Compile-time evaluation of constexpr functions [duplicate]

Consider the only cases in which it is possible for the compiler to evaluate a constexpr function at compile-time. In these cases, it's not clear from the standard (paragraph 5.19) whether it is always mandatory for the compiler to evaluate constexpr functions at compile-time or not; if it is not always mandatory, I would eventually like to see some examples where the standard does give strong guarantees about compile-time evaluations of constexpr functions.

constexpr int f(int n) {
   return n*n;
}