Description
TypeScript Version: Playground
Code
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?
}
}If compiler knows that the type of right hand expression is the same as narrowed type of parameter (in my case) why does compiler expand type of x to the initial type? Maybe it is possible to check for this type of assignments?