> What does that even mean in the absence of a formal standard and competing compiler implementations? (Hint: nothing.) Undefined behaviour is stuff that you c

> What does that even mean in the absence of a formal standard and competing compiler implementations? (Hint: nothing.) Undefined behaviour is stuff that you compiler's optimizers have been promised can never occur, so they are allowed to transform your code based on that assumption. Here's a post I made on Reddit in 2019 with a list of resources on what undefined behaviour is: https://www.reddit.com/r/rust/comments/dpxswt/rust_2020_lets_embrace_the_eternal_september/f5zwxuv/ https://www.reddit.com/r/rust/comments/dpxswt/rust_2020_lets... ...and here's the link to an example of what invoking undefined behaviour can do in C++: https://gcc.godbolt.org/z/0ubnbS https://gcc.godbolt.org/z/0ubnbS ...and here's an explanation of why it does what it does: https://kristerw.blogspot.com/2017/09/why-undefined-behavior-may-call-never.html https://kristerw.blogspot.com/2017/09/why-undefined-behavior... (TL;DR: It injects a call to EraseAll because calling Do while it's still null is undefined behaviour, and Do is a static, so the optimizer determines that the only possible answer within the rules it was given is that code outside that compilation unit will have called NeverCalled to set Do = EraseAll before invoking main().)