[mjevans]

As a hypothetical, lets assume there's a compiler that has a deep transformation around alloc/reallocs to optimize them when they're only fed 'static' input sizes.

    // Source by nlewycky
    template <typename T> struct vector<t> {
      size_t len = 0;
      size_t storage_len = 0;
      T *storage = nullptr;

      void push_back(T t) {
        if (len + 1 > storage_len) {
          // FIXME: what if storage_len > (size_t_max >> 1) ??
          // ? Define maximum expected growth unit? 1K? 4K? 64K?
          // Assignment from trinary op visually confusing, add ( )
          storage_len = ( storage_len ? storage_len * 2 : 1 );
          storage = realloc(storage, storage_len);
        }
        storage[len] = t;
        ++len;
      }
    };
    // 
    vector<t> v;
    v.push_back(a);
    v.push_back(b);
    v.push_back(c);
    v.push_back(d);

After inlining all 4 times, the compiler might notice that in this code section vector<t> v always reaches the final size of storage = realloc(storage, 8); and optimize for that.

    // Pseudo code
    template <typename T> vector<T> v = {
      // final state at end of code section
      size_t len = 4;
      size_t storage_len = 8;
      T *storage = realloc(NULL, storage_len);
    };
    // Compiled and included, forgot template function syntax.
    void template <typename T> vector<T>.push_back(T t);
    (v[0], v[1], v[2], v[3]) = (a, b, c, d);

Even in this case, all the code the programmer wrote was evaluated and had side effects at least once at compile time. No code was eliminated as unreachable / impossible to reach. Instead it was optimized into operations that would always occur, barring realloc failure (which wasn't specified in the toy source, but would probably be a fatal error).