Cannot use template parameters to remove null

I'm not sure where this is breaking down. Maybe type parameters are always nullable???

```js
/**
* @param {?T} v
* @return {!T}
* @template T
*/
function notNull(v) {
if (v === null) {
throw new Error("null");
}
return v;
}

/**
* @param {!string} s
*/
function f(s) {
alert(s);
}

/**
* @param {?string} s
*/
function g(s) {
f(notNull(s)); // ERROR!
}

```

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