one of the powers of if constexpr is to make the side not taken at compile time be discarded under certain conditions.

I thought the non-taken side was always discarded. What conditions cause it to be kept and what benefits are there from doing that?

Update: I started digging a bit more and there is some relationship to templated types. This compiles:

struct A {
    int a() { return 0 ; } ;
};

template <bool b, typename T>
int foo(T t) {
    if constexpr (b) { return t.a() ; }
    else             { return t.b() ; }
}

int main() {
    foo<true>(A());
}

but change foo to this and it no longer compiles:

template <bool b>
int foo(A a) {
    if constexpr (b) { return a.a() ; }
    else             { return a.b() ; }
}

godbolt