In TypeScript, how can i get a generic type parameter to be inferred in a method that has multiple type parameters?

Question

I'm trying to get the type of T3 to be inferenced properly in the example below, but it is just being shown as unknown . I can't understand how it can properly infer the base type T2 when calling Testing.testIt , but not the type parameter for the class it extends ( T3 ). Here is the running example . I was trying to figure out if the infer keyword would be useful here, but I can't see how that would fit into this.

export interface Type<T> extends Function {
    new (...args: any[]): T;
}

class GenericClass<T> {
}

class ChildClass extends GenericClass<string> {

}

class Testing {
    static testIt<T3, T2 extends GenericClass<T3>>(testClass: Type<T2>): T3 {
      console.log('testIt called');
      return '' as any;
    }

    static testIt2(val: string): void {
      console.log(val);
    }
}

const result = Testing.testIt(ChildClass);
Testing.testIt2(result);

Answer 1

You'll want to define a helper to extract the type:

type ExtractGenericClassType<T> = T extends GenericClass<infer TInner> ? TInner : never;

class Testing {
    static testIt<T extends GenericClass<any>>(testClass: Type<T>): ExtractGenericClassType<T> {
      ...
    }
}

Moreover, you need to ensure that your generic class actually uses the generic type parameter, otherwise this will not work.