19 General utilities library [utilities]

19.15 Metaprogramming and type traits [meta]

19.15.10 Constant evaluation context [meta.const.eval]

constexpr bool is_constant_evaluation() noexcept;
Returns: true if and only if evaluation of the call occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated ([expr.const]).
constexpr void f(unsigned char *p, int n) {
  if (std::is_constant_evaluated()) {           // should not be a constexpr if statement
    for (int k = 0; k<n; ++k) p[k] = 0;
  } else {
    memset(p, 0, n);                            // not a core constant expression
end example