I was under the impression that accessing a union member other than the last one set is UB, but I can't seem to find a solid reference (other than answers claiming it's UB but without any support f...

Hello,  I’m reading about type punning and undefined behavior, then fall into a situation where I still not sure is there any undefined behavior. Given the following:  #[repr(C)] union Foo {     b: Bar,     v: u64, }  #[repr(C)] #[derive(Clone, Copy)] struct Bar {     lo: u32,     hi: u32, }  The idea is to profit the union Foo to convert two u32 values into a u64 value, as:  use std::mem::MaybeUninit;  fn convert(lo: u32, hi: u32) -> u64 {     let mut foo = MaybeUninit::<Foo>::uninit();     let...