Homomorphic mapped types can't work on arrays while using constrained helper types

[DanielRosenwasser]

I'm on TypeScript 3.1, and Brian Terlson (@bterlson) asked if there was a way to construct all constructors inside of an array. However, trying to do this in a way that worked for object properties as well as array-likes seems impossible

type ConstructAll<T> = {
 [K in keyof T]: InstanceType<T[K]>
};

type Foo = ConstructAll<[typeof Date, typeof RegExp]>;

This gives an error on the first type argument to InstanceType:

[ts] Type 'T[K]' does not satisfy the constraint 'new (...args: any[]) => any'.

Ugh. Okay, fine.

type AnyConstructor = new (...args: never[]) => unknown;

type ConstructAll<T extends Record<string, AnyConstructor>> = {
 [K in keyof T]: InstanceType<T[K]>
};

type Foo = ConstructAll<[typeof Date, typeof RegExp]>;

Which gives me an error on the first type argument to ConstructAll:

[ts]
Type '[Date, RegExp]' does not satisfy the constraint 'Record<string, new (...args: any[]) => any>'.
 Index signature is missing in type '[Date, RegExp]'.

I'll save you time; the following doesn't work either:

type AnyConstructor = new (...args: never[]) => unknown;

type ConstructAll<T extends AnyConstructor[] | Record<string, AnyConstructor>> = {
 [K in keyof T]: InstanceType<T[K]>
};

type Foo = ConstructAll<[typeof Date, typeof RegExp]>;

You end up with the original scenario. I end up not being able to reuse InstanceType at all because it has a constraint that we can't dive into. I have to instead use some unconstrained conditional type.

type Constructor<T> = new (...args: any[]) => T;

type ConstructAll<T extends Constructor<unknown>[] | Record<string, Constructor<unknown>>> = {
 [K in keyof T]: T[K] extends Constructor<infer T> ? T : never;
};

type Foo = ConstructAll<[typeof Date, typeof RegExp]>

It seems problematic that we didn't want to "split the world" into array mapped types and object mapped types, but you end up with that problem anyway in some cases.

[ahejlsberg]

Any reason you don't like this?

type ConstructAll<T> = {
 [K in keyof T]: T[K] extends new (...args: any[]) => any ? InstanceType<T[K]> : undefined;
};

type Foo = ConstructAll<[typeof Date, typeof RegExp]>;

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.