Describing a Typescript constructor for a class with generic types

Question

There's a function I use in testing:

export function cast<T>(obj: any, klass: new (...args: any[]) => T): T {
  if (!(obj instanceof klass)) {
    throw new Error(`Not an instance of ${klass.name}: ${obj.constructor.name}`);
  }
  return obj;
}

which is used like this.

interface I {...}
class C implements I {...};

const i: I = ...;
const c = cast(i, C);

However, if I try to use generics, the compiler complains string in the call.

interface I<T> {...}
class C<T> implements I {...};

const i: I<string> = ...;
const c = cast(i, C<string>);
                    ~~~~~~

Type 'any' has no signatures for which the type argument list is applicable. ts(2635)

Any thoughts on how I can make a version of cast that is friendly to generics? Even if it can't verify that instances in i are of type string, finding a way to get it to compile would be useful (otherwise I have to use as anyway.)

TypeScript cast type using generics and constructor? looks close, but seems bound to a specific type hierarchy.

I cannot reproduce this TS Playground Link. Can you create a minimal reproducible example? (IE providing a playground link would be best). — Cody Duong
– Cody Duong, Commented Jun 26, 2022 at 4:39
I'm using Typescript 4.4.4 and with a small change, the minimal example fails to compile. So whatever it is, has changed in between. Specifically, changing from 4.6.4 to 4.7.4 removes the error. — kberg
– kberg, Commented Jun 26, 2022 at 5:41
This is because of the Instantiation expression pattern added in TS4.7.0 github.com/microsoft/TypeScript/pull/47607. C<string> fails to compile because before 4.7 there was no support to instantiate generic functions/classes with a specific type parameter. Short of upgrading version, there isn't really a terse or compact workaround (I know of). I have done a workaround for instantiation expressions here, but for functions, and it may or may not be of any use. — Cody Duong
– Cody Duong, Commented Jun 26, 2022 at 6:02
That sounds about right. Thanks. I'm prevented from upgrading due to downstream dependencies. In the meantime you want to make that an answer, I'll accept it. — kberg
– kberg, Commented Jun 28, 2022 at 4:25

Cody Duong · Accepted Answer · 2022-06-29 06:44:04Z

Answer

Requires Instantiation expression pattern added in TS4.7.0 https://github.com/microsoft/TypeScript/pull/47607. C<string> fails to compile because before 4.7 there was no support to instantiate generic functions/classes with a specific type parameter. Short of upgrading version, there isn't really a terse or compact workaround (I know of). I have done a workaround for instantiation expressions here, but for functions, and it may or may not be of any use.

Workaround

You'll have to explicitly declare C<string> as the generic parameter.

const c = cast<C<string>>(i, C)

Or you can create an instantiator function (although this will only work on class C)

export const I = <T>(
  Base: new (...args: any[]) => C<T>
) => { 
  return Base
}

export const CString = I<string>(C)

const c = cast(i, I<string>(C))
const c2 = cast(i, CString)

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