Use in keyword for enums

[pocesar]

Although enums are both numbers and strings, it's not possible to use a type like this:

enum Values {
  A, B, C
}

type ValuesConstraint = {
  [index: Values]: string; // An index signature parameter type must be 'string' or 'number'
}

const Mapped: ValuesConstraint = {
   [Values.A]: 'Alpha',
   [Values.B]: 'Beta',
   [Values.C]: 'Gamma'
}

const f = Mapped[Values.C] // f: any

Tried to use in as in [index in Values] but it says Type 'Values' is not assignable to type 'string'. Trying to use a type ValuesType = Values.A | Values.B | Values.C also doesn't work, is there any workaround?

[RyanCavanaugh]

See #2491, #13042