Apply type inference to mapObject[s.kind]

[mhchem]

TypeScript Version: 2.6.1

Description

I am very impressed by the type inference TypeScript can do with switch and for...else statements.
I miss, however, type inference for map object lookups.

Code

type Named = { kind: "named", id: string };
type Decimal = { kind: "decimal", id: number };
type Hex = { kind: "hex", id: number };
type Entity = Named | Decimal | Hex;
let areaFunctions = {
    "named": function (s: Named) { return "&"+s.id+";"; },
    "decimal": function (s: Decimal) { return "&#"+s.id+";"; },
    "hex": function (s: Hex) { return "&#x"+s.id+";"; },
    "something else": {}
}
// works
let printEntity1 = function (s: Entity) {
    switch (s.kind) {
        case "named": return areaFunctions["named"](s);
            // This works perfectly, TS knows that s can only be to type 'named', here
            // It also knows the function definition and sees that both match
        case "decimal": return areaFunctions["decimal"](s);
        case "hex": return areaFunctions["hex"](s);
    }
}
// does not work
let printEntity2 = function (s: Entity) {
    return areaFunctions[s.kind](s);  // error, although it is equivalent to printEntity1
}
// does not work, although a common JS pattern
let printEntity3 = function (s: Entity) {
    if (areaFunctions[s.kind]) {
        return areaFunctions[s.kind](s);  // type inference does not work
    }
    return "-";
}

Expected behavior:
printEntity2 would compile

Actual behavior:
But it doesn't. I am forced to expand a single line into a long switch statement (just 3 lines here, but a lot more -- and often used -- in my project).

[aluanhaddad]

As a workaround, you can write

let printEntity2 = function <K extends Entity['kind']>(s: Entity & {kind: K}) {
    return areaFunctions[s.kind](s); 
};

[mhegazy]

There are multiple features that are not supported today that contribute to blocking this scenario.. first, The main is that a union property is not callable unless all variants have the same signature. so we would need #7294 first to be able to address this issue. Second we will need to distribute the union over the whole call expression, i,.e. the type of areaFunctions[s.kind](s) would be typeof areaFunctions["named"](s) | tyoeof areaFunctions["decimal"](s) | | tyoeof areaFunctions["hex"](s); then we will need to do some narrowing on s in each branch based on the type of s.kind.

#7294 is the main issue, we currentlly have now way of resolving a union call.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.