The C standard says that accessing a value of one type through a pointer of a different type (except char) is undefined behavior (note: the actual semantics are a bit more involved, this is the ELI15 version).
So, if there's pointer casting going on under the hood, sufficiently aggressive optimization can lead to undefined behavior. This is a potential problem especially for a lot of memory allocators (not just in Nim), which do that kind of stuff a lot (the official way to do it in a defined way is to use a pointer to a union). The reason why compilers are increasingly making use of it is that it gives them some guarantees about aliasing (or rather, lack thereof), which provides additional optimization opportunities. The problem is that a lot of code hasn't actually been written with these assumptions in mind. Note that _undefined_ behavior in the C/C++ standards is different from _unspecified_ or _implementation-defined_ behavior. Implementation-defined and unspecified behavior is still assumed to have some semantics, just not something that you can rely on (such as the value of `sizeof(int)`). Undefined behavior has been increasingly exploited by compiler writers to mean "this cannot happen" and create code based on the assumption that this is an impossibility. This is an issue both for people who write C/C++ code manually and for compilers that emit C/C++ code.
