Single-element enums do not narrow to never

[krryan]

TypeScript Version: 2.1.1 / nightly (2.2.0-dev.201xxxxx)

Code

In playground

const enum One {
    Foo = 1,
}

const enum Two {
    Foo = 1,
    Bar = 2
}

function neverFn(n: never) { }

function oneSwitch(one: One) {
    switch (one) {
        case One.Foo: break;
        default: neverFn(one);
    }
}

function twoSwitch(two: Two) {
    switch (two) {
        case Two.Foo: break;
        case Two.Bar: break;
        default: neverFn(two);
    }
}

function oneIfElse(one: One) {
    if (one === One.Foo) {

    }
    else {
        neverFn(one);
    }
}

function twoIfElse(two: Two) {
    if (two === Two.Foo) {

    }
    else if (two === Two.Bar) {

    }
    else {
        neverFn(two);
    }
}

Expected behavior:
one and two are both narrowed to never in the default case for their respective Switch functions, and in the else case for their respective IfElse functions, making them valid arguments to the neverFn call.

Actual behavior:
two narrows correctly, but the neverFn call in oneSwitch and oneIfElse has the error

Argument of type One is not assignable to parameter of type never.

[aluanhaddad]

Aren't these contradictions in terms?

[normalser]

See #12771