Allow "T extends enum" generic constraint

TypeScript has a discrete enum type that allows various compile-time checks and constraints to be enforced when using such types. It would be extremely useful to allow generic constraints to be limited to enum types - currently the only way to do this is via T extends string | number which neither conveys the intent of the programmer, nor imposes the requisite type enforcement.

export enum StandardSortOrder {
    Default,
    Most,
    Least
}

export enum AlternativeSortOrder {
    Default,
    High,
    Medium,
    Low
}

export interface IThingThatUsesASortOrder<T extends enum> { // doesn't compile
    sortOrder: T;
}

[seanlaff]

I have a use case for this as well. I have a Select component that I want to make generic by taking any enum. I expect the caller to pass in a map of enumValue -> string labels.

If this feature existed, I could make a component like

function EnumSelect<T extends Enum>(options: { [e in T]: string }) {
    ...
}

Which would guarantee type-safety. (The compiler would prevent the developer ever forgetting to map a certain enum value to a user-friendly string representation)

[IanKemp]

why don’t

T extends StandardSortOrder | AlternativeSortOrder

Because I want to allow IThingThatUsesASortOrder to be used with any enum type.