Type narrowing for generic types

[laszlojakab]

Hi,

Type narrowing not working in the following situation (maybe caused by generic type?):

interface Options<T> {
 opt: T|(() => T);
}

function MyFunc<T>(options: Options<T>) {
 var opt = options.opt;
 if (typeof opt === "function")
 {
 var k = opt; // k is "T | (() => T)" but should be "() => T"
 }
}

///////////////////////////////////

var options: Options<number>;
var opt = options.opt;

if (typeof opt === "function")
{
 var k = opt; // k is "() => number", it is OK
}

[ahejlsberg]

This is by design. In the type T | (() => T), T itself could be a function type so we can't conclude anything from the typeof test.

However, when the type is number | () => number we can indeed tell the difference.