if_t<is_floating_point<Float>::value>> {
    int f() { ... }
};

// other template specializations for other categories of types

I am trying to turn it into something like this in C++17:

template <typename T>
int f() {
    if constexpr (is_floating_point<T>::value) {

    } else if constexpr(...) {
       ...
    } else {
       // compile-time error with useful message ?
    }
}

What is the proper way to generate a useful compile-time error message in the final else statement for all the types that couldn't be handled? static_assert seems to be triggered regardless of whether the else block is considered by the compiler. Simply removing the final else statement works, but gives a "no return statement ..." error, which is not informative, and also if my function had returned void, it would not have led to any error at all.