Mapped type loses type bound when used with arithmetic operator

TypeScript Version: nightly (2.4.0-dev.20170501)

Code

interface I { x: number; }

type IdentityMap<T> = {
    [P in keyof T]: T[P];
};

function f<T extends I>(i: IdentityMap<T>) {
    const n: number = i.x; // No error, good
    i.x * 2; // Error
}

Expected behavior:

No error.

Actual behavior:

src/a.ts(9,5): error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.

May be related to #14360.

[mhegazy]

This is not really special to mapped types. this is true for all generic variables:

function f2<T extends number>(x: T) {
    const n: number = x; // No error, good
    x * 2; // Error
}

the check we perform for these operators is that it has to be the type number, and not just a number.

I am sure we have another issue for strings somewhere, but could not find it.