Array mapped type change breaks ember array 'get'

[ghost]

TypeScript Version: 3.1.0-dev.20180801
(works in 3.1.0-dev.20180731)

Code

Simplified from the compile error in DefinitelyTyped/types/ember:

type ComputedPropertyGetters<T> = { [K in keyof T]: ComputedProperty<T[K]> | T[K] };

declare class ComputedProperty<T> { t: T; }

interface Array<T> {
    get<T, K extends keyof T>(this: ComputedPropertyGetters<T>, key: K): T[K];
    firstObject: ComputedProperty<T | undefined>;
}

const n: number | undefined = [0].get("firstObject");

Expected behavior:

No error.

Actual behavior:

src/a.ts:10:7 - error TS2322: Type 'ComputedProperty<number | undefined>' is not assignable to type 'number'.

10 const n: number | undefined = [0].get("firstObject");

The intention was for the following type arguments to be inferred:

[0].get<{ firstObject: number | undefined }, "firstObject">("firstObject");

The code still works when the above type is explicitly provided.

I think this change is due to #26063 (CC @ahejlsberg).

[RyanCavanaugh]

@andy-ms I can't repro on master - any additional details needed?

[ahejlsberg]

@DanielRosenwasser Looks to me like we're doing the right thing here. You mentioned you'd discuss with the Ember folks.

[typescript-bot]

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