Generic constraint with type list

[NN---]

It is a generalization of #2049 .
I would like to see generic specifying concrete types:

// Can specify only A<number> or A<string>
interface A<T is number|string|Object> { }  

interface B<T is number|string> extends A<T> {} // number|string is subtype of number|string|Object .

[RyanCavanaugh]

In order to be considered, we need suggestions to have at least some motivating use cases, examples of desired usage, examples of things that would be / would not be errors and why, etc.. We don't have resources to ask clarifying questions teasing out what these things mean.

[NN---]

One of use cases is the following:

interface KeyValueBase<K> {
  f(): K;
}

interface KeyValue<K is number|string, V> extends KeyValueBase<K> {
 [key: K]: V
}

The key type must be string or number , it is impossible to define this generic right now.
It can be possible to define it and use the type if the feature existed.

[DanielRosenwasser]

You could also use a type alias.

type Key = number | string;

interface KeyValueBase {
  f(): Key;
}

interface KeyValue extends KeyValueBase {
 [key: Key]: V
}

[NN---]

@DanielRosenwasser It is not the same since I don't know the exact type.
Also the indexer is one of possible samples.

[litera]

Object indexer is the perfect example and use case for is genetic type parameter constraint. This hasn't been updated in a while. Is there any progress?

[litera]

@DanielRosenwasser And btw even when using type aliasing TSLint still throws an error.

[massimonewsuk]

One issue I had is with Object.entries, but the type definition for that was pretty simple:

interface ObjectConstructor {
    entries<TValue>(object: { [key: string]: TValue }): [string, TValue][];
    entries<TValue>(object: { [key: number]: TValue }): [string, TValue][];
}

I understand it won't work for everything (objects that aren't strictly used as maps), so it would probably be better to just have your own "wrapper function" around Object.entries, but I thought I'd mention it in case it helps anyone else who finds this issue.