Type argument inference does not take call's contextual type into account

[zakjan]

TypeScript Version: 2.0.0

Code

function map(array: number[]): [number, number][] {
 return array.map((x: number) => [x, x * x]);
}

Expected behavior:
no error

Actual behavior:

error TS2322: Type 'number[][]' is not assignable to type '[number, number][]'.
 Type 'number[]' is not assignable to type '[number, number]'.
 Property '0' is missing in type 'number[]'.

[DanielRosenwasser]

What's happening here is that the call to array.map doesn't try to infer its return type from its contextual type. It only tries to infer from its arguments.

A workaround would be to give [number, number] as a type argument to map:

function map(array: number[]) {
 return array.map<[number, number]>(x => [x, x * x]);
}

[manueledones]

Daniel Rosenwasser (@DanielRosenwasser) Here some other examples:

export interface DataModel {
 prop1: string;
 prop2: number;
}

const array = [1,2,3,4];

// NO ERROR
const datamodels2: DataModel[] = array.map(i => ({
 prop1: 'asdf',
 prop2: 1234,
 extra: 'extra'
}))

// NO ERROR
const datamodels4: DataModel[] = array.map(i => {
 return {
 prop1: 'asdf',
 prop2: 1234,
 extra: 'extra'
 } 
});

// NO ERROR
const datamodels5: DataModel[] = array.map(i => {
 return {
 prop1: 'asdf',
 prop2: 1234,
 extra: 'extra'
 } as DataModel
});

// NO ERROR
const datamodels6: DataModel[] = array.map(i => ({
 prop1: 'asdf',
 prop2: 1234,
 extra: 'extra'
}) as DataModel);

// NO ERROR
const datamodels7: DataModel[] = array.map<DataModel>(i => ({
 prop1: 'asdf',
 prop2: 1234,
 extra: 'extra'
}));

`
Is this the expected behavior or a bug?

[RyanCavanaugh]

@whiteblackkeys Unrelated. Root cause is #241 - function return expression widening causes the object literal freshness to be lost, causing excess properties to not be checked.

[ahejlsberg]

Fixed by #29478.