Local Meaning of Union Types (safe assignments)  #17188

class A {
    x: number;
}

class B extends A {
    y: number;
}

class C extends A {
    z: number;
}

declare function process(c:C);
function f(x:  B | C) {
    
    if (x instanceof C) {
        //we memorize x in c
        let c = x; //here x of type C        
      
        //restore x
        x = c;  //here x loses the narrowed type and x is now B | C again and is's by design    
        x.z = 1; //no error, what? 
        process(x);  //it have to be an error, what?
    }
}