Infer type inside switch/case statement using instanceof

[maxkoretskyi]

I'm not sure if it's a bug or a feature request. I have the following simple example:

class T1 { a: string; }
class T2 { b: string; }

f(new T1());

function f(p: T1 | T2) {
    switch(true) {
        case (p instanceof T1) : {
            console.log(p.a); // Error:(12, 27) TS2339:  Property 'a' does not exist on type 'T2'
            break;
        }
        case (p instanceof T2) : {
            console.log(p.b); // Error:(12, 27) TS2339:  Property 'b' does not exist on type 'T1'
            break;
        }
    }
}

I get an error on the logging lines because typescript can't know whether p is of type T1 or T2. However I use instanceof operator so it should be able to infer the correct type as it works fine with if statement:

function f(p: T1 | T2) {
    if (p instanceof T1) {
        console.log(p.a);   // no error
    }
}

TS version 2.6.2

I know I can fix it using explicit type casting like this:

console.log((<T1>p).a);

but that seems redundant. Also interesting that type guard doesn't work:

function isT1(t: any): t is T1 {
    return (t instanceof T1);
}

function f(p: T1 | T2) {
    switch(true) {
        case (isT1(p)) : {
            console.log(p.a); // Error:(12, 27) TS2339:  Property 'a' does not exist on type 'T2'
            break;
        }

[RyanCavanaugh]

Duplicate #8934

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.