AFAIK, assert() is not undefined behavior, so it can't be used for optimization. It is either implementation-defined in debug mode, or does nothing in release mode.

For example:

  assert(a >= 0);
  if (a < 0) printf("a is negative");

In release mode, assert() will be gone, so the if/printf() will stay. If we used "if (a < 0) unreachable();" instead of assert(), it would optimize away both lines.