Yeah sure, there's nothing preventing you from using "constexpr" so it's compile-time evaluated and inlined:

(Please no more macros, it's 202- I mean, 2023!)

  static constexpr bool DEBUG = true;

  [[demo::invariant]] [[gnu::used]]
  void check_invariants()
  {
    if constexpr (!DEBUG) return;

    assert(top >= 0 && top <= MAX_SIZE);
  }

Note that `assert`s are disabled if you define the macro `NDEBUG`, e.g. https://godbolt.org/z/hMWo8KM7q

CMake defines the macro in release builds: https://github.com/Kitware/CMake/blob/e1eacbe2c522a8bf9a82af...

Would be nice to have a non-macro solution for controlling behavior at configure time, but the `NDEBUG` macro is basically already your `DEBUG` constexpr.

Better to define a macro that evaluates to the contract/invariants on GCC and empty on other compilers, rather than copying the ifdef/else/endif logic around to every usage.