If you're on one of the platforms with sane exception handling, it's a matter of emitting different assembly code for the landing pad so that when unwinding it calls std::terminate instead of running destructors for the local scope. Zero additional overhead. If you're on old 32-bit Microsoft Windows using MSVC 6 or something, well, you might have problems. One of the lesser ones being increased overhead for noexcept.
It's zero runtime overhead in the good case but still has an executable size overhead for functions that previously did not need to run any destructors.
Very true. Then again, if you don't need to tear down anything (ie. run destructors) during error handling you're either not doing any error handling or you're not doing any useful work.
