Compiler allows violation of a generic function type interface

[albrow]

TypeScript Version: 2.0.6

Code

interface GenericIdentityFn {
    <T>(arg: T): T;
}

function identity(n: number): string {
    return n.toString();
}

let myIdentity: GenericIdentityFn = identity;

Expected behavior:

The code should not compile because identity does not satisfy the constraints of GenericIdentityFn. Namely, it accepts an argument of type number and returns type string, whereas the interface GenericIdentityFn describes a function that accepts an argument of type T and returns type T.

Actual behavior:

The code compiles.

Other notes:

This is based on an example from https://www.typescriptlang.org/docs/handbook/generics.html.

[HerringtonDarkholme]

Related #11537

[mhegazy]

Tracked by #5616