Generic type of superclass cannot be inferred from function call on a subclass

[bryanforbes]

TypeScript Version: 2.7.0-dev.20171129

Code

class Evented<M> {
    // protected __typeMap__?: M; 
    on<K extends keyof M>(type: K, listener: (event: M[K]) => void) { }
}

function on<M, K extends keyof M>(target: Evented<M>, type: K, listener: (event: M[K]) => void) { }

interface EventMap {
    change: { type: 'change'; };
}

const e = new Evented<EventMap>();
e.on('change', (event) => { }); // type of event: { type: 'change' }
on(e, 'change', (event) => { }); // type of event: { type: 'change' }

class SubEvented extends Evented<EventMap> { }

const s = new SubEvented();
s.on('change', (event) => { }); // type of event: { type: 'change' }
on(s, 'change', (event) => { }); // type of event: M[K]

Expected behavior:
event is inferred as { type: 'change' } on line 20 (last line)

Actual behavior:
Line 20 is flagged as an error:

error TS2345: Argument of type '"change"' is not assignable to parameter of type 'never'.

Currently, the only way to get line 20 to work as expected is to uncomment line 2.

[mhegazy]

Should be fixed by #20126

[mhegazy]

it is worth noting that the inference here for M will not be EventMap, since the there is no manifestation for this type to be inferred from. however, with #20126, M will be inferred from the use M[K] to be a type with a property named K (or "change" in the example above).

[RyanCavanaugh]

This is the intended behavior -- there is no inference site for M