Control flow not formed when using consts

[NoHomey]

TypeScript Version: 2.0.3

Code

interface Releasable {
    release(): void;
}

const typeOfNumber: string = 'number';

function release(releasable: number | Releasable): void {
    if(typeof releasable === typeOfNumber) {
         allocated.splice(allocated.indexOf(releasable), 1);
    } else {
         releasable.release();
    }
}

Expected behavior:

interface Releasable {
    release(): void;
}

const typeOfNumber: string = 'number';

function release(releasable: number | Releasable): void {
    if(typeof releasable === typeOfNumber) {
         allocated.splice(allocated.indexOf(releasable), 1); // Ok, typeof releasable is number here
    } else {
         releasable.release(); // Ok, typeof releasable implements Releasable here
    }
}

Actual behavior:

interface Releasable {
    release(): void;
}

const typeOfNumber: string = 'number';

function release(releasable: number | Releasable): void {
    if(typeof releasable === typeOfNumber) {
         allocated.splice(allocated.indexOf(releasable), 1); // [ts] Argument of type 'number | Releasable' is not assignable to parameter of type 'number'. Type 'Releasable' is not assignable to type 'number'.
     } else {
         releasable.release(); // [ts] Property 'release' does not exist on type 'number | Releasable'.
     }
}

Currently type guards are only limited to literals. Type guards are consumed at compile time and from this taken in mind variables defined with let or var seems natural to not result in proper control flow but type guards using constants defined with const should result in a proper type guard. Since the compiler already knows that consts will not change their value ans so their value should be checked dose it evaluate to known typeof return value to form a proper control flow else to result in a [ts] error ...

[RyanCavanaugh]

Why are you using a const rather than the literal "number" here?

We could think about this as a suggestion but you'd definitely need to not have the : string annotation of typeOfNumber since that explicitly discards our knowledge of its initializer

[NoHomey]

I'm using const as a refactor of typeof type guards and as well for little memory optimization. And I'm using string as type for the consts since typeof returns string: The typeof operator returns a string indicating the type of the unevaluated operand.

[NoHomey]

After reading the TypeScript Language Specification; 4.18.6 The typeof operator I've changed the type of typeOfNumber from string to primitive string literal 'number' but this dose not change the behavior (the same [ts] errors are emitted).

[RyanCavanaugh]

Approved change for next release