instanceof-guard-based narrowing of sum types produces the type "never"

[pbazant]

TypeScript Version: TS 2.4
Code

class C1 {
    foo: number;
}

class C2 {
    foo: number;
    bar: number;
}

function f_not_C2(a: C1 | C2) {
    if (!(a instanceof C2)) {
        a.foo; // a has type C1, so only foo is available
    }
}

function f_not_C1(a: C1 | C2) {
    if (!(a instanceof C1)) {
        a; // a should have type C2, but has type never!
    }
}

Expected behavior:
The variable "a" in f_not_C1(...) should have type C2.
Actual behavior:
The variable "a" in f_not_C1(...) has type never, making all properties unavailable.

[jcalz]

Duplicate of #10934 and many others. TypeScript's lack of nominal typing (#202) causes instanceof to behave oddly. Workarounds include adding some optional property to the classes to help TypeScript distinguish them:

class C1 {
    __brand?: 'C1'
    foo: number;
}

class C2 {
    __brand?: 'C2'
    foo: number;
    bar: number;
}

function f_not_C1(a: C1 | C2) {
    if (!(a instanceof C1)) {
        a; // a is C2 now
    }
}

[mhegazy]

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