type not inferred correctly in generic function

[yaymukund]

TypeScript Version: 3.5.2

Search Terms: generic type constraint undefined enum

Code

enum MyEvent {
    Start = 'start',
    Stop = 'stop',
}

interface MyEventOptions {
    [MyEvent.Start]: { startTime: number },
    [MyEvent.Stop]: { endTime: number },
}

class EventService {
    private eventCounts: { [key in MyEvent]?: number };

    constructor() {
        // I don't want to have to list every possible enum member here
        this.eventCounts = {};
    }

    private startEvent() {
        this.report(MyEvent.Start, { startTime: 0 });

        // This should be invalid:
        // 
        // this.report(MyEvent.Start, { endTime: 0 });
    }

    private report<T extends keyof MyEventOptions>(event: T, options: MyEventOptions[T]) {
        // errors with `Type 'undefined' is not assignable to type 'number'.`
        const count: number = this.eventCounts[event] || 0;

        // This would do, for example:
        //
        // request(mixpanelUrl, { name: event, options });
    }
}

It doesn't complain if you change it to const count: number = (this.eventCounts.get(event) as number | undefined) || 0;

Expected behavior: Compiles without a problem.

Actual behavior:

Type '{ start?: number | undefined; stop?: number | undefined; }[T]' is not assignable to type 'number'.
  Type 'number | undefined' is not assignable to type 'number'.
    Type 'undefined' is not assignable to type 'number'.

Playground Link: https://is.gd/S57reZ

Related Issues: #32017

[sandersn]

@yaymukund typescript@next behaves the way you want it to, as far as I can tell. Can you confirm?

[yaymukund]

@sandersn Thank you for taking a look.

It's not fixed even in typescript@next. It looks like it only errors when strictNullChecks is true. I've made a repository that reproduces the error.

[RyanCavanaugh]

@yaymukund see #31908

[yaymukund]

Thanks for the heads up— I'll keep an eye on that issue!