RHS expression type in assignment incorrectly inferred

[yjbanov]

To completely fix #1587 TypeScript should also support:

function repro(message: Object | Object[]) {
    if (message instanceof Array) {
        message = message.filter;  // error
    }
}

Currently it results in Property 'entries' does not exist on type 'Object | Object[]'. It seems message on the RHS switches from Array back to Object | Object[] prematurely.

Workaround:

function workaround(message: Object | Object[]) {
    var theMessage = message;
    if (theMessage instanceof Array) {
        message = theMessage.filter;
    }
}

[danquirk]

The current design is that type guards narrow down the known constituent types of a union type, but they don't narrow to anything just based on the instanceof operator. Otherwise there'd be no need to use a union type here at all, you could just type message as any and let type guards do the rest.

[yjbanov]

I thought that was what my example did, narrow down the constituents of the union type. Here's another example:

class Uri {
    uriString: String;
 }

function repro(uri: String | Uri) {
    if (uri instanceof Uri) {
        uri = uri.uriString; // error
    }
}

function workaround(uri: String | Uri) {
    var theUri = uri;
    if (theUri instanceof Uri) {
        uri = theUri.uriString;
    }
}

I certainly wouldn't want to type uri as any.

[DanielRosenwasser]

I believe the reason you're getting issues in both examples is because type-guards don't narrow variables in scopes where you assign to those variables.

[yjbanov]

@DanielRosenwasser, that's what it feels like to me too. It seems to assign one type to the entire code block, in this case the if block. It doesn't recognize that before the assignment the type guard is in effect, and after the assignment the variable acquires a different type.

[danquirk]

Sorry, I misread the original example. @DanielRosenwasser is right, the issue is related to the assignment rule we have for type guards.

[RyanCavanaugh]

Basically duplicate of #2388, then?

[yjbanov]

@RyanCavanaugh, pretty close. Could be used as another test case.