Type deduction using mapped types and generics

[k34iml0ch]

TypeScript Version: 2.5.3, 2.6.0rc

Please consider the following simple type definitions.

type PropTypes = {
    'prop1': number,
    'prop2': string
}

interface Handler<T> {        
    handle(value: T): T;
}

type HandlerMap = {
    [K in keyof PropTypes]: Handler<PropTypes[K]>
}

let map: HandlerMap;

The following is not working. Typescript cannot deduce the actual type of the handler.
Listing 1:

function main1(props: PropTypes) {
    Object.keys(props).forEach((key: keyof PropTypes) => {
        map[key].handle(props[key]); // <--  Cannot invoke an expression whose type lacks a call signature...       
    });
}

main1({
    'prop1': 42 ,
    'prop2': 'Hello World' 
})

The followings works.
Listing 2:

function main2(props: PropTypes) {
    Object.keys(props).forEach((key: keyof PropTypes) => {
        let handler = map[key] as Handler<{}> // <-- Works fine using a "cast"
        handler.handle(props[key]);        
    });
}

main2({
    'prop1': 42 ,
    'prop2': 'Hello World' 
})

Listing 3:

function main3(props: PropTypes) {
    Object.keys(props).forEach((key: keyof PropTypes) => {
        helper(map[key], props[key]);  // <-- Works fine using a "helper"
    });
}

function helper<K extends keyof PropTypes>(handler: Handler<PropTypes[K]>, value: PropTypes[K]) {
    handler.handle(value);
}

main3({
    'prop1': 42,
    'prop2': 'Hello World'
})

Expected behavior:

Code in Listing 1 should compile without error

Actual behavior:

Cannot invoke an expression whose type lacks a call signature. Type '((value: number) => number) | ((value: string) => string)' has no compatible call signatures.

link to playground

[mhegazy]

Duplicate of #7294

[mhegazy]

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