CS-FSSA-025
Some operators like return are assigned to a Never type also denoted with !. This represents
expressions whose evaluation cannot be assigned if they're on the right-hand-side. When never is
unifified with another type, then the other type should dominate. Consider the following example:

fn main(){
 let mut v: u32 = 1; // (1)
 v = return; // (2)
}

In the example, the type of v is u32 from (1) and is also assigned a ! from (2). Since ! is always
unified with any type the snippet successfully type checks. However, the following does not type check,
even though it's a similar case.

fn main(){
 let mut v: u32 = return;
 v = 1;
}