keyof widening to string

[dpogue]

TypeScript Version: Version 2.7.0-dev.20171118

Code

class Foo<DataType> {
    public set<K extends keyof DataType>(prop : K, value : DataType[K]) {
        this.update({[prop]: value}); // Error here
    }
  
    update(data: Partial<DataType>) {
        return Promise.resolve(this);
    }
}

Expected behavior:
Compile with no errors.

Actual behavior:
Argument of type { [x: string]: DataType[K]; } is not assignable to parameter of type Partial<DataType>.

This is very similar (but maybe not identical) to #13948 and #15534

[mhegazy]

Please see #16687 (comment) for more details

[RyanCavanaugh]

TypeScript has no other way to represent the type of the value produced by {[prop]: value}. We'd need some proposal for an all-encompassing feature for representing that along with use cases and proposed behavior.