Invalid type coercion with == operator and strings

[Wintermoose]

TypeScript Version: typescript@3.0.0-dev.20180609, 2.9.2, playround

Search Terms:
== as typeguard
number typeguard
number type coercion

Code

function test(level: number | string):number {
    if (level == +level) {
        const q2 = level; //number
        return level;
    }
    return 0;
}
alert(test(5) + 1);
alert(test("5") + 1) 

Expected behavior:
Compiler error

Actual behavior:
The code compiles. As mentioned in the comment, typescript assumes that level inside the if block is a number; however in reality that condition passes also for number-like string ("5")

Playground Link:
http://www.typescriptlang.org/play/index.html#src=function%20test(level%3A%20number%20%7C%20string)%3A%20number%20%7B%0D%0A%20%20%20%20if%20(level%20%3D%3D%20%2Blevel)%20%7B%0D%0A%20%20%20%20%20%20%20%20const%20q2%20%3D%20level%3B%20%2F%2Fnumber%0D%0A%20%20%20%20%20%20%20%20return%20level%3B%0D%0A%20%20%20%20%7D%0D%0A%20%20%20%20return%200%3B%0D%0A%7D%0D%0A%0D%0Aalert(test(5)%20%2B%201)%3B%0D%0Aalert(test(%225%22)%20%2B%201)

[Kingwl]

seems narrow type in narrowTypeByEquality not handle the Relax Equal Operator
i would like to add a new type comparable similar areTypesComparable,
that handle the Relax Equal Operator and narrow type
follow the rule: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness