Type inference problem in generic subtypes of index signature types

[kazimuth]

TypeScript Version: 2.1.4

Code

// Type with index signature
type StringToNumber = {[id: string]: number};

// This works!
type ShouldBeNumber = StringToNumber[keyof StringToNumber];
let x: ShouldBeNumber = <number>0;
let y: number = x;

// This doesn't work.
class Pred<X extends StringToNumber> {
 test(value: X[keyof X]) {
 // error TS2322: type 'X[keyof X]' is not assignable to type 'number'
 let slot: number = value;

 // error TS2352: Type 'X[keyof X]' cannot be converted to type 'number'.
 let slot2 = <number> value;
 }
}

Expected behavior:
If I understand the spec correctly, all of X's properties must be subtypes of their corresponding properties in StringToNumber. All of StringToNumber's properties are of type number, so X[keyof X] should be a subtype of / assignable to number.

Actual behavior:
As far as I can tell, no relationship is inferred between the types X[keyof X] and number.