Cannot extend generic type with a mixin

[dpogue]

There appears to be no way to extend a generic type using in a mixin-style pattern, because the type arguments cannot be passed through.

TypeScript Version: 2.7.1

Search Terms: base constructor, type arguments

Code

function subclass<T>(klasstype : T) : T {
  // This is our mixin, but for demo purposes just return the class
  return klasstype;
}

class DerivedPromise<T> extends subclass(Promise) { }

Expected behavior:
A class that extends Promise.

Actual behavior:
TS2508: No base constructor has the specified number of type arguments.

Playground Link:
Here

[DanielRosenwasser]

TypeScript takes what's called an ExpressionWithTypeArguments in an extends clause:

function subclass<T>(klasstype : T) : T {
  // This is our mixin, but for demo purposes just return the class
  return klasstype;
}

class DerivedPromise<T> extends subclass(Promise)<T> { }

[dpogue]

Thanks, that works perfectly, but is not what I would have expected the syntax to be