Incorrect type inference when passing union type to a method or indexer

[DavidKDeutsch]

Not sure if this is a bug or just a feature that has not been implemented (yet), but given the following:

interface Foo {
    member1: number;
    member2: boolean;
}

function bar(foo: Foo, member: "member1" | "member2") {
    return foo[member]; // return type is inferred as any, whereas it should be number | boolean
}

I would expect the return type of Foo to be number | boolean instead of any. I see the same issue with function overloads:

declare function bar2(field: "field1"): string;
declare function bar2(field: "field2"): number;
declare function bar2(field: string): any;

var field: "field1" | "field2" = "field1";
var result = bar2(field); // inferred type of result is any, whereas it should be string | number

Note that his behavior is not limited to string literals; it also happens when overloading on type:

declare function bar3(field: Date): string;
declare function bar3(field: number): number;
declare function bar3(field: any): any;

var field2: Date | number = 6;
var result2 = bar3(field2); // inferred type of result2 is any, whereas it should be string | number

This is with typescript 1.8.2

[mhegazy]

The first sample is a duplicate of #7730

The last two samples are by design. It becomes easier to see why this is not possible for the general case once you have more than one parameter.

declare function bar2(field: "field1", a: number): string;
declare function bar2(field: "field2", b: string): number;

var result = bar2(field, "some string"); // number, or string, or number|string, or error?

This is already suggested in #1805, and you can find more discussions about these at #6735