Type issue

example.ts

// using TS V3.7.5

type ClassType<T> = new () => T;

class DefaultClass {}

class RedClass {}

class OrangeClass {}

class GreenClass {}

const enum MyEnum {
  RED = 'RED',
  ORANGE = 'ORANGE',
  GREEN = 'GREEN',
}

const getColour = (): MyEnum => {
  // just an example, could be any of the enum values (in reality comes from a request)
  return MyEnum.GREEN;
};

const myFunction = <T, S>(defaultClassType: ClassType<T>, overrides: { colour: MyEnum; classType: ClassType<S> }[]) => {
  const colour = getColour();
  const colourOverride = overrides.find(or => or.colour === colour);

  if (colourOverride) {
    return new colourOverride.classType();
  } else {
    return new defaultClassType();
  }
};

// Query: this infers the type of instance as 'DefaultClass | GreenClass' but we want it to be 'DefaultClass | RedClass | OrangeClass | GreenClass'
const instance = myFunction(DefaultClass, [
  { colour: MyEnum.RED, classType: RedClass },
  { colour: MyEnum.ORANGE, classType: OrangeClass },
  { colour: MyEnum.GREEN, classType: GreenClass },
]);

type ClassType<T> = new () => T;

class DefaultClass {}

class RedClass {}

class OrangeClass {}

class GreenClass {}

const enum MyEnum {
    RED = 'RED',
    ORANGE = 'ORANGE',
    GREEN = 'GREEN',
}

const getColour = (): MyEnum => {
    // just an example, could be any of the enum values (in reality comes from a request)
    return MyEnum.GREEN;
};

const myFunction = <
    T,
    O extends Array<{ colour: MyEnum; classType: ClassType<unknown> }>,
>(
    defaultClassType: ClassType<T>,
    overrides: O,
): T | InstanceType<O[number]['classType']> => {
    const colour = getColour();
    const colourOverride = overrides.find((or) => or.colour === colour);

    if (colourOverride) {
        return new colourOverride.classType() as InstanceType<
            O[number]['classType']
        >;
    } else {
        return new defaultClassType();
    }
};

const instance = myFunction(DefaultClass, [
    { colour: MyEnum.RED, classType: RedClass },
    { colour: MyEnum.ORANGE, classType: OrangeClass },
    { colour: MyEnum.GREEN, classType: GreenClass },
]);

@OliverJAsh 🙏 thank you this is amazing