Index signature arguments are not type checked based on the specialized key type they are constrained to

TypeScript Version:

1.8.7

Code

type OnlyNumberKeys = {[key: number]: any};
let a: OnlyNumberKeys = {};

a["abcd"] = 1; // No error?
let x = a["abcd"]; // No error?

type OnlyStringKeys = {[key: string]: any};
let b: OnlyStringKeys = {};

b[1234] = 1; // No error?
let y = b[1234]; // No error?

Expected behavior:
Should error

Actual behavior:
Doesn't error

(Sorry if this is a duplicate but a search did not yield anything on this)

Provide feedback