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Generic type of superclass cannot be inferred from function call on a subclass #20348
Description
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.
Activity
mhegazy commentedon Nov 29, 2017
Should be fixed by #20126
mhegazy commentedon Nov 29, 2017
it is worth noting that the inference here for
Mwill not beEventMap, since the there is no manifestation for this type to be inferred from. however, with #20126,Mwill be inferred from the useM[K]to be a type with a property namedK(or "change" in the example above).RyanCavanaugh commentedon Dec 16, 2021
This is the intended behavior -- there is no inference site for
M