Lookup types do not work well with type guards

[panhania]

TypeScript Version: 2.1.4

Consider this simple property getter function (defined in What's New of TypeScript 2.1):

function prop<T, K extends keyof T>(obj: T, key: K): T[K] {
    return obj[key];
}

I would expect that prop(foo, "bar") works the same way foo.bar does. However, if the type of the property is narrowed using type guards, the prop function stops working as expected:

interface Foo {
    bar?: number;
}

function quux(foo: Foo) {
    if (foo.bar) {
        let baz: number = prop(foo, "bar"); // error TS2322: Type 'number | undefined' is not assignable to type 'number'.
    }
}

Expected behavior: The code should compile just fine.
Actual behavior: Compilation ends with a type error.

[RyanCavanaugh]

Tentatively a duplicate of #10065 - type guards affect properties, but don't synthesize the kinds of on-the-fly new types that would be required to identify this in the general case. But here we should be ending up at the same symbol and resolving this correctly

[RyanCavanaugh]

@ahejlsberg any cheap fix possible?

[ahejlsberg]

@RyanCavanaugh Nothing cheap comes to mind. The real fix would be to address the issue in #10065 (which would fix several other issues as well).