Compiles without error when the return type signature of variable doesn't match that of the generic function that it is being assigned to

[prateekbhatt]

Compiling the code below:

function identity<T>(arg: T): T {
    return arg;
}

var myIdentity: <T>(arg: T) => string = identity;

var a = myIdentity<string>("string")
var b = myIdentity<number>(100)

outputs the following javascript:

function identity(arg) {
    return arg;
}
var myIdentity = identity;
var a = myIdentity("string");
var b = myIdentity(100);

I put the type signature of the variable myIdentity as <T>(arg: T) => string
where the return type is string instead of T (that I expect to be correct).

I expected the Typescript compiler to show an error because the return types
don't match. However, the code compiled without any error.

Specs:

Mac OS X 10.11

tsc --version outputs:
message TS6029: Version 1.6.2

[RyanCavanaugh]

Technically a duplicate of #3410, but I want to discuss this again so we can understand why we don't do this.